board-invited review: intrauterine growth … review: intrauterine growth retardation: implications...

BOARD-INVITED REVIEW: Intrauterine growth retardation:Implications for the animal sciences1

G. Wu,*2 F. W. Bazer,* J. M. Wallace,† and T. E. Spencer*

*Department of Animal Science, Texas A&M University, College Station, TX 77843; and†Rowett Research Institute, Bucksburn, Aberdeen, AB21 9SB, UK

ABSTRACT: Intrauterine growth retardation(IUGR), defined as impaired growth and developmentof the mammalian embryo/fetus or its organs duringpregnancy, is a major concern in domestic animal pro-duction. Fetal growth restriction reduces neonatal sur-vival, has a permanent stunting effect on postnatalgrowth and the efficiency of feed/forage utilization inoffspring, negatively affects whole body compositionand meat quality, and impairs long-term health andathletic performance. Knowledge of the underlyingmechanisms has important implications for the preven-tion of IUGR and is crucial for enhancing the efficiencyof livestock production and animal health. Fetal growthwithin the uterus is a complex biological event influ-enced by genetic, epigenetic, and environmental factors,as well as maternal maturity. These factors impact onthe size and functional capacity of the placenta, utero-placental blood flows, transfer of nutrients and oxygenfrom mother to fetus, conceptus nutrient availability,the endocrine milieu, and metabolic pathways. Alter-ations in fetal nutrition and endocrine status may re-

Key words: fetal growth, fetal programming, intrauterine growth retardation, nutrition, pregnancy

©2006 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2006. 84:2316–2337doi:10.2527/jas.2006-156

INTRODUCTION

Growth (an increase in the number and size of cellsor in the mass of tissues) and development (changes inthe structure and function of cells or tissues) of thefetus are complex biological events influenced by ge-netic, epigenetic, maternal maturity, as well as environ-

1Supported by National Research Initiative Competitive GrantsNo. 2001-35203-11247, 2003-35206-13694, and 2005-35203-16252,National Institutes of Health Grants No. 1R01HD38274,2R01HD32534, and 5P30ES09106,and the Scottish Executive Envi-ronment and Rural Affairs Department grant-in-aid to the RowettResearch Institute.

2Corresponding author: [email protected] March 19, 2006.Accepted May 8, 2006.

2316

sult in developmental adaptations that permanentlychange the structure, physiology, metabolism, and post-natal growth of the offspring. Impaired placental syn-theses of nitric oxide (a major vasodilator and angio-genic factor) and polyamines (key regulators of DNAand protein synthesis) may provide a unified explana-tion for the etiology of IUGR in response to maternalundernutrition and overnutrition. There is growing evi-dence that maternal nutritional status can alter theepigenetic state (stable alterations of gene expressionthrough DNA methylation and histone modifications)of the fetal genome. This may provide a molecular mech-anism for the role of maternal nutrition on fetal pro-gramming and genomic imprinting. Innovative inter-disciplinary research in the areas of nutrition, repro-ductive physiology, and vascular biology will play animportant role in designing the next generation of nutri-ent-balanced gestation diets and developing new toolsfor livestock management that will enhance the effi-ciency of animal production and improve animal wellbeing.

mental and other factors (Redmer et al., 2004; Goot-wine, 2005). These factors affect the size and functionalcapacity of the placenta, uteroplacental transfer of nu-trients and oxygen from mother to fetus, conceptus nu-trient availability, the fetal endocrine milieu, and meta-bolic pathways (Bell and Ehrhardt, 2002; Fowden et al.2005; Reynolds et al., 2005).

The effects of uterine capacity, which can be definedas the physiological and biochemical limitations im-posed on conceptus growth and development by theuterus (Bazer et al., 1969a,b) and maternal nutrition onfetal growth have clearly been demonstrated by studiesinvolving embryo transfer (Dickinson et al., 1962; Fer-rell, 1991; Allen et al., 2002) and altered maternal nutri-ent intake (Redmer et al., 2004), respectively. Further,uterine environment can affect the size of the fetus, asdemonstrated in different breeds of pigs (Wilson et al.,

Published December 8, 2014

Board-invited review: Fetal growth restriction 2317

1998). There are a plethora of studies aimed at identi-fying nutritionally sensitive periods of conceptus (i.e.,embryo/fetus, associated placental membranes, and fe-tal fluids) development. Available evidence suggeststhat the prenatal growth trajectory of all eutherians(placental mammals) is sensitive to the direct and indi-rect effects of maternal nutrition at all stages betweenoocyte maturation and birth (Robinson et al., 1999; Reh-feldt et al., 2004; Ferguson, 2005).

Intrauterine growth retardation (IUGR) can be de-fined as impaired growth and development of the mam-malian embryo/fetus or its organs during pregnancy.Because it is easy to measure practically on farms andin clinics, fetal weight or birth weight relative to gesta-tional age is often used as a criterion to detect IUGR.Naturally occurring and environmentally (e.g., over-and underfeeding, heat stress, disease, and toxins) in-duced IUGR are well documented for livestock (includ-ing cattle, goat, horse, pig, and sheep; Pond et al., 1969;Baker et al., 1969; Wallace et al., 2005b) and litter-bearing small mammals (e.g., dog, mouse, and rat;Wootton et al., 1983).

Despite improvement of management techniques andintensive research on mammalian nutrient require-ments over the past half-century, IUGR remains a sig-nificant problem in animal agriculture because of ourincomplete knowledge concerning the impact of nutri-tion on the mechanisms regulating fetal growth. Themajor objective of this article is to critically review theliterature on IUGR in domestic animals, its implica-tions for the animal sciences, its putative biologicalmechanisms, and its potential solutions. Readers arereferred to recent reviews for discussion of IUGR inrodents and humans (Wu et al., 2004a; McMillen andRobinson, 2005; Murphy et al., 2006).

INTRAUTERINE GROWTH RETARDATIONIN LIVESTOCK

Significant losses of embryos/fetuses occur duringearly, mid, and late gestation (Geisert and Schmitt,2002; van der Lende and van Rens, 2003; Jonker, 2004).In addition to the genetic contribution from both par-ents, fetal growth and development are affected by avariety of environmental and other factors. These in-clude maternal nutrition (low or high feed intake, andnutrient imbalance), maternal intestinal malabsorp-tion, inadequate provision of amniotic and allantoicfluid nutrients, the ingestion of toxic substances, envi-ronmental temperature and stress, disturbances in ma-ternal or fetal metabolic and homeostatic mechanisms,insufficiency or dysfunction of the uterus, endome-trium, or placenta, and poor management (Mellor,1983; McEvoy et al., 2001; Redmer et al., 2004; Wu etal., 2004a).

The outcome of stressful conditions in utero dependson their nature, severity, stage of gestation, and dura-tion. Thus, multiple factors regulate conceptus growth

Figure 1. Regulation of mammalian fetal growth. Intra-uterine growth is regulated by genetic, epigenetic, andenvironmental factors. These factors affect placentalgrowth and therefore the availability of nutrients for fe-tal growth.

and contribute to IUGR (Figure 1). Insufficient uterinecapacity and inadequate maternal nutrition are 2 majorfactors that impair fetal growth.

Uterine Capacity and IUGR

Experimentally Induced Uterine Insufficiency. Al-though the fetal genome plays an important role ingrowth potential in utero, increasing evidence suggeststhat the intrauterine environment is an important de-terminant of fetal growth (Wilson, 2002). Further, theintrauterine environment of the individual fetus maybe of greater importance in the etiology of chronic dis-eases in adults than the genetics of the fetus (Wu et al.,2004a). Additionally, in domestic animals (includingsheep, cattle, and horses), when the embryo from agenetically larger mother was transferred to a recipientdam with a lower uterine capacity, IUGR was a preg-nancy outcome (Dickinson et al., 1962; Ferrell, 1991;Allen et al., 2002). Conversely, when the embryo froma genetically smaller mother was transferred to a recipi-ent dam with a greater uterine capacity, fetal growthwas enhanced (Dickinson et al., 1962; Ferrell, 1991;Allen et al., 2002) but cardiovascular dysfunction oc-curred (Giussani et al., 2003). Notably, genetic selectionof gilts for high uterine capacity led to increased litter

Wu et al.2318

size and total litter weight without a change in averagepiglet weight (Vallet et al., 2002).

Production-Imposed Uterine Insufficiency.Through improving reproductive technologies (e.g., em-bryo transfer and hormonal induction of ovulation),twinning in cattle offers an important means to increasethe efficiency of beef production, because the overheadcosts for maintaining single-calving cows account formore than 50% of the total costs of beef production(Guerra-Martinez et al., 1990). Herd input costs perunit of beef output value can be reduced by 24% in twincompared with single births (Guerra-Martinez et al.,1990). However, twin-bearing heifers and cows can lose10 to 12% of empty BW during the last one-third ofpregnancy. Also, twinning reduces fetal growth and calfbirth weight. In sheep, high prolificacy is a desirabletrait under intensive management systems, and in-creasing the prolificacy of ewes through genetic selec-tion is an effective means to increase the profitabilityof lamb production (Gootwine et al., 2001). However, anincreased number of fetuses within the uterus results inrelative placental insufficiency and low birth weights(Gootwine et al., 2006). These observations indicate achallenge for reducing the risk of IUGR associated withcurrent reproductive technologies that increase ovula-tion rates.

Maternal or gynecological immaturity is a prototypefor production-imposed uterine insufficiency in live-stock. Domestic animals are often bred at immatureBW to maximize their production performance. For ex-ample, it is commonly recommended and practiced thatewe lambs be bred with fertile rams at two-thirds oftheir mature BW with the goal of first lambing at 12to 13 mo of age (Chappell, 1993). Similarly, heifers andgilts enter the first pregnancy at 70 to 80% of theirmature BW. The birth weights of the first-parity prog-eny (e.g., in lambs, calves, piglets, and foals) from im-mature dams are generally 10 to 15% lower comparedwith the offspring born from dams of mature adult BW(Bellows and Short, 1978; Quiniou et al., 2002; Wilsherand Allen, 2003). This can be explained by the fact thatmother and fetus grow substantially and compete fornutrients during pregnancy (Redmer et al., 2004; Wuet al., 2004a). Interestingly, when heifers began thefirst pregnancy at a more mature BW achieved at 21rather than 15 mo of age, the parity of the dam had noeffect on fetal growth rate (Tudor, 1972). Thus, it is thematurity of the dam rather than parity that affectsintrauterine growth.

Natural Uterine Insufficiency. Natural IUGR is fre-quently observed in dams with multifetal pregnancies(Wootton et al., 1983). Although total placental weightis increased in these animals, placental mass per fetusis reduced, resulting in relative placental insufficiency(Redmer et al., 2004). Thus, in ewes, the individualbirth weight of a lamb in triplet and twin pregnancies isonly 62 and 78%, respectively, of a singleton pregnancy(Gootwine, 2005). Twins account for 38 to 52% of allpregnancies in sheep (USDA, 2003). Even in well-fed

ewes, multifetal pregnancy impairs fetal growth, in-cluding reduction in the skeletal muscle mass and myo-fiber number of neonates (Greenwood et al., 2000). Simi-lar findings have been observed for heifers and cows(Guerra-Martinez et al., 1990) as well as horses (Ross-dale and Ousey, 2002) when natural multifetal preg-nancy occurs.

Among domestic animals, pigs exhibit the most se-vere, naturally occurring IUGR. Before d 35 of gesta-tion, porcine embryos are uniformly distributed withinthe uterine horn (Anderson and Parker, 1976). Afterthis date, uterine capacity becomes a limiting factor forfetal growth, even though the fetuses are distributedrelatively uniformly (Knight et al., 1977). Conse-quently, porcine fetal development may depend on theposition and number of fetuses in the uterus, in thatfetuses near both ends of the uterus (i.e., the uterotubaljunction and the cervix) are generally larger than thosein the middle of the horn (Perry and Rowell, 1969). Thisdifferential growth of porcine fetuses in relation to theirposition within the uterus is evident, particularly dur-ing late gestation, in pregnancies in which the numberof fetuses exceeds 5 per horn (Perry and Rowell, 1969).

However, runts can be present at any position withinthe uterus and are more related to the size of the pla-centa. At birth, runt piglets may weigh only one-halfor even one-third as much as the largest littermates(Widdowson, 1971). The small intestine, liver, and skel-etal muscle of the runt pig are disproportionatelysmaller than those of the largest littermates at birth(Widdowson, 1971). Whereas IUGR could be consideredas a natural mechanism to protect the dam in cases ofundernutrition, it may not be beneficial for the survivaland growth performance of the progeny or the efficiencyof livestock production in animal agriculture.

Undernutrition and IUGR

Undernutrition Under Practical Production Con-ditions. Animals of agricultural importance are raisedunder various production conditions (e.g., intensive andextensive systems), depending on the species, region,and season. Pigs are commonly housed in pens and fedprimarily plant-based formulated diets (an intensivesystem). Ruminants (e.g., cattle, sheep, goats, and deer)and horses are allowed to graze pasture in rangelandand consume forages (an extensive system); receivefeedlot diets containing various supplemental levels ofenergy, protein, vitamins, and minerals; or a combina-tion of these. Pasture grazing is the most common prac-tice for managing dairy cows worldwide, and it is gain-ing renewed interest in the United States (Boken et al.,2005; Fontaneli et al., 2005). Also, beef herds in theUnited States and worldwide are managed under condi-tions varying from confinement cow-calf productionunits to the more common grazing systems. However,the quality of forages and roughages is often poor, par-ticularly in dry and winter seasons, and is inadequatefor optimal nutrition of growing, gestating, and lactat-


ing herbivores (including ruminants and horses) with-out high-quality protein and energy supplements (Lip-pke, 1980; Hoaglund et al., 1992; Huston et al., 1993;Fontaneli et al., 2005). In extensive production systemsworldwide, there is little or no supplement provided forgrazing ruminants (Fontaneli et al., 2005).

Thus, fetal undernutrition frequently occurs in ani-mal agriculture, leading to reduced fetal growth. Forexample, Thomas and Kott (1995) reported that, with-out any supplement, the nutrient uptake of grazingewes in the western United States is often less than50% of the National Research Council (NRC) recom-mendations (NRC, 1985). Unsupplemented, grazingewes lose a significant amount of BW during pregnancy,and their health, fetal growth, and lactation perfor-mance are seriously compromised (Thomas and Kott,1995). Also, the content of MP in the grazed forage,particularly during winter, is low (often <8% on a DMbasis) and is inadequate for supporting optimal repro-ductive performance of beef heifers or cows (Pattersonet al., 2003; Ferguson, 2005). Additionally, the sheepis a seasonal breeder. In the United States, ewes usuallyenter pregnancy in late fall or early winter seasons,and therefore, most of the gestational period coincideswith winter, when the grazed forage is of low quality(Hoaglund et al., 1992). Also, gestating heifers in feedlotsituations often have inadequate intakes of nutrientsand poor pregnancy outcomes (including reduced fetalgrowth; Kreikemeier and Unruh, 1993). Another exam-ple of production-imposed fetal undernutrition is short-ening of the postpartum-to-breeding or interpregnancyintervals. Although this practice is desirable for in-creasing the potential economic return from livestockproduction (Ferguson, 2005), it results in maternal nu-tritional depletion at the outset of pregnancy (Wu etal., 2004a).

Finally, in tropical or subtropical regions, high envi-ronmental temperatures reduce feed intake by preg-nant dams that graze pasture in open rangelands orby pigs housed without air conditioning. The thermalstress will cause IUGR in animals (Reynolds et al.,1985; 2005; Wallace et al., 2005c). Conversely, exposureto a cold climate can increase the utilization of dietaryenergy for maintaining maternal and fetal body temper-atures, thereby reducing the availability of nutrientsfor fetal growth (Ferguson, 2005).

