B R A I N R E S E A R C H 1 1 3 2 ( 2 0 0 7 ) 7 8 – 8 3

ava i l ab l e a t www.sc i enced i rec t . com

www.e l sev i e r. com/ loca te /b ra in res

Research Report

Growth-deficient vomeronasal organs in the naked mole-rat(Heterocephalus glaber )

Timothy D. Smitha,b,⁎, Kunwar P. Bhatnagarc, John C. Dennisd,Edward E. Morrisond, Thomas J. Parke

aSchool of Physical Therapy, Slippery Rock University, Slippery Rock, PA 16057, USAbDepartment of Anthropology, University of Pittsburgh, Pittsburgh, PA, USAcDepartment of Anatomical Sciences and Neurobiology, University of Louisville School of Medicine, Louisville, KY, USAdDepartment of Anatomy, Physiology, and Pharmacology, Auburn University, Auburn, AL, USAeDepartment of Biological Sciences, University of Illinois at Chicago, Chicago, IL, USA

A R T I C L E I N F O

⁎ Corresponding author. School of Physical The+1 724 738 2113.

E-mail address: timothy.smith@sru.edu (T

0006-8993/$ – see front matter © 2006 Elsevidoi:10.1016/j.brainres.2006.11.021

A B S T R A C T

Article history:Accepted 8 November 2006Available online 26 December 2006

The naked mole-rat (Heterocephalus glaber) is unusual in numerous life historycharacteristics as well as its eusocial organization. This species demonstrates widespreadsexual suppression and prominent scentmarking, behaviors that have been associated withpheromonal communication involving the vomeronasal organ in other rodents. Yet,previous studies indicate that urinary signals do not mediate sexual suppression inHeterocephalus. Surprisingly, no previous studies have examined the vomeronasal organ inthis species. Here, we show that Heterocephalus is unique among rodents in showing noevidence of postnatal volumetric growth in the vomeronasal neuroepithelium. Subadultsfrom birth to weaning fell within the same volume range as adults regardless of breeding/non-breeding status of the latter. A comparison of existing ontogenetic data on othermammals suggests that the proportionally small VNOs of Heterocephalus may be explainedby a deficiency in VNNE growth. Growth deficiency of the vomeronasal organ inHeterocephalus may relate to a diminished role that pheromones play in certain socialinteractions for this species, such as breeding suppression. In light of the unique aspects ofthe vomeronasal organ in Heterocephalus, comparative studies of rodents may provide amodel for understanding variation of this sensory system in other mammalian ordersincluding primates, an order which shows a range from vestigial to demonstrably functionalvomeronasal organs.

© 2006 Elsevier B.V. All rights reserved.

Keywords:Accessory olfactionNeuroepitheliumVNO

1. Introduction

In addition to olfaction, most mammals possess an “acces-sory” olfactory sense. The peripheral sense organ for acces-sory olfactory system is the vomeronasal organ (VNO),

rapy, 204 Physical Therap

.D. Smith).

er B.V. All rights reserved

bilateral epithelial tubes found in the base of the nasal septumof most terrestrial vertebrates (Halpern and Martínez-Marcos,2003; Wysocki, 1979). Although it is absent or nonfunctional insome species (e.g., humans, whales, numerous bats), the VNOplays a critical role in mediating sociosexual behaviors in

y Bldg, Slippery Rock University, Slippery Rock, PA 16057, USA. Fax:

.

Fig. 1 – In adult (a, female) and infant (b, 4-week-old male) mole-rats, the vomeronasal organ (VNO) is found at thebase of the nasal septum (ns) within a capsule that is mostly cartilaginous (vnc, vomeronasal cartilage). The cross-sectionalarea of the neuroepithelium (*) is similar between adults and subadults in most cases (a, b) or smaller in adults(c, adult male; d, 2-week-old). The largest cross-sectional area of VNNE was found in a 2-week-old mole-rat (d). In mostanimals, olfactory neuroepithelium appears to be thicker with more numerous rows of receptor nuclei (orn) compared tothe VNNE (Fig. 1e; same animal as c). L, lumen of vomeronasal organ; oe, olfactory neuroepithelium; rfe, receptor-freeepithelium; sc, nuclei of supporting cells. All slides prepared with Gomori trichrome staining.

Fig. 2 – In adult (a) and infant (b) mole-rats, beta tubulin (BT) reactivity is present in the vomeronasal organ (vno) and olfactoryneuroepithelium (oe). Note the increased height of the nasal cavity (nc) in the adult compared to infant mole-rat. Both olfactoryneuroepithelium (oe, Figs. 2b, c) and vomeronasal neuroepithelium (vnne, d, e) are BT+. The VNO has a thicker, medialneuroepithelium and thinner receptor-free epithelium (rfe) at all ages (d, adult male; e, P7). Beta tubulin immunoreactivityreveals bipolar neurons (d, e, open arrows) and their dendrites (arrows) in the VNNE. Based on the superimposed 20×20 μmbox, note that the cell bodies of bipolar neurons in the VNNE of the adult (d) are larger than those of the 1-week-old (e). Also notethat neuron cell bodies in the VNNE (e) are larger than those of the olfactory neuroepithelium (c, same adult male, showing thejunction of respiratory epithelium (re) and oe). L, lumen; ns, nasal septum; vnc, vomeronasal cartilage.

79B R A I N R E S E A R C H 1 1 3 2 ( 2 0 0 7 ) 7 8 – 8 3

80 B R A I N R E S E A R C H 1 1 3 2 ( 2 0 0 7 ) 7 8 – 8 3

some other mammals, especially rodents. Relative to bodysize, the neuroepithelial part of the VNO (VNNE) is propor-tionally large in rodents compared to other mammals (Smithet al., 2005). In rats and mice, rapid proliferation andmaturation of the receptor neuron population occur duringinfancy and overall VNNE volume increases with body size(Weiler et al., 1999; Wilson and Raisman, 1980). Here we reportan exception: the African naked mole-rat (Heterocephalusglaber), a eusocial rodent in which nearly all members oflarge colonies are reproductively suppressed (Jarvis, 1981).

In some other rodents (e.g., mice, voles), reproductivesuppression is mediated by urinary substances—a pheromo-nal effect (Halpern and Martínez-Marcos, 2003; Thompson etal., 2004). However, suppression of mole-rat reproduction innon-breeding females is thought to be meditated by beha-vioral, rather than pheromonal, cues from the breedingfemales (Smith et al., 1997). Fossorial mammals are noted forsensory specializations, such as exquisite tactile senses(Catania, 2005; Crish et al., 2003). Chemosensory systemsremain uninvestigated in Heterocephalus despite demonstrablefunctions of scent marking in common nesting and toilet loci(Jarvis and Sherman, 2002). Herein, we investigated size andimmunoreactivity of the VNNE in adult and infant nakedmole-rats from captive breeding colonies.

Fig. 3 – (a) Volume (in mm3) of the vomeronasalneuroepithelium (vnne) in infant and adult mole-rats(breeding and non-breeding). Error bars indicate standarderror of the mean for 2-week-olds (nn=3), adult males (nn=3)and adult females (nn=3). (b) Existing data for rats on VNNEvolume (±SEM) in rats, from birth to sexual maturity. (c)Comparison of mean vomeronasal neuroepithelial (vnne)volume in adult nakedmole-rats compared to published dataon other rodents. Data for all species are from animals ofadult body mass and include both sexes combined. Errorbars, standard error of the mean.

2. Results

2.1. Morphology and immunohistochemistry

The VNOs are found within cartilaginous-osseous capsules(Figs. 1a, b). At all ages, a medial VNNE is clearlydistinguished from the thinner receptor-free epithelium ofthe lateral wall (Figs. 1c, d). Compared to VNNE, theolfactory neuroepithelium is usually thicker (Figs. 1c, e)and has more numerous rows of receptor neuron nuclei(Fig. 1e). The thickness of the VNNE appears to be greater inmole-rats from P0 to P14 compared to those of olderanimals (Figs. 1c, d) with the exception of one adult male(Fig. 2a, non-breeding male). The greatest cross-sectionalarea of the VNNE is found in a 2-week mole-rat (Fig. 1d). Incontrast to increased dimensions of the nasal cavity, theVNO shows little change in cross-sectional area betweenage groups (Figs. 1a, b; 2a, b).