Undernutrition Due to Maternal PhysiologicalExtremes. Various physiological extremes of gestatingor lactating dams often result in fetal undernutrition.Ewes commonly exhibit ketosis during late gestationdue to an energy deficit (Wastney et al., 1982), andacidotic conditions are associated with increased catab-olism of branched-chain AA and glutamine by skeletalmuscle and kidneys, respectively (Wu and Marliss,1992). Indeed, ditocous ewes fed a 12%-CP diet (currentNRC requirement) exhibited negative protein balancein maternal tissues between d 110 and 140 of gestation,indicating significant mobilization of protein reserves(McNeill et al., 1997). Similarly, bovine fetal undernu-

trition often occurs during late pregnancy, particularlyin multiparous cows. In heifers and mature cows, volun-tary feed intake usually decreases by 30 to 35% duringthe last 3 wk before calving (Grummer, 1995), whenthe absolute rate of fetal growth is most rapid (Ferrell,1991). The reduced feed intake during this transitionperiod is further decreased by conditions such as twinpregnancies, increase in body condition, primiparouspregnancy, and thermal stress (Grummer, 1995). Com-plicating the metabolic challenge during late preg-nancy, much of gestation is concurrent with lactationin multiparous cows, where additional amounts of nu-trients are required for conceptus growth (Knight,2001). Low protein intake (e.g., 80% of NRC require-ment) prepartum further reduces DMI in pregnantcows (Chew et al., 1984). These metabolic interplayscause negative energy and protein balances prepartumin heifers and cows (Grummer, 1995; Bell et al., 2000).

Low precalving BW of the cow is associated with lowbirth weight of the calf (Bellows et al., 1971). There isalso a nutritional inadequacy before mating in lactatingheifers and cows beginning at the second or greaterparity. In these animals, milk output generally peaksat about 2 mo postpartum, but feed intake usually takesat least 2 mo (in some cases up to 4 to 5 mo despiteprovision of a high-quality diet) to reach its maximum,therefore resulting in negative nutrient balance (partic-ularly energy and protein deficits) during early or mid-lactation (Bauman and Currie, 1980; Bar-Peled et al.,1998). This period usually coincides with the early stageof pregnancy in multiparous cows, and undernutritionaffects embryonic and fetal development (Redmer etal., 2004). The findings that calf birth weight was in-creased in heifers and cows in response to dietary sup-plementation with protein and energy concentratesduring late gestation (Clanton and Zimmerman, 1970;Bellows and Short, 1978) suggest that undernutritioncaused by the maternal physiological extremes impairsfetal growth in unsupplemented dams.

Besides the ruminant, low feed intake remains a sig-nificant problem for lactating sows before breeding,when the mobilization of nutrient reserves for milk pro-duction results in a severe catabolic state and a pro-longed interval from farrowing to estrus (Cole, 1990).A 3-yr study of 10,200 lactating sows on 120 farms inthe United States showed that feed consumption couldbe as low as 70% of the NRC requirements (Johnson,1993). Inadequate nutrition increased losses of BW andbackfat in lactating sows and also prolonged weaning-to-estrus intervals (Johnson, 1993). When sows enterpregnancy, the suboptimal nutritional status (namelypremating maternal undernutrition), coupled with re-stricted feed intake (Ji et al., 2005), may negativelyaffect the growth and development of early embryosand fetuses (Vinsky et al., 2006).

Maternal insulin resistance gradually develops incows (Bell et al., 2000), ewes (Wastney et al., 1982),horses (Hoffman et al., 2003), and sows (Kemp et al.,1996) during late pregnancy (Bell et al., 2000), likely

Wu et al.2320

because of the inability of the liver and skeletal muscleto oxidize the fatty acids released from adipose tissuein response to a negative energy balance (Ferguson,2005). An increase in plasma and tissue levels of freefatty acids is a major factor contributing to the occur-rence of insulin resistance (Jobgen et al., 2006). Thereis evidence that low glucose tolerance of pregnant sowsis associated with high postnatal mortality of piglets(Kemp et al., 1996).

Whereas insulin resistance in the dam may have thepotential to increase the availability of glucose and AAfor the fetus, the transfer of nutrients from mother tofetus may be impaired under this condition. Becauseinsulin stimulates muscle protein synthesis and inhib-its muscle protein degradation, insulin resistance in-creases the net rate of whole-body proteolysis and thusplasma levels of methylarginines (protein-derived in-hibitors of endothelial nitric oxide [NO] synthesis; Mar-liss et al., 2006). Because NO is a major regulator ofuteroplacental blood flows (Bird et al., 2003), severeinsulin resistance likely compromises the placental de-livery of nutrients and oxygen during late gestation. Insupport of this view, IUGR is associated with elevatedconcentrations of plasma asymmetric dimethylargininein obese subjects (Savvidou et al., 2003).

Experimentally Induced Undernutrition. In addi-tion to the above practical production and physiologicalconditions of undernutrition that can result in IUGR,well-controlled experimental studies have demon-strated that maternal undernutrition during the peri-conceptual or gestational periods reduces fetal growthin sheep (Mellor, 1983; Osgerby et al., 2002; Vonnahmeet al., 2003), cows (Tudor, 1972), pigs (Pond et al., 1969),and horses (Pugh, 1993). In adult sheep, severe under-nutrition during the periconceptual period acceleratesmaturation of the fetal hypothalamic-pituitary-adrenalaxis and causes preterm delivery (Fowden et al., 1994).Low prepregnancy weights, followed by undernutritionduring midpregnancy, result in reduced placentalgrowth and lower birth weights at term (Redmer etal., 2004). Studies involving the restricted intake ofnutrients solely during midgestation reveal variableeffects on the placental and fetal growth trajectory;however, if undernutrition is prolonged during latepregnancy, fetal growth is compromised, particularlyin twin pregnancies (Redmer et al., 2004; Luther et al.,2005a). Reduced provision of all nutrients to the ovinefetus through a combination of reduced maternal feedintake and carunclectomy also resulted in IUGR andparticularly impaired growth of the fetal gastrointesti-nal tract (Trahair et al., 1997). Intrauterine growthretardation in undernourished sheep is often associatedwith fetal hypoglycemia and hypoxemia as well as withincreased risks of fetal death and premature birth (Mel-lor, 1983).

In Hereford cows, submaintenance levels of nutritionduring the last trimester reduced calf birth weight (Tu-dor, 1972). In heifers bred at 15 mo of age, reducingnutrient intake from high to maintenance to low levels

via decreasing the amounts of feedlot rations or pastureavailability during the last 3 mo of pregnancy alsocaused a progressive decrease in birth weights of calves(Kroker and Cummins, 1979). Likewise, in beef heifersfed a low-level protein diet, a BW loss of 0.5 kg/d duringthe last trimester was associated with weak labor, in-creased incidence of dystocia, increased perinatal mor-tality, reduced postnatal growth of calves, and pro-longed postpartum anestrus (Kroker and Cummins,1979). Also, in heifers, decreasing daily TDN from 6.4to 3.4 kg for 90 d before calving led to a substantialloss of maternal tissues during pregnancy, reduced calfbirth weight, and prolonged postpartum-to-estrus in-tervals (Bellows and Short, 1978). In mares whose fetushas a limited ability to synthesize glucose during theentire gestation, maternal fasting caused an increaseduteroplacental production of PGF2α and uterine con-tractility, impaired fetal growth, premature delivery ofnonviable foals in most animals (>80% of pregnancies)during late gestation, and low birth weight (Fowden etal., 1994).

In contrast to the ruminant and horse, the pig gener-ally has a remarkable ability to mobilize maternal nu-trient reserves to support placental and fetal develop-ment during prolonged inanition in the presence of ade-quate progesterone and estrogen (Anderson, 1975).Thus, a modest reduction in the dietary intake of energyalone is not sufficient to cause IUGR in pigs. For exam-ple, in gilts fed adequate amounts of protein, vitaminsand minerals, restriction of dietary energy intake (50%of controls) did not affect birth weight of piglets (Atinmoet al., 1974). However, with a more severe reduction inenergy intake by gilts during the entire gestation from8.0 to 2.2 Mcal of DE/d caused a reduction in birthweights, the number of gastrocnemius muscle fibers,muscle weight, liver weight, liver glycogen content, andserum protein concentrations of newborn piglets (Bui-trago et al., 1974).

Energy deficiency likely reduces protein synthesis inthe liver and skeletal muscle. Results of the followingextensive studies indicate that maternal underfeedingof energy and protein impairs embryonic/fetal growthin pigs. First, reducing the intake of complete rationsby 50% for 2 estrous cycles before mating decreasedfetal weight at d 30 of pregnancy in gilts (Ashworth,1991). Similarly, in primiparous sows, restricting feedintake by 50% during lactation (a reduction from 5.0to 2.5 kg/d between d 14 and 21 of lactation) beforemating reduced the weight of both male and femalefetuses as well as the survival of female embryos at d 30of gestation (Vinsky et al., 2006). Second, birth weight ofpiglets decreased in response to restriction of feed in-take (e.g., 0.9 vs. 1.9 kg/d) or increased litter size (Bakeret al., 1969). Third, decreasing feed intake after d 80of gestation reduced fetal growth in gilts (Noblet et al.,1985). Finally, birth weights as well as brain and liverweights were reduced in the progeny of gilts fed a pro-tein-deficient diet throughout gestation (Pond et al.,1969; Atinmo et al., 1974). These findings suggest that


porcine fetal growth can be influenced by a severe ma-ternal protein-energy imbalance during pregnancy.

Overnutrition and IUGR

Increasing energy intake increases the rate of ovula-tion in farm animals (including cattle, sheep, pigs, andhorses). Thus, the practice of increasing feed intakeduring a short period of time (termed flushing) aroundthe time of conception has been employed by producersin an attempt to increase the number of embryos/fe-tuses (Cole, 1990). Overnutrition can result from in-creased intake of energy, protein, or both. Thus, ov-erfeeding of livestock and companion animals occurswhen excess amounts of diets (particularly concen-trates) are provided to dams before breeding or duringpregnancy (Han et al., 2000; Luther et al., 2005b). In-deed, overconditioning of cows during the dry periodstill occurs on many farms, particularly among high-producing herds (Ferguson, 2005).

Maternal overnutrition (high energy, high proteinfeeding, or both) during the premating period or earlypregnancy often results in increased porcine embryoand fetal mortality (Ashworth, 1991; Einarsson andRojkittikhun, 1993). Interestingly, like underfeeding,overfeeding once pregnancy is established retards fetalgrowth in pigs (Cole, 1990) and adolescent sheep (Wal-lace et al., 2004). Strikingly, feeding mares to obesitybefore or after mating can also reduce fetal growth andcause fetal death (Pugh, 1993). Overfeeding of dairycows during late pregnancy is associated with an in-creased risk of metritis, ketosis, milk fever, cystic ova-ries, and subsequent infertility. Further, overcondi-tioned cows are more susceptible to a prepartum de-crease in voluntary feed intake, thereby compromisingnutritional status in the mother and fetus (Ferguson,2005).

Increased feed intake by sows during all or part ofgestation has a negative effect on feed intake duringlactation (Han et al., 2000). In multiparous sows, in-creasing dietary intakes of both protein and energy by43% during the first 50 d of gestation, relative to astandard gestational diet (10.7 MJ of DE/kg and 12.0%CP), decreased the birth weights of the 2 lightest and2 heaviest piglets in litters (Bee, 2004). Likewise, ov-erfeeding both energy and protein between d 25 and 50of gestation had no beneficial effect on muscle fibernumber or area in the offspring but instead reducedskeletal muscle weight of newborn piglets due tosmaller fiber size (Nissen et al., 2003). Furthermore,overfeeding gilts by 40% of the NRC requirements(NRC, 1998) during the entire gestation impaired fetaldevelopment and postnatal survival (Han et al., 2000).These results indicate that overfeeding during all orpart of the gestation has a detrimental effect on preg-nancy outcomes in domestic animals.

IMPLICATIONS OF IUGR FOR THEANIMAL SCIENCES

The major goals of animal production are to enhancethe efficiency of feed/forage utilization, produce abun-

dant, healthy meats, eggs, milk, and wools for increas-ingly health-conscious consumers, and improve thequality of human life. Fetal growth restriction is a sig-nificant obstacle to achieving these goals. The availableevidence suggests that IUGR has permanent negativeimpacts on neonatal adjustment, preweaning survival,postnatal growth, feed utilization efficiency, lifetimehealth, body composition, and meat quality, as well asreproductive and athletic performance (Table 1). Thus,IUGR has important implications for the animalsciences.

Neonatal Survival and Adjustment

Low birth weight is associated with high neonatalmorbidity and mortality rates in domestic animals, par-ticularly under adverse climatic conditions (Mellor,1983; Azzam et al., 1993; Van Rens et al., 2005). Recentdata show that preweaning deaths in neonates of do-mestic animals in the United States remain high, withmost mortality occurring within the first days of postna-tal life (Table 2). These high preweaning mortality ratesresult in economic loss and emotional stress for animalowners, particularly on farms raising heifers/cows andhorses with long gestational periods (280 and 335 d, re-spectively).

Intestinal and respiratory dysfunctions, which occurin IUGR neonates (Thornbury et al., 1993; Trahair etal., 1997; Rossdale and Ousey, 2002), are major factorscontributing to preweaning mortality in livestock (Ta-ble 2). Neonates with IUGR that survive the first daysof life are often at increased risk for subsequent neuro-logical, respiratory, intestinal, and circulatory disor-ders during the neonatal period (Wu et al., 2004a). Neo-nates whose intrauterine growth is retarded due tosmall placentae or severe malnutrition often becomehypoglycemic and hypoxemic, and are susceptible tofatal hypothermia in response to cold stress due to im-paired thermogenic mechanisms (both shivering andnon-shivering) and low energy reserves (Mellor, 1983).In addition, the reduced quality (nutrient compositionand immunoglobulin content) and quantity of colostrumproduced at parturition by dams underfed or overfedduring pregnancy also negatively affects neonatal sur-vival (Mellor, 1983; Wallace et al., 2001).

Of note, the physical appearance or behavior mayappear to be normal in some neonates (e.g., foals), buttheir various organs may not be functionally mature(Ginther and Douglas, 1982). Thus, special care is re-quired for managing young animals that experienceIUGR, which adds additional costs to animal produc-tion. This is compounded when fetal growth restrictionis accompanied by a major reduction in gestation length(premature delivery), which frequently occurs in over-nourished adolescent sheep (Wallace et al., 2001) andin underfed mares (Rossdale and Ousey, 2002). Thus,future research is warranted to identify the proportionof neonatal death caused by IUGR in livestock.