Both VNNE and olfactory neuroepithelium label positivelyfor beta tubulin (BT+) in mole-rats of all ages (Figs. 2a, b, c).Beta tubulin also delineates individual bipolar neurons in theVNNE (Figs. 2d, e). In adult mole-rats (Fig. 2d), the cell bodiesof bipolar neurons of the VNNE are generally larger than insubadults (Fig. 2e). In addition, bipolar neurons of the VNNEwere larger than those of the olfactory neuroepithelium ofadults (Figs. 2c, d).

2.2. Quantitative and statistical results

Length of the VNNE is significantly (p<0.05) greater in adults(mean: 1.68 mm±standard deviation: 0.08) compared tosubadults (0.97 mm±0.2). In contrast, VNNE thickness issignificantly (p<0.05) greater in subadults (0.066 mm±0.012)compared to adults (0.047 mm±0.009). Volume of the VNNE is

remarkably similar among ages (Fig. 3a), despite greatlyincreased dimensions of the nasal chambers between sub-adults and adults (compare Figs. 1a, b; 2a, b). The range ofVNNE volume among subadults (0.008 mm3 and 0.022 mm3) is

Table 1 – Comparative ontogenetic data on volume of the vomeronasal neuroepithelium (VNNE)

Species (common name) Perinatal volume (in mm3)/source Adult volume (in mm3)/source

Mus musculus (laboratory mouse) 0.023/this study 0.15 to 0.17/Wilson and Raisman, 1980Microtus ochrogaster (prairie vole) 0.023/this study 0.15/Maico et al., 2003Rattus norvegicus (laboratory rat) 0.08/Weiler et al., 1999 0.66 to 0.70/Weiler et al., 1999Oryctolagus cuniculus (laboratory rabbit) 0.142/this study 0.96/Negus, 1958Otolemur garnetti (greater bushbaby) * 0.073/Smith et al., 2005 0.36/Smith et al., 2005

* Data presented for male bushbabies only.

81B R A I N R E S E A R C H 1 1 3 2 ( 2 0 0 7 ) 7 8 – 8 3

entirely within the range for adults (0.006 mm3 and0.027 mm3). The means VNNE volumes were 0.012 mm3

(±0.005 mm3) for subadults and 0.015 mm3 (±0.007 mm3) foradults. A t-test reveals no significant (p>0.05) differencesbetween subadult and adult VNNE volume.

No age or sex-related trends are discernable, althoughsample sizes are small for this purpose. The largest VNNEvolume (1-year-old male) is only 20% larger than that of thelargest subadult (a 2-week-oldmale). Breeding adults establishthe lower limit for males (0.006 mm3). The adult breedingfemale has the largest VNNE (0.015 mm3), but the variationamong females is small (Fig. 3a). VNNE nuclear density is19,607±2968 nuclei per mm2 in subadults and 15,040±1315 nuclei per mm2 in adults.

In perinatal Mus and Mictrotus, VNNE volume on theright side is 0.023 mm3 in both cases. In the perinatal Or-yctolagus, right VNNE volume is 0.142 mm3. A comparison ofthese data to published measurements in adults is shownin Table 1.

3. Discussion

In the most commonly studied rodents (e.g., mice, rats, voles),experimental removal of the VNO produces changes in somesexual behaviors, territorial marking, aggression, as well asthe release of some urinary substances that delay puberty(Halpern and Martínez-Marcos, 2003; Thompson et al., 2004;Wysocki and Lepri, 1991). There is likely a functional overlapwith the main olfactory pathway for some of these behaviors(Halpern and Martínez-Marcos, 2003). Nonetheless, therodents studied to date have the most well documentedVNO mediation of social interactions among all mammals.Correspondingly, the neuroepithelium of the VNNE is pre-cocious in early postnatal growth, acquiring most of its adultsize prior to sexual maturity in rats and mice (Fig. 3b; Weileret al., 1999; Wilson and Raisman, 1980).