Wu et al.2322

Table 1. Postnatal consequences of intrauterine growth retardation in domestic animals

Item Species References

Body composition and meat quality Pig and sheep Pond et al., 1969;Decreased skeletal muscle fiber number, increased Powell and Aberle, 1980;whole-body and intramuscular fat mass, increased Greenwood et al., 1998, 2000;connective tissue content, and reduced meat quality Bee, 2004; Gondret et al., 2005

Cardiovascular disorders Sheep Ozaki et al., 2000;Coronary heart disease, hypertension, and endothelial Giussani et al., 2003;dysfunction Fowden et al., 2005

Growth performance Pig, sheep, and horse Hegarty and Allen, 1978;Reduced whole-body and skeletal muscle growth rates, Greenwood et al., 1998, 2000;and reduced efficiency of feed/forage utilization Allen et al., 2004

Athletic performance, reduced Horse Rossdale and Ousey, 2002Hormonal imbalance Sheep Wallace et al., 2001, 2003b, 2004;

Increased plasma levels of glucocorticoids and renin; Fowden et al., 2005decreased plasma levels of insulin, growth hormone,IGF-I, and thyroid hormones

Metabolic disorders Sheep Wallace et al., 1996, 2005c;Insulin resistance, β-cell dysfunction, dyslipidemia, glucose Da Silva et al., 2001;intolerance, impaired energy homeostasis, obesity, type-II Fowden et al., 1994, 2005diabetes, oxidative stress, and mitochondrial dysfunction

Neonatal health and adjustment Pig, sheep, and horse Ginther and Douglas, 1982;Increased morbidity and mortality, reduced survival, Mellor, 1983;maladjustment to the extrauterine life, and increased Rossdale and Ousey, 2002;stillbirths Quiniou et al., 2002

Organ dysfunction and abnormal development Pig and sheep Widdowson, 1971;Testes, ovaries, brain, heart, skeletal muscle, liver, thymus, Wigmore and Stickland, 1983;small intestine, wool follicles, and mammary gland Da Silva et al., 2001, 2002, 2003

Compared with high birth weight offspring, IUGRnewborn lambs (Greenwood et al., 1998), calves (Bel-lows et al., 1971), piglets (Milligan et al., 2002; Quiniouet al., 2002), and foals (Ginther and Douglas, 1982)suffered from greater rates of neonatal mortality andtook a longer period of time to adapt to postnatal life.Heavier offspring (including calves, lambs, and piglets)at birth are more viable and more rapidly adjust to theextrauterine environment (Cundiff et al., 1986). Below0.8 kg of birth weight, 35% of piglets are stillborn, incomparison with 4% for birth weights ranging from 1.2to 1.4 kg (Quiniou et al., 2002). Preweaning survivalrates decrease progressively from 95 to 15% as pigletbirth weights decrease from 1.80 to 0.61 kg (Quiniouet al., 2002). Approximately 15 to 20% of piglets are

Table 2. Preweaning death in neonates of domestic animals in the United States

PreweaningSpecies death, % Comments Reference

Cattle1 10.5 Preweaning death as percentage of heifer calves born alive, Azzam et al., 1993; USDA, 2003awith 70% occurring within the first 7 d

Horse2 5.0 Foal death at <6 mo of age, USDA, 1998with 34% occurring within the first 2 d of life

Pig3 11.8 Preweaning death as percentage of piglets born alive, USDA, 2005with 75% occurring within the first 7 d of life

Sheep4 8.3 Preweaning lamb death as percentage of lambs born alive, USDA, 2003bwith 69.6% occurring within the first 7 d of life

1Cattle: Digestive and respiratory problems account for 62.1% of preweaning deaths in heifer calves.2Horse: Birth defects, prematurity, low milk yield, and infection account for 46.9% of deaths, and unknown factors account for 33.2% of

deaths.3Pigs: Digestive, starvation, and laid-on (being weak) account for 80.4% of all deaths.4Sheep: Digestive, respiratory, and lambing problems contribute to 49.7% of nonpredator deaths.

born with a birth weight less than 1.1 kg, and theirsurvival and postnatal growth rates are severely re-duced (Wu et al., 2004a).

Newborn piglets with IUGR suffer from necrotizingenterocolitis (a serious disorder of the small intestine;Thornbury et al., 1993), which impairs intestinal func-tion, including the synthesis of arginine, an essentialAA for neonatal pigs, but remarkably deficient in sow’smilk (Wu et al., 2004d). Necrotizing enterocolitis is amajor cause of death in neonates, including piglets(Thornbury et al., 1993), and can be ameliorated bydietary arginine supplementation (Wu et al., 2004c).Foals with IUGR exhibit organ dysfunction (e.g., skele-tal and respiratory problems, and reduced immunefunction). Twin foals have poor prospects for postnatal


survival of one or both foals (Ginther and Douglas,1982). The second-day syndrome in neonatal horses,defined as a condition in which foals markedly deterio-rate on the second day postpartum, may result fromIUGR due to impaired pulmonary and metabolic func-tions (Rossdale and Ousey, 2002).

Postnatal Growth and Efficiencyof Feed/Forage Utilization

The gut and muscle coordinate nutrient metabolismin animals. The small intestine plays an important rolein terminal digestion and absorption of nutrients and,therefore, in postnatal growth of animals (Wu, 1998).In growing animals, protein deposition in skeletal mus-cle is a high priority, and accounts for approximately15% of total energy expenditure (Wu and Self, 2005).In contrast to fat (a hydrophobic substance), proteindeposition is associated with retention of a largeamount of water, with a ratio of approximately 1 to 3on a gram basis (Wu and Marliss, 1992). Thus, withwater content of 75 to 80% in the body, muscle proteinbalance is the major determinant of postnatal growthrate in young livestock. In other words, skeletal musclegrowth is energetically more efficient than fat synthesisand accretion. Interestingly, natural or experimentallyinduced IUGR is associated with abnormal gastrointes-tinal morphologies and gastrointestinal dysfunction(Thornbury et al., 1993; Trahair et al., 1997; Wang etal., 2005) as well as the impaired development of skele-tal muscle (Hegarty and Allen, 1978; Greenwood et al.,2000). These conditions will contribute to a reducedefficiency of nutrient utilization in the IUGR progeny.

Studies have shown that IUGR has a permanentstunting effect on postnatal growth and reduces theefficiency of feed/forage utilization. Some researchershave reported reduced postnatal growth of IUGR lambsunder artificial rearing (Schinckel and Short, 1961; Vil-lette and Theriez, 1981) or practical production condi-tions (Gootwine et al., 2006). Compared with high birthweight lambs, the IUGR newborn lambs grew slowerwithin the first 2 wk, exhibited lower rates of efficiencyof energy utilization for protein and fat deposition(Greenwood et al., 1998), and had lower intramuscularconcentrations of DNA and lower rates of postnatalskeletal muscle growth (Greenwood et al., 2000). Re-duced myofiber number in IUGR lambs limits the ca-pacity for postnatal compensatory growth of skeletalmuscle. Under feedlot and forage grazing conditions,the efficiency of feed utilization is lower for twins thansingletons (Guerra-Martinez et al., 1990), and smallbirth weight calves grew more slowly before weaningthan high birth weight calves (Cundiff et al., 1986).Recent studies involving embryo transfer have shownthat IUGR led to a permanent stunting effect on postna-tal growth of horses throughout life (Allen et al., 2004).These results indicate a negative effect of IUGR onpostnatal nutrient utilization and growth performancein animals.

In addition to herbivores, maternal undernutritionduring gestation stunted the postnatal growth and de-velopment of swine (Schoknecht et al., 1993). Low birthweight pigs also fail to increase their muscle fiber num-ber or muscle growth during the postnatal period evenwhen fed adequately (Hegarty and Allen, 1978). Thus,the small intestine, liver, and skeletal muscle of therunt pig continued to be disproportionately smallerthan the largest littermates at 3 yr of age (Widdowson,1971). Runt or fostered runt pigs exhibited lower ratesof skeletal muscle and whole-body growth betweenbirth and slaughter, and utilized feeds less efficientlyfor growth, compared with high birth weight lit-termates (Hegarty and Allen, 1978; Powell and Aberle,1980). The lower piglet birth weight is associated withlower ADG during the suckling, nursing, and growing-finishing periods (Quiniou et al., 2002). Compared withprogeny of gilts fed a diet containing adequate protein,postnatal growth rates between birth and weaning (5wk of age) and between weaning and slaughter (90 kg)were markedly reduced in all progeny of gilts fed aprotein-deficient, isocaloric diet during all or part ofgestation, regardless of birth weights (Pond et al., 1969;Atinmo et al., 1974).

As with IUGR piglets born from underfed dams, prog-eny from sows overfed between d 0 and 50 of gestationexhibited slower growth rates during both lactation andgrowing-finishing periods, as well as lower efficiency offeed utilization for gain (G:F) in comparison with pigsborn from underfed sows (Bee, 2004). Similarly, ov-erfeeding both energy and protein between d 25 and 50of gestation reduced the postnatal ADG, muscle deposi-tion rate, and carcass weight at slaughter weight (104kg; Nissen et al., 2003) of the offspring. During thenewborn period, the fractional rate of protein synthesis(%/d) did not differ in tissues (skeletal muscle, heart,liver, pancreas, and jejunum) between normal andIUGR piglets under fasting or fed conditions (Davis etal., 1997). The inability of the IUGR piglets to increasetissue protein synthesis beyond that of the normal lit-termates explains their incomplete compensatorygrowth after birth. The longer the period of intrauterinenutrient deprivation, the lesser the ability of IUGR pigsto recover from the insult (Pond et al., 1969).

Body Composition and Meat Quality

The function of an animal critically depends on itsbody composition of protein, fat, carbohydrates, miner-als, vitamins, and water, which in turn influences therate of postnatal growth. In addition, the contents ofskeletal muscle, fat, and connective tissue as well asmuscle fiber number and area are major factors thataffect the postmortem quality of meat. Further, theintramuscular concentration of glycogen and the glycol-ysis rate postslaughter affect the production of lacticacid and the pH of meat, as well as its water-holdingcapacity. Also, an increase in the amount of intramus-cular fat promotes lipid peroxidation postslaughter

Wu et al.2324

(Fang et al., 2002). This results in the oxidation of mus-cle proteins, including oxymyoglobin (the main pigmentresponsible for the bright red color of fresh meat) and,therefore, changes in the color and taste of meat (Gore-lik and Kanner, 2001). A greater amount of connectivetissue results in tougher meat. Fast-growing animalscontaining a high number of muscle fibers with a smallcross-sectional area generally yield a greater qualitymeat (Gondret et al., 2005).

There is evidence showing that IUGR is associatedwith altered composition of the whole body and muscle,as well as the distribution of muscle fiber type, of theoffspring (Wigmore and Stickland, 1983). During lategestation, growth-retarded fetuses from overfed adoles-cent mothers have greater relative fetal carcass fat con-tent and perirenal fat mass than normally growing con-trol fetuses (Matsuzaki et al., 2006). In contrast, themore modest fetal growth restriction in undernourishedadolescent pregnancies is associated with preservationof fetal skeletal growth and depletion of fetal fat stores(Luther et al., 2005a). In lambs, low birth weight isassociated with lower percentages of bone and muscleand a greater percentage of fat in the slaughter-weight(46 kg) carcass (Makarechian et al., 1978). Comparedwith high birth weight lambs, low birth weight lambshad more fat and less minerals in the whole body re-gardless of whether they exhibited slow or fast postna-tal growth rates (achieved by feeding different levels ofhigh-quality liquid diet; Greenwood et al., 1998).

In comparison with the average-sized littermate, in-tramuscular fat (within and perhaps also between mus-cle fibers) and connective tissue (collagen I) contentsare greater in the small porcine fetus at d 86 of gestationand in postnatal pigs with prior experience of IUGR(Karunaratne et al., 2005). At similar adult weights,runt pigs had larger muscle fiber diameters and largequantities of intramuscular fat (Hegarty and Allen,1978; Powell and Aberle, 1980) and lighter muscledcarcasses (Powell and Aberle, 1980). Also, at slaughter(105 kg BW), semitendinosus muscle of piglets with thelowest birth weights had fewer fast glycolytic fibers butmore oxidative fibers and more fast-oxidative glycolyticfibers compared with littermates with the heaviestbirth weight (Bee, 2004). In addition, progeny from sowsoverfed between d 0 and 50 of gestation had greatercontent of adipose tissue at birth and at adult slaughterweight, compared with pigs born from underfed sows(Bee, 2004). The change in muscle composition doestranslate into an adverse effect on meat quality, aspiglets that had experienced IUGR exhibited elevatedlevels of intramuscular lipids and low scores for meattenderness (Gondret et al., 2005). Thus, the prenataldevelopment of muscle fibers and adipocytes has a pro-found impact on meat quality when the animal isslaughtered at or near adult BW.

Long-Term Consequences for Health as Well asReproductive and Athletic Performance

Most domestic animals are raised for producing meatat relatively young ages when muscle protein deposition

approaches a plateau. For those animals selected forbreeding, lactation, or both, IUGR may influence theirsubsequent reproductive performance. For example,under some production conditions (a combination ofgrazing and feed supplement), ewe lambs born as sin-gletons with a greater birth weight reach puberty atboth a younger age and a heavier weight than twin-born lambs (Southam et al., 1971). Additionally, femalefetuses from overfed adolescent sheep pregnancies havefewer ovarian follicles than normally growing fetusesat mid- and late gestation (Da Silva et al., 2002; 2003)and hence a limited pool for follicular recruitment inadult life. Similarly, follicular development is delayedin IUGR piglets at birth (Da Silva-Buttkus et al., 2003).Furthermore, low birth weight lambs produced by anin utero crowding model have fewer uterine carunclesthan normal birth weight lambs, and this may affectsubsequent placental growth and uterine capacity (Ait-ken et al., 2003). For male lambs, low birth weight isassociated with a delay in the onset of endocrine pu-berty and attenuated testis growth (Da Silva et al.,2001).

All of the above findings suggest that selection ofIUGR offspring for breeding purposes is best avoidedin animal production. Additionally, the lifespan of ani-mals (including horses, cats, and dogs) that are raisedfor racing or for human companionship is increasingdue to improved medical and nutritional care. BecauseIUGR results in smaller skeletal muscle and liver (Wid-dowson, 1971), and these organs play a crucial rolein the metabolism of energy substrates (Jobgen et al.,2006), their reduced functional capacity may help ex-plain impaired glucose utilization and dyslipidemia inthe adult life of IUGR offspring. Likewise, abnormalcomposition and the reduced size of muscle fibers inthe animal that experiences IUGR (Hegarty and Allen,1978; Powell and Aberle, 1980) may impair energy me-tabolism, protein turnover, force generation, locomo-tion, strength, endurance, and coordination of skeletalmuscle. Thus, IUGR likely has an adverse impact onlifetime health, reproductive performance, and athleticperformance under practical livestock production andmanagement conditions. In addition, results from well-controlled experiments with sheep show that IUGRprogeny develop metabolic abnormalities, including re-duced insulin secretion, insulin resistance, dyslipide-mia, and cardiovascular dysfunction in adult life (Table1). There is also evidence indicating that IUGR hasa negative impact on athletic performance in horses(Rossdale and Ousey, 2002).

Fetal Programming

The compelling evidence summarized in the preced-ing sections suggests that the intrauterine environmentof the conceptus may alter expression of the fetal ge-nome and have lifelong consequences. This phenome-non is termed fetal programming, which has led to therecent theory of the fetal, or developmental, origins of


adult disease (Barker and Clark, 1997). Namely, alter-ations in fetal nutritional and endocrine status mayresult in developmental adaptations that permanentlychange the structure, physiology, and metabolism ofthe offspring, thereby predisposing individuals to meta-bolic, endocrine, and cardiovascular diseases in adultlives of animals and humans. Because growth perfor-mance, which depends on both the rates and the effi-ciency of metabolic transformations of nutrients, is alsoa major concern in animal agriculture, the theory offetal programming can be extended to include fetal ori-gins of postnatal growth retardation, reduced feed effi-ciency, and reduced meat quality. This concept of fetalprogramming has far-reaching implications for the ani-mal sciences.

MECHANISMS OF IUGR

The lack of knowledge about the mechanisms forIUGR has prevented the development of effectivemeans to enhance fetal growth in animals. Due to thelack of intervention options, the current management ofIUGR livestock fetuses is only empirical and primarilyaimed at improving neonatal care and adopting artifi-cial rearing with liquid replacer milk. Artificial rearingis costly at present because it requires expensive facili-ties and diets (Wu et al., 2004d). Only by elucidatingthe mechanisms of IUGR, can we design effective meansto prevent fetal growth restriction. Available evidencesuggests that impaired placental growth (including vas-cular growth) or function, possibly owing to reducedplacental synthesis of vasodilators and metabolic regu-lators, may contribute primarily to IUGR in responseto undernutrition and overnutrition.