Although most adult naked mole-rats do not becomesexually mature (Jarvis, 1981; Jarvis and Sherman, 2002), ourfindings show that animals from birth to weaning age fallwithin the VNNE volume range for breeding and non-breedingadults alike, indicating slight or no volumetric growth. Adultmole-rats do surpass subadults by about 75% in rostrocaudallength of the VNNE. Yet, this only reflects shape change: as theVNO of Heterocephalus elongates postnatally, it decreases inapical–basal thickness. Further work must establish whetherneuronal numbers differ, but the similar density of nuclei inthe VNNE between ages suggests little change in cell numbers.Indeed, our estimates indicate a decrease in nuclear density

between subadult and adult sample, which is consistent withdata on other mammals (Smith et al., 2005; Weiler et al., 1999).The similar nuclear density and VNNE volume suggest thatthe number of neurons may be relatively static after birth. Afuture investigation of cell dynamics in the early postnatalVNNE of Heterocephalus might reveal significant differencescompared to other rodents.

Weiler et al. (1999) relate VNNE volume, in part, tovariation in body size (but see discussion in Maico et al.,2003; Smith et al., 2005). In this light, it may not seemsurprising that in adult Rattus norvegicus (up to 600 g) VNNEvolume is larger by 45-fold than that of adult Heterocephalus(30–70 g) (Fig. 3c). Yet in the meadow vole (Microtuspennsylvanicus), body mass is similar to that of Heterocepha-lus, but VNNE volume is larger by 11-fold (Table 1; Maico etal., 2003). In laboratory mice (Mus musculus), body mass issmaller than in Heterocephalus, but adult VNNE volume is 9.5-fold greater. Thus, VNNE volume of adult Heterocephalusappears proportionally small compared to other rodentsstudied to date (Table 1; Fig. 3c). Since Heterocephalus is along-lived rodent species and most individuals are reproduc-tively suppressed (Jarvis, 1981; Jarvis and Sherman, 2002), ourobservations deserve further scrutiny with a broader agerange and larger samples of breeding pairs.

An ontogenetic comparison of VNNE volume amongmammals suggests that the proportionally small VNOs ofHeterocephalus may be explained by a deficiency in VNNEgrowth (Table 1). The completely overlapping volumetricranges between age groups in Heterocephalus are unprece-dented in rodents, which typically exhibit pronouncedvolumetric increase of the VNNE from birth to sexualmaturity. In Rattus, a nearly 8-fold increase occurs betweenP1 and P66 (Table 1; Fig. 3b; Weiler et al., 1999). No otherrodents have been studied from P0 to adult ages. However,VNNE volumes obtained from perinatal M. musculus, Microtusochrogaster (prairie vole) and Oryctolagus. cuniculus (laboratoryrabbit) indicate a similar picture if compared to previouslypublished data from adults. Adult VNNE volumes publishedon these species are 5.5-fold to 6-fold larger than the VNNEvolumes obtained from the perinatal specimens studiedherein (Table 1). No other fossorial rodents have beenstudied ontogenetically in this regard, but in the blindmole-rat (Spalax ehrenbergi) VNOs are described as welldeveloped with adults exceeding infants in VNO length bymore than 7.5-fold (Zuri et al., 1998). Ontogenetic VNNEvolume data from other orders of mammals are rarer. Smithet al. (2005) showed that mean adult VNNE volume in malegreater bushbabies was 4-fold larger than a neonatalbushbaby (Table 1). The lack of clear age differences of

82 B R A I N R E S E A R C H 1 1 3 2 ( 2 0 0 7 ) 7 8 – 8 3

VNNE volume in Heterocephalus is also unusual compared toa report on the vestigial human VNO of adults, in whichadult VNO volume is 10-fold larger than that of the largestfetal VNO (Smith et al., 1998).

The uniquely small and apparently growth-deficient VNOof Heterocephalus is intriguing in light of this species' dis-tinctive social and ecological characteristics, even amongother fossorial mammals. The most notable traits in thisregard are the naked mole-rat's eusocial organization and thewidespread reproductive suppression that holds nearly alladults in a colony in a sexually immature state (Jarvis, 1981).Unlike other rodents, urinary signals do not mediate thissuppression (Faulkes and Abbott, 1993; Smith et al., 1997).Thus, the small VNNE in adult Heterocephalus may correlatewith a diminished role in sociosexual behaviors.

Further study is warranted to elucidate VNO function inHeterocephalus, including any differences with other rodents.Rodents are themost utilizedmodel of VNO function as nearlyall species have well-developed VNOs. Given the uniquecharacteristics of the VNO in Heterocephalus, comparisons toother rodents may augment an understanding of variations inVNNE ontogeny and physiology. Furthermore, a comparativerodent model may help to explain the significance of theVNNE variation that typifies some other mammals, such asprimates.

4. Experimental procedures

4.1. Sample

All procedures were approved by the IACUC at University ofIllinois at Chicago. The naked mole-rats were part of abreeding colony; all but one animal were born in captivity.The sample was composed of a range of postnatal stages thatincluded birth to weaning (P0, P7, P14 (n=3), P28 (weaning) andadults that were breeding or non-breeding). Adult ages rangedfrom 1 year to 4+ years (estimated age of one wild caughtanimal). Six subadult and six adult mole-rats (three males;three females), including one breeding pair, were deeplyanesthetized and sacrificed by exsanguination or decapita-tion. Adults were perfused through the heart with 4%paraformaldehyde prior to sacrifice. Heads of all animalswere subsequently stored in 4% paraformaldehyde, decalci-fied using a sodium citrate–formic acid solution, transferred to10% buffered formalin for 12–24 h, and processed for paraffinembedding. Blockswere serially sectioned at 10 μm. Every fifthsection was stained alternately with hematoxylin–eosin orGomori trichrome; intervening sections were saved forimmunohistochemical procedures. In order to unequivocallydemonstrate the shape and size of bipolar neurons fordescriptive purposes, randomly selected specimens wereprepared for immunohistochemical study using antibodiesto neuron-specific beta tubulin. Unstained sections weremounted and processed for immunoreactivity as describedin Smith et al. (2005).

Volumetric data on the VNNE of Heterocephalus werecompared to published data on other mammals includingother rodents, a lagomorph and a primate (Maico et al., 2003;Negus, 1958; Smith et al., 2005; Weiler et al., 1999; Wilson and

Raisman, 1980). Since few ontogenetic studies of VNNEvolume exist, the present study also included new observa-tions on perinatal VNNE volume in the laboratory mouse(M. musculus), prairie vole (M. ochrogaster) and laboratoryrabbit (O. cuniculus). The specimens were previously sectionedat 10 to 20 μm and stained as described above for Heteroce-phalus. The Mus specimen was at 19 days gestation and wassectioned in a previous study (Smith, 1995). The Microtusspecimen was a 25.3 mm crown rump length (CRL) fetus froma sample of wild caught voles (Maico et al., 2003). Althoughthe specimen is of unknown age, this CRL is the same as thatof neonatal M. pennsylvanicus, a species with the same lengthof gestation and similar birth weight (see Nadeau, 1985). TheOryctolagus was P2 and was acquired from an archival sampleof animals that died perinatally in a captive colony at theUniversity of Pittsburgh (courtesy of MP Mooney). The animalwas a normal rabbit in a colony that exhibits nonsyndromiccoronal suture craniosynostosis (Mooney et al., 1994). Thenormal rabbits in this colony exhibit no significant differ-ences in craniofacial form compared to wild type rabbits(Mooney et al., 1994).