Impaired Placental Growth and IUGR

Crucial Role for Placental Growth and Uteropla-cental Blood Flows in Fetal Growth. The placenta isthe organ that transports nutrients, respiratory gases,and the products of their metabolism between the ma-ternal and fetal circulation. Placental growth (includingvascular growth) is crucial for fetal growth and develop-ment (Gootwine, 2004; Reynolds et al., 2005). Thus, anincrease in placental growth through elevated expres-sion of placental anabolic proteins (e.g., prolactin andplacental lactogen) is associated with enhanced fetalgrowth in sheep (Gootwine, 2004). During normal preg-nancy, uterine and placental blood flows increasethroughout gestation to meet the metabolic needs ofthe growing conceptus (Reynolds et al., 2005). Umbilicalblood flow also increases markedly during late gestationin livestock (including sows, ewes, and cows) to satisfythe metabolic needs of the rapidly growing fetus (Ford,1995; Pere and Etienne, 2000). Thus, uteroplacentalblood flow is a major factor that influences the availabil-ity of nutrients for fetal growth and development. Avail-able evidence from well-controlled studies shows thatimpaired placental growth is associated with IUGR

(Mellor, 1983; Schoknecht et al., 1994; Wallace et al.,1996, 2003a).

Rates of uteroplacental blood flows depend in largepart on placental vascular growth, which results fromangiogenesis (the growth of new vessels from existingones) and placental vascularization (Vonnahme andFord, 2004; Reynolds et al., 2005). Consistent with in-creased uterine and placental blood flows (Ford, 1995),placental angiogenesis increases markedly from thefirst to the second third of gestation and continues toincrease further during late gestation (Reynolds andRedmer, 2001). Both nutrient restriction of adult ewes(Redmer et al., 2004) and overnourishment of adoles-cent ewes (Redmer et al., 2005) during pregnancy re-duced placental proliferation in the fetal trophectodermand placental expression of angiogenic factors. In ov-erfed adolescent ewes, these changes at midgestationmay underlie the attenuated uteroplacental blood flowsand IUGR that characterize late pregnancy (approxi-mately d 130) in these rapidly growing animals (Wal-lace et al., 2002).

Indeed, we recently found that uterine blood flow wasreduced by 56% as early as on d 90 of gestation, whichoccurred before any reduction in fetal or placentalweight was observed (J. M. Wallace, unpublished data).In other ovine models of IUGR induced by heat stressor multiple fetuses, decreases in placental angiogenesisand vascularity are also associated with reduced utero-placental blood flows as well as reduced placental andfetal growth (Reynolds et al., 2005). Thus, placentalefficiency is not reflected just by placental weight orsize but also depends on other factors, such as placentalmicrovascular density, interdigitation of the placentawith the maternal endometrium to increase surface,and placental blood flow.

Nutrient uptake by the uterus or the fetus can bedetermined experimentally on the basis of the Fickprinciple: Uptake = Blood Flow Rate × (A-V), where (A-V) represents the difference in arteriovenous concentra-tion across the uterus or the fetus (Bell and Ehrhardt,2002). Thus, the transuterine or transplacental ex-change of a substance is determined by both blood flowrate and its concentrations in the arterial and venousblood (Reynolds et al., 2006). Blood concentrations ofmetabolites in the uterine artery and vein as well asthe umbilical vein and artery are regulated by 1) theactivities and amounts of nutrient transporters on theplasma membranes of cells of the uteroplacental unit, 2)the amounts of the substances entering the circulationfrom dietary and endogenous sources, and 3) rates ofoxidation of the substances. There is evidence that re-ductions in placental growth, angiogenesis, and pre-sumably placental vascularization are associated withdecreased placental transport of O2 and nutrients frommother to fetus in compromised ovine pregnancies(Wallace et al., 2002, 2005c).

Insufficient Uteroplacental Blood Flows and Re-duced Transport Activity in Natural Uterine Insuf-ficiency. Fetal growth restriction in ruminants car-

Wu et al.2326

rying multiple fetuses is associated with reduced utero-placental blood flows and placental function (Ferrelland Reynolds, 1992). In sows, at d 77 to 110 of gestation,there are significant correlations between placentalweight and placental blood flow, between placentalweight and fetal weight, and between placental bloodflow and fetal weight (Wootton et al., 1977). Betweend 44 and 111 of gestation, total blood flow to the porcineuterus does not increase linearly with an increase inthe number of fetuses, and uterine blood flow per fetusdecreases with increasing litter size (Pere and Etienne,2000). In comparison with its littermate, the runt fetalpig is associated with a small placenta and a low rateof placental blood flow (Wootton et al., 1977). In additionto the compromised placental blood flow, placentaltransport of leucine was reduced in the small porcinefetus compared with the average-size fetus at d 45, 60,and 100 of gestation because of the impaired develop-ment of transport systems and their reduced capacity(Finch et al., 2004).

NO and Polyamines and IUGR

Crucial Roles of NO and Polyamines in Placentaland Fetal Growth. Arginine is a common substratefor NO and polyamine syntheses via NO synthase(NOS) and ornithine decarboxylase, respectively (Wuand Morris, 1998). Nitric oxide is a major endothelium-derived vasorelaxing factor, and plays an importantrole in regulating placental-fetal blood flows and, thus,the transfer of nutrients and O2 from mother to fetus(Bird et al., 2003). Likewise, polyamines regulate DNAand protein synthesis and, therefore, cell proliferationand differentiation (Flynn et al., 2002). Growing evi-dence shows that NO and polyamines are key regula-tors of angiogenesis and embryogenesis as well as pla-cental and fetal growth (Reynolds and Redmer, 2001;Zheng et al., 2006).

Excitingly, we recently discovered that arginine isparticularly abundant in porcine allantoic fluid (4.1 to6 mM) at d 40 of gestation (term = 114 d; Wu et al.,1996, 1998a). Remarkably, concentrations of arginineand its precursor ornithine in porcine allantoic fluidincreased by 23- and 18-fold, respectively, between d30 and 40 of gestation (Figure 2), with their N account-ing for approximately 50% of the total free α-amino acidN in allantoic fluid (Wu et al., 1996). The absence ofarginase activity from the porcine placenta ensuresmaximum transfer of arginine from mother to fetus(Wu et al., 2005). Most recently, we found that citrulline(an immediate precursor of arginine) is unusually rich(10 mM) in ovine allantoic fluid at d 60 of gestation(term = 147 d; Kwon et al., 2003). Concentrations ofcitrulline and its precursor glutamine in ovine allantoicfluid increase by 34- and 18-fold, respectively, betweend 30 and 60 of gestation (Figure 3), with their N repre-senting 60% of total α-amino acid N in ovine allantoicfluid (Kwon et al., 2003a). Citrulline derived from theuterus and/or placenta is effectively converted into argi-

Figure 2. Concentrations of arginine, ornithine, andglutamine in porcine allantoic fluid during pregnancy.These arginine-family AA are highly enriched in porcineallantoic fluid between d 35 and 45 of gestation. Maternalplasma concentrations of arginine, ornithine, and gluta-mine are 0.13 to 0.14, 0.08 to 0.09, and 0.30 to 0.40 mM,respectively, between d 30 and 110 of gestation. Data arepresented as means ± SEM and are adapted from Wu etal. (1995, 1996). Allantoic fluid nutrients can be absorbedby the allantoic epithelium into the fetal-placental circula-tion to support placental and fetal development (Bazer,1989).

nine via argininosuccinate synthase and lyase in fetaltissues (Wu and Morris, 1998). Because the ovine pla-centa contains a high arginase activity (Kwon et al.,2004b) that would catabolize arginine, the placentaltransfer of citrulline and its storage in allantoic fluidprovide an effective strategy to conserve arginine in theovine conceptus. The unusual abundance of the argi-nine-family AA in fetal fluids is associated with thegreatest rates of NO and polyamine syntheses in theovine and porcine placentae during the first half of preg-nancy, when its growth is most rapid (Kwon et al.,2003b, 2004b; Self et al., 2004; Wu et al., 2005). Thesenovel findings from the 2 diverse animal models areconsistent with the proposed crucial roles of the argi-nine-dependent metabolic pathways in conceptusgrowth and development (Figure 4).

Evidence from studies with pregnant pigs and sheepindicates impaired syntheses of NO and polyamines inthe conceptuses of underfed and overfed dams (Wu etal., 2004a). Dietary protein deficiency or hypercholes-terolemia reduces the availability of arginine and orni-thine in maternal plasma, fetal plasma, amniotic fluid,and allantoic fluid, as well as the placental synthesisof NO and polyamines in pregnant pigs (Wu et al.,1998a; 1998b). In addition, we found that maternal un-dernutrition in sheep (50% of NRC requirements; NRC,1985) between d 28 and 78 of gestation decreased con-centrations of arginine, citrulline, and polyamines inmaternal plasma, fetal plasma, amniotic fluid, and al-lantoic fluid at d 78 of gestation (Kwon et al., 2004a).Notably, concentrations of biopterin (an indicator of de


Figure 3. Concentrations of arginine, ornithine, andcitrulline in ovine allantoic fluid during pregnancy. Cit-rulline is most abundant in ovine allantoic fluid at d60 of gestation. During late gestation, concentrations ofcitrulline and arginine in the fluid are high. The pooledSEM values for arginine, ornithine, and citrulline are0.075, 0.206, and 0.034 mM. Maternal plasma concentra-tions of arginine, ornithine, and citrulline are 0.10 to 0.19,0.03 to 0.10, and 0.13 to 0.21 mM, respectively, betweend 30 and 140 of gestation. Data are adapted from Kwonet al. (2003a).

novo synthesis of tetrahydrobiopterin, BH4, an essen-tial cofactor for NOS; Shi et al., 2004) in amniotic andallantoic fluids are reduced by 32 to 36% in underfedewes, compared with control ewes (G. Wu, unpublisheddata). These changes would impair placental and fetalNO synthesis, thereby theoretically resulting in re-duced placental-fetal blood flows in underfed ewes (Re-dmer et al., 2004).

Consistent with these findings, maternal undernutri-tion impairs NO-dependent vasodilation and increasesarterial blood pressure in the ovine fetus (Ozaki et al.,2000). Similarly, uterine and umbilical blood flows arereduced in overnourished adolescent sheep (Wallace etal., 2002, 2003a), suggesting a reduction in NO genera-tion by vascular endothelial cells of the uterus and pla-centae. The underlying mechanisms may include 1) re-duced availability of BH4 due to oxidative stress, 2)reduced expression of NOS, 3) inactivation of NOS dueto its close association with caveolin-1, and 4) increasedplasma concentrations of homocysteine, saturated lip-ids, and asymmetric dimethylarginine, which are allinhibitors of endothelial NO generation (Wu and Mei-ninger, 2002; Fu et al., 2005). Collectively, these resultshave generated the novel hypothesis that impaired pla-cental syntheses of NO and polyamines may provide aunified explanation for the same pregnancy outcome(namely, IUGR) in response to the 2 extremes of nutri-tional problems (both maternal undernutrition andovernutrition) during gestation (Wu et al., 2004a).

Possible Role of NO and Polyamines in theGrowth and Development of Fetal Muscle Cells andAdipocytes. Whereas the endocrine system plays an

Figure 4. Roles of arginine, NO, and polyamines in fetalgrowth. Both maternal undernutrition and overnutritionmay impair placental synthesis of NO and polyamines,and therefore placental development and uteroplacentalblood flows. This may result in reduced transfer of nutri-ents and O2 from mother to fetus, and thus fetal growthrestriction. The ornithine used for polyamine synthesis isderived from proline catabolism via proline oxidase inporcine placental and other tissues (Wu et al., 2005) aswell as from arginine hydrolysis via arginase in a varietyof porcine tissues, including the small intestine, liver, andkidneys (Wu and Morris, 1998). Glutamine is a commonsubstrate for the synthesis of both citrulline and prolinein pigs (Wu, 1998). Arg = arginine; AS-AL = argininosuc-cinate synthase and argininosuccinate lyase; BH4 = tetra-hydrobiopterin; Cit = citrulline; Gln = glutamine; mTOR =mammalian target of rapamycin; GTP-CH = GTP cyclohy-drolase-I; ODC = ornithine decarboxylase; NO = nitricoxide; NOS = nitric oxide synthase; Orn = ornithine; PO-OAT = proline oxidase and ornithine aminotransferase;and SAM = S-adenosylmethionine.

important role in fetal growth (Fowden et al., 2005),the changes in fetal muscle growth rate in responseto maternal undernutrition and overnutrition is notalways correlated with changes in growth regulatoryhormones (Davis et al., 1997; Rehfeldt et al., 2004).Thus, we have been prompted to investigate the role ofspecific nutrients in the growth and development offetal muscle cells. Because AA are the only source ofboth C and N for energy metabolism and protein synthe-sis, we focused on these macronutrients using our por-cine and ovine models of IUGR (Wu et al., 1998a).

Myocytes and adipocytes are derived from a commonmesenchymal precursor (Sordella et al., 2003). Thus,excessive amounts of adipose tissue are developed atthe expense of skeletal muscle when embryonic myo-

Wu et al.2328

genesis is impaired (Kablar et al., 2003). There are 2developing types of muscle fibers in fetal pigs: primaryfibers (formed by the rapid fusion of primary myoblastsbetween d 25 and 50 of gestation) and secondary fibers(formed on the surface of primary fibers between ap-proximately d 50 and 90 of gestation). In late gestation,the number of secondary fibers is much greater thanthat of primary fibers. During prenatal development,muscle fibers undergo contractile differentiation, re-sulting in the formation of slow-twitch oxidative, fast-twitch oxidative-glycolytic, and fast-twitch glycolyticfibers. At birth, most muscle fibers are oxidative(Bee, 2004).

The numbers of secondary muscle fibers, but not pri-mary muscle fibers, are affected by conditions in utero(Dwyer et al., 1994). The total number of muscle fibersis fixed at birth and is a major factor affecting thegrowth of skeletal muscle and thus the postnatalgrowth of the animal. Postnatally, skeletal muscle de-velopment continues such that there are more glycolyticfibers with increasing age (Bee, 2004). Comparison be-tween small and large littermates in pigs indicates thatthe differences in their prenatal and postnatal growthrates are related to a lower ratio of secondary to primarymuscle fibers as well as a smaller size of the fibers inthe former (Handel and Stickland, 1987).

Polyamines are necessary for the proliferation anddifferentiation of cells (Flynn et al., 2002) and likelymediate the growth and development of fetal musclefibers and adipocytes. In support of this suggestion, werecently noted that concentrations of arginine, orni-thine, and polyamines were reduced in skeletal muscleof IUGR fetal pigs compared with average-weight lit-termates (G. Wu, unpublished data). Similarly, concen-trations of arginine, putrescine, and spermidine werelower in the gastrocnemius muscle of IUGR fetal lambsin response to maternal undernutrition (G. Wu, unpub-lished data). Further, elevated levels of NO inhibit thegrowth of adipocytes (Fu et al., 2005; Jobgen et al.,2006). Because adipose tissue of fetal lambs in underfedewes have reduced levels of endothelial NOS (G. Wu,unpublished data), reduced NO availability is expectedto facilitate the growth of preadipocytes in IUGR lambs.

Possible Role of AA in mTOR Signaling andIUGR. Fetal muscle growth may be regulated by thesignaling mechanism of the mammalian target of rapa-mycin (mTOR; Du et al., 2005). This serine/threonineprotein kinase is an evolutionarily conserved memberof the phosphoinositol kinase-related kinase family ofproteins. The phosphorylation of mTOR in response tonutrients (e.g., amino acids and glucose) results in thephosphorylation of p70 S6 kinase and eukaryotic initia-tion factor 4E-binding protein-1, which promotes theformation of the active initiation complex for polypep-tide synthesis (Meijer and Dubbelhuis, 2004).

In mature cows, maternal undernutrition (50% offeed intake for the control group) during early gestation(d 30 to 125) had no effect on the content of calpains Iand II (calcium-dependent cysteine proteases) in mater-

nal and fetal skeletal muscles (Du et al., 2004). How-ever, underfeeding increased and reduced the concen-tration of calpastatin (a specific inhibitor of calpains)in maternal and fetal muscles, respectively (Du et al.,2004). Malnutrition also reduced the concentrations ofphosphorylated mTOR in both maternal and fetal skele-tal muscle and increased concentrations of ubiquiti-nated proteins in maternal muscle (Du et al., 2005).

Importantly, the activation of the mTOR signalingpathway in skeletal muscle is under the control of thearginine-family amino acids (e.g., arginine and gluta-mine) and leucine (Meijer and Dubbelhuis, 2004),whose concentrations are reduced in the IUGR fetus(Wu et al., 1998a; Kwon et al., 2004b). Glutamine andleucine also inhibit protein degradation in skeletal mus-cle (Wu and Thompson, 1990; Meijer and Dubbelhuis,2004). Thus, during pregnancy, reduced concentrationsof AA in the conceptus in response to maternal undernu-trition may contribute to an increase in protein degra-dation and a decrease in protein synthesis in maternalskeletal muscle. Such a mechanism may also reduceboth protein degradation and protein synthesis in fetalmuscle. These differential effects of maternal un-derfeeding may help to mobilize the maternal proteinreserve to supply AA for metabolic utilization by themother and the fetus, thereby providing a protectivemechanism for ensuring fetal growth.

MECHANISMS OF FETAL PROGRAMMING

As noted above, nutritional insults during a criticalperiod of gestation may have a permanent effect onthe progeny throughout postnatal life. There is alsoevidence that fetal undernutrition due to placental in-sufficiency impaired vascular function in two genera-tions of rats (Anderson et al., 2006), indicating an inter-generational effect. Interestingly, some of the effects,such as embryo survival (Vinsky et al., 2006) and endo-thelium-dependent relaxation (Anderson et al., 2006),appear to be sex-specific. These effects likely result fromgenomic imprinting, which is defined as the parent-of-origin-dependent expression of a single allele of a genein the embryo/fetus, namely a parental influence onthe gene expression of the progeny. There is growingevidence that maternal or fetal nutritional status canalter the epigenetic state of the fetal genome and geneexpression of imprinted genes (e.g, Igf2 and H19),where the methylation of DNA and proteins plays acrucial role. Interestingly, Igf2 is paternally expressedand maternally silent, whereas H19 is paternally silentand preferentially expressed from the maternal allele(Doherty et al., 2000).

Epigenetic alterations (i.e., stable alterations of geneexpression through covalent modifications of DNA andcore histones) in early embryos may be carried forwardto subsequent developmental stages (Waterland andJirtle, 2004). Two mechanisms mediating epigenetic ef-fects are DNA methylation (occurring in the 5′-positionsof cytosine residues within CpG dinucleotides through-


Figure 5. Roles of nutrients in the provision of methyl group donors as well as DNA and protein methylation.Amino acids and vitamins play crucial roles in the provision of S-adenosylmethionine for methylation of DNA andprotein as well as the synthesis of creatine, cysteine, taurine, and polyamines. Enzymes catalyzing the indicatedreactions are: 1, methionine S-adenosyltransferase; 2, guanidinoacetate N-methytransferase; 3, DNA methyltransferase;4, protein methyltransferase; 5, S-adenosylhomocysteine hydrolase; 6, betaine:homocysteine methyltransferase; 7, N5-methyltetrahydrofolate:homocysteine methyltransferase; 8, serine hydroxymethyltransferase; 9, N5,N10-methylenetet-rahydrofolate reductase; 10, folate dehydrogenase; 11, enzymes of protein degradation (including calpains, ubiquitin-dependent proteases, proteasome, and lysosomal proteases); 12, enzymes of protein synthesis; 13, S-adenosylmethio-nine decarboxylase; 14, spermidine synthase and spermine synthase; 15, choline dehydrogenase; 16, enzymes of taurinesynthesis (cysteine dioxygenase and cysteinesulfinate decarboxylase); 17, enzymes of N5-formimino-tetrahydrofolateformation (histidase, urocanase, imidazolonepropionase, and glutamate formiminotransferase); 18, formiminotetrahy-drofolate cyclodeaminase; and 19, methylene-tetrahydrofolate reductase. B6 = vitamin B6; B12 = vitamin B12; CH3-DNA = methylated DNA; CH3-protein = methylated protein; DCAM = decarboxylated S-adenosylmethionine; 5-FTF =N5-formimino-tetrahydrofolate; GA = guanidinoacetate; Glu = glutamate; Gly = glycine; His = histidine; 5-MTF = N5-methyl-tetrahydrofolate; 5,10-MTF = N5,N10-methylene-tetrahydrofolate; MTHF = N5,N10-methenyl-tetrahydrofolate;Ser = serine; SPD = spermidine; SPM = spermine; and THF = tetrahydrofolate.

out the mammalian genome) and histone modification(acetylation, methylation, etc.; Jaenisch and Bird, 2003;Oommen et al., 2005). The CpG methylation can regu-late gene expression by modulating the binding ofmethyl-sensitive DNA-binding proteins, thereby affect-ing regional chromatin conformation. Histone modifi-cations can alter the positioning of histone-DNA inter-actions and the affinity of histone binding to DNA,thereby affecting gene expression (Jaenisch and Bird,2003).

The DNA and protein methylation are catalyzed byspecific DNA and protein methyltransferases, with S-adenosylmethionine (SAM) being the methyl donor inthese reactions (Jaenisch and Bird, 2003). S-adenosyl-methionine is synthesized from methionine and ATPby methionine adenosyltransferase, and its placentalconcentration is greatest when placental growth is mostrapid (Wu et al., 2005). Besides methylation reactions,SAM is utilized for the synthesis of spermidine andspermine from putrescine through the generation ofdecarboxylated SAM (Figure 5).

The synthesis of creatine is quantitatively the mostimportant pathway for SAM utilization and thus is a

major regulator of methyl donor availability in the body(Stead et al., 2001). When the diet is deficient in cyste-ine/taurine or contains excess methionine, an increasein cysteine/taurine synthesis from methionine con-sumes a large amount of SAM. One-carbon-unit metab-olism, which depends on serine, glycine, histidine, cho-line, and B vitamins (including folate, vitamin B12, andvitamin B6), in addition to methionine, plays an im-portant role in regulating the availability of SAM (Fig-ure 5). Thus, DNA methylation and histone modifica-tions may be altered by the overall availability of AAand micronutrients (Oommen et al., 2005). Epigeneticsmay provide a molecular mechanism for the impact ofmaternal nutrition on the fetal programming of postna-tal growth performance and disease susceptibility.

POTENTIAL SOLUTIONS TO PREVENT IUGR

Because of the incomplete knowledge about the mech-anisms of IUGR, attempts to alleviate the detrimentaleffects of IUGR on postnatal growth performance inlivestock have so far achieved only limited success. Therecognition of fetal programming suggests that strate-

Wu et al.2330

gies to promote postnatal growth and health of livestockshould be initiated at the key stages of prenatal devel-opment (Finch et al., 2004). Thus, targeting an effectivewindow of opportunity during a specific period of preg-nancy would be most beneficial for preventing IUGR.Despite much failure, the largely trial-and-error ap-proaches to treating pregnant dams have generatedsome promising results. These approaches include hor-monal therapy; dietary supplementation of energy, pro-tein concentrates, or both; provision of antioxidant nu-trients; and manipulations of the arginine-NO path-way. Although these methods are diverse in nature,they appear to directly or indirectly promote placentalgrowth and uteroplacental blood flow in pregnant damsvia increasing the availabilities of arginine, NO, orboth.

Hormonal Therapy

Changes in endocrine systems during prenatal orpostnatal life in response to altered maternal nutritionmay contribute to the programming of metabolism andphysiology in later life (Fowden et al., 2005). Specifi-cally, maternal concentrations of progesterone are usu-ally low in severely underfed and overfed pregnantdams, likely because of a decrease and an increase in itssynthesis and catabolism, respectively (Dziuk, 1992).Thus, administration of progesterone is required tomaintain pregnancy and fetal growth in pigs duringprolonged inanition (Anderson, 1975). Although proges-terone treatment may not affect litter size or fetalgrowth in gilts fed a restricted diet (2 kg/d) duringpregnancy (Yu et al., 1997), some evidence indicatesthat, beginning 24 h after the onset of estrus, adminis-tration of progesterone to gilts fed a diet providing 2-times maintenance requirements enhanced embryonicsurvival (Jindal et al., 1997). Progesterone also maybe required for maintaining adequate placental growth(including vascular growth) via NO- and polyamine-dependent mechanisms (Chwalisz and Garfield, 1997;Kwon et al., 2003b). In this regard, it is noteworthythat treatment with progesterone partially amelioratedfetal growth restriction in overfed adolescent ewes(Wallace et al., 2003b).

Alternatively, maternal growth hormone treatmentfrom d 35 to 80 of gestation alters maternal nutrientpartitioning in favor of uteroplacental growth in theovernourished adolescent sheep (Wallace et al., 2004).When overnourished adolescent dams were treatedwith growth hormone in late pregnancy once placentalgrowth was complete, a modest increase in fetal weightwas observed but was associated with a major increasein fetal adiposity, which may have negative implica-tions for long-term health (Wallace et al., 2005a). Simi-larly, daily administration of growth hormone to preg-nant sows during late or a large part of gestation in-creased fetal weight (Rehfeldt et al., 2004). Further,intramuscular administration of growth hormone toewes at breeding led to a more efficient placenta, larger

birth weight lambs, and more rapid postnatal growth(Costine et al., 2005). In addition to its effects on mater-nal carbohydrate metabolism, growth hormone may in-crease the availability of arginine in the conceptus bystimulating maternal and fetal intestinal synthesis ofcitrulline (the precursor of arginine) and inhibiting he-patic degradation of amino acids in pregnant dams, aspreviously reported for growing pigs (Bush et al., 2002).Significantly, a single injection of human chorionic go-nadotropin on d 12 postmating increased placental andfetal growth in sheep carrying two fetuses (Cam andKuran, 2004), further supporting the view that placen-tal growth positively influences fetal growth.

Dietary Supplementation with Energy, ProteinConcentrates, or Both

Maintenance represents 75 to 85% of the total re-quirement in pregnant dams because of substantial in-creases in maternal body and tissue weights as well asmetabolic rates (Cole, 1990; Grummer, 1995). Nutrientsprovided beyond the maintenance requirement are usedfor maternal tissue accretion and fetal growth. As notedabove, forages are often deficient in both protein andenergy and, therefore, they are inadequate for provid-ing both macro- and micronutrients to support the max-imal growth performance of unsupplemented rumi-nants and horses during pregnancy (Pugh, 1993; Bellet al., 2000). In addition, dams are usually in a severecatabolic state during late gestation because of eitherinadequate voluntary feed intake for ruminants or re-stricted feed provision for sows. Thus, dietary supple-mentation with energy, protein concentrates, or bothmay provide a means to enhance fetal growth.

For example, realimentation of underfed ewes begin-ning from midgestation to 100% of the NRC nutrientrequirements is effective in preventing IUGR (Kwon etal., 2004a). In beef cattle, supplementing protein andenergy concentrates to low-quality forages containing<8% MP reduced the loss of maternal BW and increasedcalf birth weight (Clanton and Zimmerman, 1970). Di-etary protein supplementation also enhanced DMI bypregnant cattle, thereby improving the overall nutri-tional status of the fetus (Chew et al., 1984). Of note,increasing feed intake to grazing heifers and cows didnot result in calving difficulties but enhanced fetalgrowth (Tudor, 1972; Bellows and Short, 1978). In pri-miparous beef heifers, dietary supplementation withprotein concentrates to meet the MP requirement be-fore and during pregnancy can increase the value ofeach bred heifer by $13.64 while improving pregnancyoutcome (Patterson et al., 2003).

In pigs, modest changes in global energy or proteinintake during pregnancy do not appear to alter thenumber of live-born piglets or total litter weight (Pondet al., 1981). Interestingly, realimentation for 5 d be-tween d 16 and 20 (period of implantation of blastocysts)of previously protein-deficient gilts, followed by feedingof a protein-free diet until parturition, resulted in


greater piglet birth weights, compared with those ofgilts fed a protein-free diet between d 0 and parturition(Pond et al., 1969). Although these results are of limitedvalue in practical swine production, they indicate thatit is feasible to regulate fetal growth through dietarymanipulations of nutrients.

Adequate Nutritional Support for ImmaturePregnant Dams

Improving maternal nutritional status can be ameans to enhance fetal growth in immature pregnantanimals (Luther et al., 2005b). For example, it is possi-ble to achieve the same prenatal growth trajectory andfinal birth weight in adolescent sheep as in maturesheep if the adolescent dam is optimally nourishedthroughout gestation (Wallace et al., 2005b). This canbe accomplished when adolescent dams have adequatenutrient reserves at conception and maternal adiposityis maintained throughout the final third of pregnancyby a stepwise increase in maternal dietary intake toprevent maternal catabolism and fully meet the fetalnutrient requirements (Wallace et al., 2005c).

Provision of Antioxidant Nutrients

Maternal and fetal metabolism during pregnancy isgreater than at any other stage in the life cycle, becauseof increased mitochondrial activity in maternal tissuesand the conceptus (Aurouseau et al., 2004). This is asso-ciated with an increase in the production of oxidants(e.g., superoxide anion, hydrogen peroxide, lipid perox-ides, and hydroxyl radicals), particularly in dairy cowsduring late gestation (Castillo et al., 2005). A low intakeof feed reduces the provision of antioxidant substancesand the endogenous synthesis of antioxidant proteinsand peptides (Wu et al., 2004b), thereby weakening theoxidative defense system, whereas overfeeding in-creases the oxidation of energy substrates, producingmore reactive oxygen species (Fang et al., 2002). Inaddition, when the diet is deficient in glycine, its de novosynthesis from choline yields formaldehyde (a potentoxidant). Thus, a deficiency of antioxidant minerals(e.g., Se, Zn, Cu, or Fe) or vitamins (e.g., folic acid,vitamin B6, and vitamin B12) reduces the survival andgrowth of embryos and fetuses (Ashworth and Anti-patis, 2001).

The state of oxidative stress during pregnancy isworsened in IUGR in response to underfeeding (Castilloet al., 2005) or overfeeding (Cole, 1990). A consequenceof oxidative stress is a reduction in the bioavailabilityof BH4 (not only an essential factor for endothelial NOsynthesis but also a potent antioxidant) and NO inmaternal and fetal tissues, particularly the vascularbed (Shi et al., 2004). This may contribute to insulinresistance in cows and sows during late gestation, be-cause NO mediates the stimulatory effect of insulinon muscle glucose uptake and metabolism (Jobgen etal., 2006).

The recognition of oxidative stress in IUGR has ledto the development of selective interventions. For exam-ple, dietary supplementation of selenium could enhanceplacental angiogenesis and fetal growth in underfedewes (Reynolds et al., 2005). This effect may result, inpart, from an increase in the bioavailability of BH4 andNO in the vascular system through an increase in theactivity of selenium-dependent glutathione peroxidasesto remove hydrogen peroxide (Shi et al., 2004; Wu et al.,2004b). Finally, increasing the biological availabilityof Zn, Cu, and Mn through attachment to short-chainpeptides has been reported to improve reproductive per-formance of swine, partly by enhancing antioxidantfunctions (Hostetler et al., 2003).

Manipulations of the Arginine-NO/PolyaminePathway

On the basis of studies with rodents, we previouslyhypothesized that modulation of the arginine-NO andpolyamine pathways could be highly effective to en-hance placental angiogenesis or placental blood flow,or both, and, therefore, improve fetal growth in IUGR(Wu et al., 2004a). There is now evidence from livestockto support this hypothesis. For example, we found thatadministration of sildenafil citrate (Viagra; a phospho-diesterase-5A inhibitor, which results in increased NOlevels) to ewes underfed between d 28 and 112 of gesta-tion prevented IUGR (M. C. Satterfield, Texas A&MUniversity, College Station, TX; G. Wu, F. W. Bazer,and T. E. Spencer, unpublished data), presumably byincreasing uteroplacental blood flow (Zoma et al., 2004).Additionally, in the ovine model of IUGR caused byplacental embolization, an increase in fetal arginineavailability via a short-term (4-h) direct infusion of argi-nine into the fetal femoral vein increased protein accre-tion in the fetus, in comparison with saline infusion (deBoo et al., 2005).

Importantly, Mateo et al. (2006) found that dietarysupplementation with 1.0% L-arginine-HCl to pregnantgilts between d 30 and 114 of gestation increased thenumber of live-born pigs by 2.1 per litter (a 23% in-crease; 9.13 vs. 11.23 piglets per litter) and the totallitter weight by 28% (12.37 vs. 15.80 kg). Thus, thearginine treatment improves the survival of porcineembryos/fetuses and enhances the provision of nutri-ents to the fetuses for supporting their in utero growth.This new exciting discovery could result in a tremen-dous economic return to swine producers, as an increasein even 1 piglet per litter has significant benefits (e.g.,net profits of approximately $45 per pig; NPPC, 2005).Therefore, dietary L-arginine supplementation may of-fer a novel, effective means to prevent IUGR inlivestock.

CONCLUSION AND PERSPECTIVES

Fetal growth is controlled by complex interactionsamong genetic, epigenetic, and environmental factors,

Wu et al.2332

as well as maternal maturity. These factors regulateplacental growth (including placental angiogenesis andvascular growth) and, therefore, uteroplacental bloodflows and the transfer of nutrients from mother to fetus.The IUGR results from disturbances of these maternaland fetal homeostatic mechanisms and occurs undervarious practical conditions of animal production. TheIUGR reduces neonatal survival, has a permanentstunting effect on postnatal growth performance andthe efficiency of feed/forage utilization in offspring, neg-atively affects whole body composition and meat qual-ity, and impairs lifetime fertility, health, and athleticperformance. Available evidence suggests that the pla-cental or fetal growth trajectory is vulnerable to mater-nal undernutrition or overnutrition throughout gesta-tion but that the most profound effects arguably occurwhen nutritional insults are applied during the periodof rapid placental development. Additionally, arginine-derived signaling and regulatory molecules (NO andpolyamines) are crucial for placental and fetal growth.New knowledge on the mechanisms regulating fetalgrowth and development will be beneficial for designingnew, rational, and effective strategies to prevent andtreat IUGR in livestock. Further, understanding themultiple roles of nutrients in DNA methylation (whichcan influence genome stability, viability, expression,and imprinting) will have a broad impact on reproduc-tive health and disease prevention. We expect that stud-ies utilizing domestic animal models of IUGR will pro-vide the necessary scientific basis for the developmentof management practices that will improve pregnancyoutcome in domestic animals. In view of the crucialroles of the arginine-dependent metabolic pathways,intravenous or oral administration of arginine may pro-vide a potentially novel solution to enhance uteropla-cental blood flows (and therefore transfer of nutrientsfrom mother to fetus), thereby ameliorating or pre-venting IUGR. Promoting an optimal intrauterine envi-ronment will not only ensure optimal fetal developmentbut also will enhance growth performance postnatallyand reduce the risk of chronic diseases in adults. Be-cause IUGR remains a major problem in mammalianpregnancies, innovative interdisciplinary research inthe areas of nutrition, reproductive physiology, and vas-cular biology are critical to design the next generationof nutrient-balanced gestational diets and develop newtools for livestock management, which will enhance theefficiency of animal production and improve the wellbeing of animals.

ACKNOWLEDGMENTS

We thank our colleagues and students for importantcontribution to the work reviewed in this article.Thanks also go to Scott Jobgen for assistance in manu-script preparation and to E. Gootwine for helpful com-ments on the paper.

LITERATURE CITED

Aitken, R. P., J. S. Milne, and J. M. Wallace. 2003. The impact ofprenatal growth restriction on the onset of puberty, ovulationrate and uterine capacity in sheep. Pediatr. Res. 53(Suppl.):34A.

Allen, W. R., S. Wilsher, C. Tiplady, and R. M. Butterfield. 2004. Theinfluence of maternal size on pre- and postnatal growth in thehorse: III. Postnatal growth. Reproduction 127:67–77.

Allen, W. R., S. Wilsher, C. Turnbull, F. Stewart, J. Ousey, P. D.Rossdale, and A. L. Fowden. 2002. Influence of maternal sizeon placental, fetal and postnatal growth in the horse: I. Develop-ment in utero. Reproduction 123:445–453.

Anderson, C. M., F. Lopez, A. Zimmer, and J. N. Benoit. 2006. Placen-tal insufficiency leads to developmental hypertension and mes-enteric artery dysfunction in two generations of Sprague-Dawleyrat offspring. Biol. Reprod. 74:538–544.

Anderson, L. L. 1975. Embryonic and placental development duringprolonged inanition in the pig. Am. J. Physiol. 229:1687–1694.

Anderson, L. L., and R. O. Parker. 1976. Distribution and develop-ment of embryos in the pig. J. Reprod. Fertil. 46:363–368.

Ashworth, C. J. 1991. Effect of pre-mating nutritional status andpost-mating progesterone supplementation on embryo survivaland conceptus growth in gilts. Anim. Reprod. Sci. 26:311–321.

Ashworth, C. J., and C. Antipatis. 2001. Micronutrient programmingof development throughout gestation. Reproduction 122:527–535.

Atinmo, T., W. G. Pond, and R. H. Barnes. 1974. Effect of maternalenergy vs. protein restriction on growth and development ofprogeny in swine. J. Anim. Sci. 39:703–711.

Aurouseau, B., D. Durand, and D. Gruffat. 2004. Control of oxidativephenomenon during gestation in monogastrics and ruminants.Prod. Anim. 17:339–354.

Azzam, S. M., J. E. Kinder, M. K. Nielsen, L. A. Werth, K. E. Gregory,L. V. Cundiff, and R. M. Koch. 1993. Environmental effects onneonatal mortality of beef calves. J. Anim. Sci. 71:282–290.

Baker, D. H., D. E. Becker, H. W. Norton, C. E. Sasse, A. H. Jensen,and B. G. Harmon. 1969. Reproductive performance and progenydevelopment in swine as influenced by feed intake during preg-nancy. J. Nutr. 97:489–495.

Barker, D. J. P., and P. M. Clark. 1997. Fetal undernutrition anddisease in later life. Rev. Reprod. 2:105–112.

Bar-Peled, U., Y. Aharoni, B. Robinzon, I. Bruckental, R. Lehrer, E.Maltz, C. H. Knight, J. Kali, Y. Folman, H. Voet, H. Gacitua,and H. Tagari. 1998. The effect of enhanced milk yield of dairycows by frequent milking or suckling on intake and digestibilityof the diet. J. Dairy Sci. 81:1420–1427.

Bauman, D. E., and W. B. Currie. 1980. Partitioning of nutrientsduring pregnancy and lactation: A review of mechanisms involv-ing homeostasis and homeorhesis. J. Dairy Res. 63:1514–1529.

Bazer, F. W. 1989. Allantoic fluid: Regulation of volume and composi-tion. Pages 135–155 in Reproductive and Perinatal Medicine.R. A. Brace, M. G. Ross, and J. E. Robillard, ed. PerinatologyPress, Ithaca, NY.

Bazer, F. W., A. J. Clawson, O. W. Robinson, and L. C. Ulberg. 1969a.Uterine capacity in gilts. J. Reprod. Fertil. 18:121–124.

Bazer, F. W., O. W. Robinson, A. J. Clawson, and L. C. Ulberg. 1969b.Uterine capacity at two stages of gestation in gilts followingembryo superinduction. J. Anim. Sci. 29:30–34.

Bee, G. 2004. Effect of early gestation feeding, birth weight, and sexof progeny on muscle fiber characteristics of pigs at slaughter.J. Anim. Sci. 82:826–836.

Bell, A. W., W. S. Burhans, and T. R. Overton. 2000. Protein nutritionin late pregnancy, maternal protein reserves and lactation per-formance in dairy cows. Proc. Nutr. Soc. 59:119–126.

Bell, A. W., and R. A. Ehrhardt. 2002. Regulation of placental nutrienttransport and implications for fetal growth. Nutr. Res. Rev.15:211–230.


Bellows, R. A., and R. E. Short. 1978. Effects of precalving feed levelon birth weight, calving difficulty and subsequent fertility. J.Anim. Sci. 46:1522–1528.

Bellows, R. A., R. E. Short, D. C. Anderson, B. W. Knapp, and O. F.Pahnish. 1971. Cause and effect relationships associated withcalving difficulty and calf birth weight. J. Anim. Sci. 33:407–415.

Bird, I. M., L. B. Zhang, and R. R. Magness. 2003. Possible mecha-nisms underlying pregnancy-induced changes in uterine arteryendothelial function. Am. J. Physiol. 284:R245–R258.

Boken, S. L., C. R. Staples, L. E. Sollenberger, T. C. Jenkins, andW. W. Thatcher. 2005. Effect of grazing and fat supplementationon production and reproduction of Holstein cows. J. Dairy Sci.88:4258–4272.

Buitrago, J. A., E. F. Walker, W. I. Snyder, and W. G. Pond. 1974.Blood and tissue traits in pigs at birth and at 3 weeks from giltsfed low or high energy diets during gestation. J. Anim. Sci.38:766–771.

Bush, J. A., G. Wu, A. Suryawan, H. V. Nguyen, and T. A. Davis. 2002.Somatotropin-induced amino acid conservation in pigs involvesdifferential regulation of liver and gut urea cycle enzyme activ-ity. J. Nutr. 132:59–67.

Cam, M. A., and M. Kuran. 2004. Effects of a single injection of hCGor GnRH agonist on day 12 post mating on fetal growth andreproductive performance of sheep. Anim. Reprod. Sci. 80:81–90.

Castillo, C., J. Hernandez, A. Bravo, M. Lopez-Alonso, V. Perreira,and J. L. Benedito. 2005. Oxidative status during late pregnancyand early lactation in dairy cows. Vet. J. 169:286–292.

Chappell, G. L. M. 1993. Nutritional management of replacementsheep utilizing southern forages: A review. J. Anim. Sci.71:3151–3154.

Chew, B. P., F. R. Murdock, R. E. Riley, and J. K. Hillers. 1984.Influence of prepartum dietary crude protein on growth hor-mone, insulin, reproduction, and lactation of diary cows. J. DairySci. 67:270–275.

Chwalisz, K., and R. E. Garfield. 1997. Regulation of the uterus andcervix during pregnancy and labor—Role of progesterone andnitric oxide. N.Y. Acad. Sci. 828:238–253.

Clanton, D. C., and D. R. Zimmerman. 1970. Symposium on pasturemethods for maximum production in beef cattle: Protein andenergy requirements for female beef cattle. J. Anim. Sci.30:122–132.

Cole, D. J. A. 1990. Nutritional strategies to optimize reproductionin pigs. J. Reprod. Fertil. Suppl. 40:67–82.

Costine, B. A., E. K. Inskeep, and M. E. Wilson. 2005. Growth hormoneat breeding modifies conceptus development and postnatalgrowth in sheep. J. Anim. Sci. 83:810–815.

Cundiff, L. V., M. V. Macneil, and K. E. Gregory. 1986. Between-and within-breed analysis of calving and survival to weaning inbeef cattle. J. Anim. Sci. 63:27–33.

Da Silva, P., R. P. Aitken, S. M. Rhind, P. A. Racey, and J. M. Wallace.2001. Influence of placentally-mediated foetal growth restrictionon the onset of puberty in male and female lambs. Reproduction122:375–383.

Da Silva, P., R. P. Aitken, S. M. Rhind, P. A. Racey, and J. M. Wallace.2002. Impact of maternal nutrition during pregnancy on pitu-itary gonadotrophin gene expression and ovarian developmentin growth-restricted and normally grown late gestation sheepfetuses. Reproduction 123:769–777.

Da Silva, P., R. P. Aitken, S. M. Rhind, P. A. Racey, and J. M. Wallace.2003. Effect of maternal overnutrition during pregnancy on pitu-itary gonadotrophin gene expression and gonadal morphologyin female and male sheep at Day 103 of gestation. Placenta24:248–257.

Da Silva-Buttkus, P., R. van den Hurk, E. R. T. Velde, and M. A. M.Taverne. 2003. Ovarian development in intrauterine growth-retarded and normally developed piglets originating from thesame litter. Reproduction 126:249–258.

Davis, T. A., M. L. Fiorotto, D. G. Burrin, W. G. Pond, and H. V.Nguyen. 1997. Intrauterine growth restriction does not alter

response of protein synthesis to feeding in newborn pigs. Am.J. Physiol. 272:E877–E884.

de Boo, H. A., P. L. Van Zijl, D. E. C. Smith, W. Kulik, H. N. Lafeber,and J. E. Harding. 2005. Arginine and mixed amino acids in-crease protein accretion in the growth-restricted and normalovine fetus by different mechanisms. Pediatr. Res. 58:270–277.

Dickinson, A. G., J. L. Hancock, G. J. R. Hovell, S. C. S. Taylor, andG. Wiener. 1962. The size of lambs at birth: A study involvingegg transfer. Anim. Prod. 4:64–79.

Doherty, A. S., M. R. W. Mann, K. D. Tremblay, M. S. Bartolomei,and R. M. Schultz. 2000. Differential effects of culture on im-printed H19 expression in the preimplantation mouse embryo.Biol Reprod. 62:1526–1535.

Du, M., M. J. Zhu, W. J. Means, B. W. Hess, and S. P. Ford. 2004.Effect of nutrient restriction on calpain and calpastatin contentof skeletal muscle from cows and fetuses. J. Anim. Sci.82:2541–2547.

Du, M., M. J. Zhu, W. J. Means, B. W. Hess, and S. P. Ford. 2005.Nutrient restriction differentially modulates the mammaliantarget of rapamycin signaling and the ubiquitin-proteasome sys-tem in skeletal muscle of cows and their fetuses. J. Anim. Sci.83:117–123.

Dwyer, C. M., N. C. Stickland, and J. M. Fletcher. 1994. The influenceof maternal nutrition on muscle fiber number development inthe porcine fetus and on subsequent postnatal growth. J. Anim.Sci. 72:911–917.

Dziuk, P. J. 1992. Embryonic development and fetal growth. Anim.Reprod. Sci. 28:299–308.

Einarsson, S., and T. Rojkittikhun. 1993. Effects of nutrition on preg-nant and lactating sows. J. Reprod. Fertil. Suppl. 48:229–239.

Fang, Y. Z., S. Yang, and G. Wu. 2002. Free radicals, antioxidants,and nutrition. Nutrition 18:872–879.

Ferguson, J. D. 2005. Nutrition and reproduction in dairy herds. Vet.Clin. Food Anim. 21:325–347.

Ferrell, C. L. 1991. Maternal and fetal influences on uterine andconceptus development in the cow: I. Growth of the tissues ofthe gravid uterus. J. Anim. Sci. 69:1945–1953.

Ferrell, C. L., and L. P. Reynolds. 1992. Uterine and umbilical bloodflows and net nutrient-uptake by fetuses and uteroplacentaltissues of cows gravid with either single or twin fetuses. J. Anim.Sci. 70:426–433.

Finch, A. M., L. G. Yang, M. O. Nwagwu, K. R. Page, H. J. McArdle,and C. I. Ashworth. 2004. Placental transport of leucine in aporcine model of low birth weight. Reproduction 128:229–235.

Flynn, N. E., C. J. Meininger, T. E. Haynes, and G. Wu. 2002. Themetabolic basis of arginine nutrition and pharmacotherapy. Bio-med. Pharmacother. 56:427–438.

Fontaneli, R. S., L. E. Sollenberger, R. C. Littell, and C. R. Staples.2005. Performance of lactating dairy cows managed on pasture-based or in freestall barn-feeding systems. J. Dairy Sci.88:1264–1276.

Ford, S. P. 1995. Control of blood flow to the gravid uterus of domesticlivestock species. J. Anim. Sci. 73:1852–1860.

Fowden, A. L., D. A. Giussani, and A. J. Forhead. 2005. Endocrineand metabolic programming during intrauterine development.Early Hum. Dev. 81:723–734.

Fowden, A. L., M. M. Ralph, and M. Silver. 1994. Nutritional regula-tion of uteroplacental prostaglandin production and metabolismin pregnant ewes and mares during late gestation. Exp. Clin.Endocrinol. 102:212–221.

Fu, W., T. E. Haynes, R. Kohli, J. Hu, W. Shi, T. E. Spencer, R. J.Carroll, C. J. Meininger, and G. Wu. 2005. Dietary L-argininesupplementation reduces fat mass in Zucker diabetic fatty rats.J. Nutr. 135:714–721.

Geisert, R. D., and R. A. M. Schmitt. 2002. Early embryonic survivalin the pig: Can it be improved? J. Anim. Sci. 80(E. Suppl.1):E54–E65.

Ginther, O. J., and R. H. Douglas. 1982. The outcome of twin pregnan-cies in mares. Theriogenology 18:237–242.

Giussani, D. A., A. J. Forhead, D. S. Gardner, A. J. Fletcher, W. R.Allen, and A. L. Fowden. 2003. Postnatal cardiovascular function

Wu et al.2334

after manipulation of fetal growth by embryo transfer in thehorse. J. Physiol. 547:67–76.

Gondret, F., L. Lefaucheur, I. Louveau, and B. Lebret. 2005. Thelong-term influence of birth weight on muscle characteristicsand eating meat quality in pigs individually reared and fedduring fattening. Arch. Tierz. Dummerstorf. 48:68–73.

Gootwine, E. 2004. Placental hormones and fetal-placental develop-ment. Anim. Reprod. Sci. 82-83:551–566.

Gootwine, E. 2005. Variability in the rate of decline in birth weightas litter size increases in sheep. Anim. Sci. 81:393–398.

Gootwine, E., A. Rozov, A. Bor, and S. Reicher. 2006. Carrying theFecB (Booroola) mutation is associated with lower birth weightand slower post-weaning growth rate for lambs, as well as alighter mature BW for ewes. Reprod. Fertil. Dev. 18:433–437.

Gootwine, E., A. Zenu, A. Bor, S. Yossafi, A. Rosov, and G. E. Pollott.2001. Genetic and economic analysis of introgression the B alleleof the FecB (Booroola) gene into the Awassi and Assaf dairybreeds. Livest. Prod. Sci. 71:49–58.

Gorelik, S., and J. Kanner. 2001. Oxymyoglobin oxidation and mem-brane lipid peroxidation initiated by iron redox cycle. J. Agric.Food Chem. 49:5939–5944.

Greenwood, P. L., A. S. Hunt, J. W. Hermanson, and A. W. Bell.1998. Effects of birth weight and postnatal nutrition on neonatalsheep: I. Body growth and composition, and some aspects ofenergetic efficiency. J. Anim. Sci. 76:2354–2367.

Greenwood, P. L., A. S. Hunt, J. W. Hermanson, and A. W. Bell.2000. Effects of birth weight and postnatal nutrition on neonatalsheep: II. Skeletal muscle growth and development. J. Anim.Sci. 78:50–61.

Grummer, R. R. 1995. Impact of changes in organic nutrient metabo-lism on feeding the transition dairy cow. J. Anim. Sci.73:2820–2833.

Guerra-Martinez, P., G. E. Dickerson, G. B. Anderson, and R. D.Green. 1990. Embryo-transfer twinning and performance effi-ciency in beef production. J. Anim. Sci. 68:4039–4050.

Han, I. K., P. Bosi, Y. Hyun, J. D. Kim, K. S. Sohn, and S. W. Kim.2000. Recent advances in sow nutrition to improve reproductiveperformance. Asian-australas. J. Anim. Sci. 13:335–355.

Handel, S. E., and N. C. Stickland. 1987. The growth and differentia-tion of porcine skeletal muscle fibre types and the influence ofbirthweight. J. Anat. 152:107–119.

Hegarty, P. V. J., and C. E. Allen. 1978. Effect of pre-natal runtingon the post-natal development of skeletal muscles in swine andrats. J. Anim. Sci. 46:1634–1640.

Hoaglund, C. M., V. M. Thomas, M. K. Petersen, and R. W. Kott.1992. Effects of supplemental protein source and metabolizableenergy intake on nutritional status in pregnant ewes. J. Anim.Sci. 70:273–280.

Hoffman, R. M., D. S. Kronfeld, W. L. Cooper, and P. A. Harris.2003. Glucose clearance in grazing mares is affected by diet,pregnancy, and lactation. J. Anim. Sci. 81:1764–1771.

Hostetler, C. E., R. L. Kincaid, and M. A. Mirando. 2003. The roleof essential trace elements in embryonic and fetal developmentin livestock. Vet. J. 166:125–139.

Huston, J. E., C. A. Taylor, C. J. Lupton, and T. D. Brooks. 1993.Effects of supplementation on intake, growth rate, and fleeceproduction by female Angora kid goats grazing rangeland. J.Anim. Sci. 71:3124–3130.

Jaenisch, R., and A. Bird. 2003. Epigenetic regulation of gene expres-sion: How the genome integrates intrinsic and environmentalsignals. Nat. Genet. 33(Suppl.):245–254.

Ji, F., G. Wu, J. R. Blanton Jr., and S. W. Kim. 2005. Changes inweight and composition in various tissues of pregnant gilts andtheir nutritional implications. J. Anim. Sci. 83:366–375.

Jindal, R., J. R. Cosgrove, and G. R. Foxcroft. 1997. Progesteronemediates nutritionally induced effects on embryonic survival ingilts. J. Anim. Sci. 75:1063–1070.

Jobgen, W. S., S. K. Fried, W. J. Fu, C. J. Meininger, and G. Wu.2006. Regulatory role for the arginine-nitric oxide pathway inenergy-substrate metabolism. J. Nutr. Biochem. doi:10.1016/j.nutbio.2005.12.001

Johnson, L. J. 1993. Maximizing feed intake of lactating sows. Com-pend. Contin. Educ. Pract. Vet. 15:133–141.

Jonker, F. H. 2004. Fetal death: Comparative aspects in large domes-tic animals. Anim. Reprod. Sci. 82:415–430.

Kablar, B., K. Krastel, S. Tajbakhsh, and M. A. Rudnicki. 2003. Myf5and MyoD activation define independent myogenic compart-ments during embryonic development. Dev. Biol. 258:307–318.

Karunaratne, J. F., C. J. Ashton, and N. C. Stickland. 2005. Fetalprogramming of fat and collagen in porcine skeletal muscles. J.Anat. 207:763–768.

Kemp, B., N. M. Soede, P. C. Vesseur, F. A. Helmond, J. H. Spooren-berg, and K. Frankena. 1996. Glucose tolerance of pregnant sowsis related to postnatal pig mortality. J. Anim. Sci. 74:879–885.

Knight, C. H. 2001. Lactation and gestation in dairy cows: Flexibilityavoids nutritional extremes. Proc. Nutr. Soc. 60:527–537.

Knight, J. W., F. W. Bazer, W. W. Thatcher, D. E. Frank, and H. D.Wallace. 1977. Conceptus development in intact and unilaterallyhysterectomized-ovariectomized gilts: Interrelationships amonghormonal status, placental development, fetal fluids, and fetalgrowth. J. Anim. Sci. 44:620–637.

Kreikemeier, K. K., and J. A. Unruh. 1993. Carcass traits and theoccurrence of dark cutters in pregnant and nonpregnant feedlotheifers. J. Anim. Sci. 71:1699–1703.

Kroker, G. A., and L. J. Cummins. 1979. The effect of nutritionalrestriction on Hereford heifers in late pregnancy. Aust. Vet. J.55:467–474.

Kwon, H., S. P. Ford, F. W. Bazer, T. E. Spencer, P. W. Nathanielsz,M. J. Nijland, B. W. Hess, and G. Wu. 2004a. Maternal undernu-trition reduces concentrations of amino acids and polyamines inovine maternal and fetal plasma and fetal fluids. Biol. Reprod.71:901–908.

Kwon, H., T. E. Spencer, F. W. Bazer, and G. Wu. 2003a. Develop-mental changes of amino acids in ovine fetal fluids. Biol. Reprod.68:1813–1820.

Kwon, H., G. Wu, F. W. Bazer, and T. E. Spencer. 2003b. Develop-mental changes in polyamine levels and synthesis in the ovineconceptus. Biol. Reprod. 69:1626–1634.

Kwon, H., G. Wu, C. J. Meininger, F. W. Bazer, and T. E. Spencer.2004b. Developmental changes in nitric oxide synthesis in theovine placenta. Biol. Reprod. 70:679–686.

Lippke, H. 1980. Forage characteristics related to intake, digestibilityand gain by ruminants. J. Anim. Sci. 50:952–961.

Luther, J. S., R. P. Aitken, J. S. Milne, M. Matsuzaki, D. A. Redmer,L. P. Reynolds, and J. M. Wallace. 2005a. The effect of under-nourishing pregnant adolescent sheep on maternal and fetalgrowth, nutrient status and body composition. Reprod. Abstr.17:68.

Luther, J. S., D. A. Redmer, L. P. Reynolds, and J. M. Wallace.2005b. Nutritional paradigms of ovine fetal growth restriction:Implications for human pregnancy. Hum. Fertil. 8:179–187.

Makarechian, M., J. V. Whiteman, L. E. Walters, and A. W. Munson.1978. Relationships between growth rate, dressing percentageand carcass composition in lambs. J. Anim. Sci. 46:1610–1617.

Marliss, E. B., S. Chevalier, R. Gougeon, J. A. Morais, M. Lamarche,O. A. J. Adegoke, and G. Wu. 2006. Elevations of plasma methyl-arginines in obesity and ageing are related to insulin sensitivityand rates of protein turnover. Diabetologia 49:351–359.

Mateo, R. D., G. Wu, J. A. Carroll, I. Shinzato, and S. W. Kim.2006. Dietary L-arginine supplementation improves pregnancyoutcome in gilts. J. Anim. Sci. 84(Suppl. 2):7–8.

Matsuzaki, M., J. S. Milne, R. P. Aitken, and J. M. Wallace. 2006.Overnourishing pregnant adolescent ewes preserves perirenalfat deposition in their growth-restricted fetuses. Reprod. Fertil.Dev. 18:357–364.

McEvoy, T. G., J. J. Robinson, C. J. Ashworth, J. A. Rooke, and K.D. Sinclair. 2001. Feed and forage toxicants affecting embryosurvival and fetal development. Theriogenology 55:113–129.


McMillen, I. C., and J. S. Robinson. 2005. Developmental origins of themetabolic syndrome: Prediction, plasticity, and programming.Physiol. Rev. 85:571–633.

McNeill, D. M., R. Slepetis, R. A. Ehrhardt, D. M. Smith, and A. W.Bell. 1997. Protein requirements of sheep in late pregnancy:Partitioning of nitrogen between gravid uterus and maternaltissues. J. Anim. Sci. 75:809–816.

Meijer, A. J., and P. F. Dubbelhuis. 2004. Amino acid signaling andthe integration of metabolism. Biochem. Biophys. Res. Commun.313:397–403.

Mellor, D. J. 1983. Nutritional and placental determinants of fetalgrowth rate in sheep and consequences for the newborn lamb.Br. Vet. J. 139:307–324.

Milligan, B. N., D. Fraser, and D. L. Kramer. 2002. Within-litterbirth weight variation in the domestic pig and its relation topre-weaning survival, weight gain, and variation in weaningweights. Livest. Prod. Sci. 76:181–191.

Murphy, V. E., R. Smith, W. B. Giles, and V. L. Clifton. 2006. Endo-crine regulation of human fetal growth: The role of the mother,placenta, and fetus. Endocr. Rev. 27:141–169.

National Research Council. 1985. Nutrient Requirements of Sheep.Natl. Acad. Press, Washington, DC.

National Research Council. 1998. Nutrient Requirements of Swine.Natl. Acad. Press, Washington, DC.

Nissen, P. M., V. O. Danielsen, P. F. Jorgensen, and N. Oksbjerg.2003. Increased maternal nutrition of sows has no beneficialeffects on muscle fiber number or postnatal growth and hasno impact on the meat quality of the offspring. J. Anim. Sci.81:3018–3027.

Noblet, J., W. H. Close, and R. P. Heavens. 1985. Studies on theenergy metabolism of the pregnant sow. I. Uterus and mammarytissue development. Br. J. Nutr. 53:251–265.

NPPC. 2005. National Pork Producers Council, Washington, DC.http://www.nppc.org Accessed Feb. 10, 2006.

Oommen, A. M., J. B. Griffin, G. Sarath, and J. Zempleni. 2005. Rolesfor nutrients in epigenetic events. J. Nutr. Biochem. 16:74–77.

Osgerby, J. C., D. C. Wathes, D. Howard, and T. S. Gadd. 2002.The effect of maternal undernutrition on ovine fetal growth. J.Endocrinol. 173:131–141.

Ozaki, T., P. Hawkins, H. Nishina, C. Steyn, L. Poston, and M. A.Hanson. 2000. Effects of undernutrition in early pregnancy onsystemic small artery function in late-gestation fetal sheep. Am.J. Obstet. Gynecol. 183:1301–1307.

Patterson, H. H., D. C. Adams, T. J. Klopfenstein, R. T. Clark, andB. Teichert. 2003. Supplementation to meet MP requirementsof primiparous beef heifers: II. Pregnancy and economics. J.Anim. Sci. 81:563–570.

Pere, M. C., and M. Etienne. 2000. Uterine blood flow in sows: Effectsof pregnancy stage and litter size. Reprod. Nutr. Dev. 40:369–382.

Perry, J. S., and J. G. Rowell. 1969. Variations in foetal weight andvascular supply along the uterine horn of the pig. J. Reprod.Fertil. 19:527–534.

Pond, W. G., D. N. Strachan, Y. N. Sinha, E. F. Walker, Jr., J. A.Dunn, and R. H. Barnes. 1969. Effect of protein deprivation ofswine during all or part of gestation on birth weight, postnatalgrowth rate and nucleic acid content of brain and muscle ofprogeny. J. Nutr. 99:61–67.

Pond, W. G., J. T. Yen, R. R. Mauer, and R. K. Christenson. 1981.Effect of doubling daily energy intake during the last two weeksof pregnancy on pig birth weight, survival and weaning weight.J. Anim. Sci. 52:535–541.

Powell, S. E., and E. D. Aberle. 1980. Effects of birth weight on growthand carcass composition of swine. J. Anim. Sci. 50:860–868.

Pugh, D. G. 1993. Feeding and nutrition of brood mares. Compend.Contin. Educ. Pract. Vet. 15:106–115.

Quiniou, N., J. Dagorn, and D. Gaudre. 2002. Variation of piglets’birth weight and consequences on subsequent performance.Livest. Prod. Sci. 78:63–70.

Redmer, D. A., R. P. Aitken, J. S. Milne, L. P. Reynolds, and J.M. Wallace. 2005. Influence of maternal nutrition on mRNAexpression of placental angiogenic factors and their receptors atmid-gestation in adolescent sheep. Biol. Reprod. 72:1004–1009.

Redmer, D. A., J. M. Wallace, and L. P. Reynolds. 2004. Effect ofnutrient intake during pregnancy on fetal and placental growthand vascular development. Domest. Anim. Endocrinol.27:199–217.

Rehfeldt, C., P. M. Nissen, G. Kuhn, M. Vestergaard, K. Ender, andN. Oksbjerg. 2004. Effects of maternal nutrition and porcinegrowth hormone (pGH) treatment during gestation on endocrineand metabolic factors in sows, fetuses and pigs, skeletal muscledevelopment, and postnatal growth. Domest. Anim. Endocrinol.27:267–285.

Reynolds, L. P., P. P. Borowicz, K. A. Vonnahme, M. L. Johnson, A.T. Grazul-Bilska, D. A. Redmer, and J. S. Caton. 2005. Placentalangiogenesis in sheep models of compromised pregnancy. J.Physiol. 565:43–58.

Reynolds, L. P., J. S. Caton, D. A. Redmer, A. T. Grazul-Bilska, K.A. Vonnahme, P. B. Borowicz, J. S. Luther, J. M. Wallace, G.Wu, and T. E. Spencer. 2006. Evidence for altered placental bloodflow and vascularity in compromised pregnancies. J. Physiol.572:51–58.

Reynolds, L. P., C. L. Ferrell, J. A. Nienaber, and S. P. Ford. 1985.Effects of chronic environmental heat stress on blood flow andnutrient uptake of the gravid bovine uterus and foetus. J. Agric.Sci. Camb. 104:289–297.

Reynolds, L. P., and D. A. Redmer. 2001. Angiogenesis in the placenta.Biol. Reprod. 64:1033–1040.

Robinson, J. J., K. D. Sinclair, and T. G. McEvoy. 1999. Nutritionaleffects on foetal growth. Anim. Sci. 68:315–331.

Rossdale, P. D., and J. C. Ousey. 2002. Fetal programming for athleticperformance in the horse: Potential effects of IUGR. Equine Vet.Educ. 14:98–112.

Savvidou, M. D., A. D. Hingorani, D. Tsikas, J. C. Frolich, P. Vallance,and K. H. Nicolaides. 2003. Endothelial dysfunction and raisedplasma concentrations of asymmetric dimethylarginine in preg-nant women who subsequently develop pre-eclampsia. Lancet361:1511–1517.

Schinckel, P. G., and B. F. Short. 1961. The influence of nutritionallevel during pre-natal and early post-natal life on adult fleeceand body characteristics. Aust. J. Agric. Res. 12:176–202.

Schoknecht, P. A., G. R. Newton, D. E. Weise, and W. G. Pond. 1994.Protein restriction in early pregnancy alters fetal and placentalgrowth and allantoic fluid proteins in swine. Theriogenology42:217–226.

Schoknecht, P. A., W. G. Pond, H. J. Mersmann, and R. R. Mauer.1993. Protein restriction during pregnancy affects postnatalgrowth in swine progeny. J. Nutr. 123:1818–1825.

Self, J. T., T. E. Spencer, G. A. Johnson, J. Hu, F. W. Bazer, andG. Wu. 2004. Glutamine synthesis in the developing porcineplacenta. Biol. Reprod. 70:1444–1451.

Shi, W., C. J. Meininger, T. E. Haynes, K. Hatakeyama, and G. Wu.2004. Regulation of tetrahydrobiopterin synthesis and bioavail-ability in endothelial cells. Cell Biochem. Biophys. 41:415–433.

Sordella, R., W. Jiang, G. C. Chen, M. Curto, and L. Settleman. 2003.Modulation of Rho GTPase signaling regulates a switch betweenadipogenesis and myogenesis. Cell 113:147–158.

Southam, E. R., C. V. Hulet, and M. P. Botkin. 1971. Factors influenc-ing reproduction in ewe lambs. J. Anim. Sci. 33:1282–1287.

Stead, L. M., K. P. Au, R. L. Jacobs, M. E. Brosnan, and J. T. Brosnan.2001. Methylation demand and homocysteine metabolism: Ef-fects of dietary provision of creatine and guanidinoacetate. Am.J. Physiol. 281:E1095–E1100.

Thomas, V. M., and R. W. Kott. 1995. A review of Montana winterrange ewe nutrition research. Sheep Goat Res. J. 11:17–24.

Thornbury, J. C., P. D. Sibbons, D. Vanvelzen, R. Trickey, and L.Spitz. 1993. Histological investigations into the relationship be-tween low-birth-weight and spontaneous bowel damage in theneonatal piglet. Pediatr. Pathol. 13:59–69.

Wu et al.2336

Trahair, J. F., T. M. DeBarro, J. S. Robinson, and J. A. Owens. 1997.Restriction of nutrition in utero selectively inhibits gastrointesti-nal growth in fetal sheep. J. Nutr. 127:637–641.

Tudor, G. D. 1972. The effect of pre- and post-natal nutrition on thegrowth of beef cattle. I. The effect of nutrition and parity of thedam on calf birth weight. Aust. J. Agric. Res. 23:389–395.

USDA. 1998. Equine morbidity and mortality. #N292.998.USDA:APHIS:VS., CEAH, National Animal Health MonitoringSystem, Fort Collins, CO.

USDA. 2003a. Dairy 2002. Part II: Changes in the United StatesDairy Industry, 1991-2002. #N388.0603. USDA:APHIS:VS.,CEAH, National Animal Health Monitoring System, Fort Col-lins, CO.

USDA. 2003b. Sheep 2001. Part III: Lambing practices, Spring 2001.#N379.0403. USDA:APHIS:VS., CEAH, Natl. Anim. HealthMonitoring System, Fort Collins, CO.

USDA. 2005. Swine 2000. Part IV: Changes in the U.S. pork industry,1990-2000. #N428.0405. USDA:APHIS:VS., CEAH, Natl. Anim.Health Monitoring System, Fort Collins, CO.

Vallet, J. L., K. A. Leymaster, and R. K. Christenson. 2002. Theinfluence of uterine function on embryonic and fetal survival. J.Anim. Sci. 80(E. Suppl. 2):E115–E125.

van der Lende, T., and B. T. T. M. van Rens. 2003. Critical periodsfor foetal mortality in gilts identified by analyzing the lengthdistribution of mummified foetuses and frequency of non-freshstillborn piglets. Anim. Reprod. Sci. 75:141–150.

Van Rens, B. T. T. M., G. de Koning, R. Bergsma, and T. van derLende. 2005. Preweaning piglet mortality in relation to placentalefficiency. J. Anim. Sci. 83:144–151.

Villette, Y., and M. Theriez. 1981. Effect of birth weight on lambperformances. I. Level of feed intake and growth. Ann. Zootech.30:151–168.

Vinsky, M. D., S. Novak, W. T. Dixon, M. K. Dyck, and G. R. Foxcroft.2006. Nutritional restriction in lactating primiparous sows selec-tively affects female embryo survival and overall litter develop-ment. Reprod. Fertil. Dev. 18:347–355.

Vonnahme, K. A., and S. P. Ford. 2004. Placental vascular endothelialgrowth factor receptor system mRNA expression in pigs selectedfor placental efficiency. J. Physiol. 554:194–201.

Vonnahme, K. A., B. W. Hess, T. R. Hansen, R. J. McCormick, D. C.Rule, G. E. Moss, W. J. Murdoch, M. J. Nijland, D. C. Skinner,P. W. Nathanielsz, and S. P. Ford. 2003. Maternal undernutri-tion from early to mid-gestation leads to growth retardation,cardiac ventricular hypertrophy and increased liver weight inthe fetal sheep. Biol. Reprod. 69:133–140.

Wallace, J. M., R. P. Aitken, and M. A. Cheyne. 1996. Nutrient parti-tioning and fetal growth in the rapidly growing adolescent ewe.J. Reprod. Fertil. 107:183–190.

Wallace, J. M., D. A. Bourke, R. P. Aitken, N. Leitch, and W. W. Hay,Jr. 2002. Blood flows and nutrient uptakes in growth-restrictedpregnancies induced by overnourishing adolescent sheep. Am.J. Physiol. 282:R1027–R1036.

Wallace, J. M., D. A. Bourke, R. P. Aitken, J. S. Milne, and W. W.Hay. 2003a. Placental glucose transport in growth-restrictedpregnancies induced by overnourished adolescent sheep. J. Phys-iol. 547:85–94.

Wallace, J. M., D. A. Bourke, P. Da Silva, and R. P. Aitken. 2001.Nutrient partitioning during adolescent pregnancy. Reproduc-tion 122:347–357.

Wallace, J. M., D. A. Bourke, P. Da Silva, and R. P. Aitken. 2003b.Influence of progesterone supplementation during the first thirdof pregnancy on fetal and placental growth in overnourishedadolescent ewes. Reproduction 126:481–487.

Wallace, J., M. Matsuzaki, J. Milne, and R. Aitken. 2005a. The effectof maternal growth hormone treatment on fetal growth andadiposity in rapidly growing adolescent sheep. Pediatr. Res.58:1030.

Wallace, J. M., J. S. Milne, and R. P. Aitken. 2004. Maternal growthhormone treatment from day 35 to 80 of gestation alters nutrient

partitioning in favor of uteroplacental growth in the overnouri-shed adolescent sheep. Biol. Reprod. 70:1277–1285.

Wallace, J. M., J. S. Milne, and R. P. Aitken. 2005b. The effect ofovernourishing singleton bearing adult ewes on nutrient parti-tioning to the gravid uterus. Br. J. Nutr. 94:533–539.

Wallace, J. M., T. R. Regnault, S. W. Limesand, W. W. Hay, Jr.,and R. V. Anthony. 2005c. Investigating the causes of low birthweight in contrasting ovine paradigms. J. Physiol. 565:19–26.

Wang, T., Y. J. Huo, F. X. Shi, R. J. Xu, and R. J. Hutz. 2005.Effects of intrauterine growth retardation on development of thegastrointestinal tract in neonatal pigs. Biol. Neonate 88:66–72.

Wastney, M. E., A. C. Arcus, R. Bickerstaffe, and J. E. Wolff. 1982.Glucose tolerance in ewes and susceptibility to pregnancy toxe-mia. Aust. J. Biol. Sci. 35:381–392.

Waterland, R. A., and R. L. Jirtle. 2004. Early nutrition, epigeneticchanges at transposons and imprinted genes, and enhanced sus-ceptibility to adult chronic diseases. Nutrition 20:63–68.

Widdowson, E. M. 1971. Intra-uterine growth retardation in the pig.Biol. Neonate 19:329–340.

Wigmore, P. M. C., and N. C. Stickland. 1983. Muscle developmentin large and small pig fetuses. J. Anat. 137:235–245.

Wilsher, S., and W. R. Allen. 2003. The effects of maternal age andparity on placental and fetal development in the mare. EquineVet. J. 35:476–483.

Wilson, M. E. 2002. Role of placental function in mediating conceptusgrowth and survival. J. Anim. Sci. 80(E. Suppl. 2):E195–E201.

Wilson, M. E., N. J. Biensen, C. R. Youngs, and S. P. Ford. 1998.Development of Meishan and Yorkshire littermate conceptusesin either a Meishan or Yorkshire uterine environment to day90 of gestation and to term. Biol. Reprod. 58:905–910.

Wootton, R., P. A. Flecknell, J. P. Royston, and M. John. 1983. Intra-uterine growth retardation detected in several species by non-normal birth weight distributions. J. Reprod. Fertil. 69:659–663.

Wootton, R., I. R. McFadyen, and J. E. Cooper. 1977. Measurementof placental blood flow in the pig and its relation to placentaland fetal weight. Biol. Neonate 31:333–339.

Wu, G. 1998. Intestinal mucosal amino acid catabolism. J. Nutr.128:1249–1252.

Wu, G., F. W. Bazer, T. A. Cudd, C. J. Meininger, and T. E. Spencer.2004a. Maternal nutrition and fetal development. J. Nutr.134:2169–2172.

Wu, G., F. W. Bazer, J. Hu, G. A. Johnson, and T. E. Spencer. 2005.Polyamine synthesis from proline in the developing porcine pla-centa. Biol. Reprod. 72:842–850.

Wu, G., F. W. Bazer, and W. Tuo. 1995. Developmental changes offree amino acid concentrations in fetal fluids of pigs. J. Nutr.125:2859–2868.

Wu, G., F. W. Bazer, W. Tuo, and S. P. Flynn. 1996. Unusual abun-dance of arginine and ornithine in porcine allantoic fluid. Biol.Reprod. 54:1261–1265.

Wu, G., Y. Z. Fang, S. Yang, J. R. Lupton, and N. D. Turner. 2004b.Glutathione metabolism and its implications for health. J. Nutr.134:489–492.

Wu, G., L. A. Jaeger, F. W. Bazer, and J. M. Rhoads. 2004c. Argininedeficiency in premature infants: Biochemical mechanisms andnutritional implications. J. Nutr. Biochem. 15:442–451.

Wu, G., D. A. Knabe, and S. W. Kim. 2004d. Arginine nutrition inneonatal pigs. J. Nutr. 134:2783S–2790S.

Wu, G., and E. B. Marliss. 1992. Interorgan metabolic coordinationduring fasting and underfeeding: An adaptation for mobilizingfat and sparing protein in man. Pages 219–244 in Biology ofFeast and Famine. G. H. Anderson and S. H. Kennedy, ed. Aca-demic Press, San Diego, CA.

Wu, G., and C. J. Meininger. 2002. Regulation of nitric oxide synthesisby dietary factors. Annu. Rev. Nutr. 22:61–86.

Wu, G., and S. M. Morris, Jr. 1998. Arginine metabolism: Nitric oxideand beyond. Biochem. J. 336:1–17.

Wu, G., W. G. Pond, S. P. Flynn, T. L. Ott, and F. W. Bazer. 1998b.Maternal dietary protein deficiency decreases nitric oxide syn-


thase and ornithine decarboxylase activities in placenta andendometrium of pigs during early gestation. J. Nutr.128:2395–2402.

Wu, G., W. G. Pond, T. Ott, and F. W. Bazer. 1998a. Maternal dietaryprotein deficiency decreases amino acid concentrations in fetalplasma and allantoic fluid of pigs. J. Nutr. 128:894–902.

Wu, G., and J. T. Self. 2005. Proteins. Pages 757–759 in Encyclopediaof Animal Science. W. G. Pond and A. W. Bell, ed. Marcel DekkerInc., New York, NY.

Wu, G., and J. R. Thompson. 1990. The effect of glutamine on proteinturnover in chick skeletal muscle in vitro. Biochem. J.265:593–598.

Yu, Z., R. N. Kirkwood, and P. A. Thacker. 1997. Effect of exogenousprogesterone following mating on embryo survival in the gilt.Can. J. Anim. Sci. 77:731–733.

Zheng, J., Y. X. Wen, J. L. Austin, and D. B. Chen. 2006. Exogenousnitric oxide stimulates cell proliferation via activation of a mito-gen-activated protein kinase pathway in ovine fetoplacental ar-tery endothelial cells. Biol. Reprod. 74:375–382.

Zoma, W. D., R. S. Baker, and K. E. Clark. 2004. Effects of combineduse of sildenafil citrate (Viagra) and 17-β-estradiol on ovine coro-nary and uterine hemodynamics. Am. J. Obstet. Gynecol.190:1291–1297.

board-invited review: intrauterine growth … review: intrauterine growth retardation: implications...

Documents