4.2. Data collection and analysis

In all Heterocephalus specimens, each section containing theVNO was digitally photographed. The rostrocaudal distancebetween the first and last sections containing VNNE wasmeasured. Scion Image (NIH) was used to measure VNNEthickness and volume based on bitmap files corresponding toeach section, scaled with a digital micrograph of a stagemicrometer at the same magnification. Using digital micro-graphs from approximately the 25th, 50th and 75th percentilesof VNNE length, the thickness of the VNNE from the apicalborder to the interface with the basement membrane wasmeasured and averaged for a VNNE thickness. The perimeterof the VNNE was traced using Scion Image software, and thetotal volume of the right VNNE was calculated for eachspecimen; the left side was used in one case where the rightVNNE was damaged. An independent sample t-test was usedto compare length, thickness and volumes of the VNNE insubadult sample to the adult sample (significance set atp<0.05). The entire age range of subadults was combinedbecause the sample had a similar range in VNNE volume andsimilar variation to the adult sample. To determine if celldensity differed between age groups, the density of nuclei wasestimated at mid-length of the VNNE as follows. A 25×50 μmrectangle was superimposed over amicrograph of the VNNE ineach specimen using Adobe Photoshop 8.0 software. Therectangle was placed within the center of nuclear rows ofVNNE in order to exclude basal and supporting cell nuclei.Subsequently, each nucleus was marked using the paintbrushtool and counted. The number of nuclei per 25×50 μm wasthen converted to nuclei per mm2 and averaged for each agegroup.

VNNE volume was measured in the perinatal Mus, Microtusand Oryctolagus specimens as described above. To account fora greater sectional thickness (20 μm), intervening sectionswere mounted and stained from the 2-day-old Oryctolagus inorder to maintain similar intervals of area measurementsamong specimens.

83B R A I N R E S E A R C H 1 1 3 2 ( 2 0 0 7 ) 7 8 – 8 3

Acknowledgments

The authors thank J Cass for sectioning some of the speci-mens. MP Mooney generously provided the 2-day-old rabbitcadaver. This study was funded in part by grants to TJ Park(NIH # R29DC02850) and EE Morrison (FAA # 01-G-022).

R E F E R E N C E S

Catania, K.C., 2005. Evolution of sensory specializations ininsectivores. Anat. Rec. 287A, 1038–1050.

Crish, S.D., Rice, F.L., Park, T.J., Comer, C.M., 2003. Somatosensoryorganization and behavior in naked mole-rats. I: Vibrissa-likebody hairs comprise a sensory array that mediates orientationto tactile stimuli. Brain Behav. Evol. 62, 141–151.

Halpern, M., Martínez-Marcos, A., 2003. Structure and function ofthe vomeronasal system: an update. Progr. Neurobiol. 70,245–318.

Faulkes, C.G., Abbott, D.H., 1993. Evidence that primerpheromones do not cause social suppression of reproductionin male and female naked mole-rats (Heterocephalus glaber).J. Reprod. Fertil. 99, 225–230.

Jarvis, J.U.M., 1981. Eusociality in a mammal: cooperative breedingin naked mole-rat colonies. Science 212, 571–573.

Jarvis, J.U.M., Sherman, P.H., 2002. Heterocephalus glaber. Mamm.Species 706, 1–9.

Maico, L.M., Burrows, A.M., Mooney, M.P., Siegel, M.I., Bhatnagar,K.P., Smith, T.D., 2003. Size of the vomeronasal organ in wildMicrotus with different mating strategies. Acta Biol. Hung. 54,263–273.

Mooney, M.P., Losken, H.W., Siegel, M.I., Lalikos, J.F., Losken, A.,Burrows, A.M., Smith, T.D., 1994. Development of a strain ofrabbits with congenital simple nonsyndromic coronal suturesynostosis. Part II: Somatic and craniofacial growth patterns.Cleft Palate-Craniofac. J. 31, 8–16.

Nadeau, J.H., 1985. Ontogeny. In: Tamarin, R.H. (Ed.), Biology ofNew World Microtus. The American Society of Mammalogists,pp. 254–285.

Negus, V., 1958. The Comparative Anatomy and Physiology of theNose and Paranasal Sinuses. Livingston, Edinburgh.

Smith, T.D., 1995. Development of the human vomeronasal organin normal and cleft lip and palate human fetuses. Ph.D.dissertation, University of Pittsburgh.

Smith, T.E., Faulkes, C.G., Abbott, D.H., 1997. Combined olfactorycontact with the parent colony and direct contact withnonbreeding animals does not maintain suppression ofovulation in female naked mole-rats (Heterocephalus glaber).Horm. Behav. 31, 277–288.

Smith, T.D., Siegel, M.I., Burrows, A.M., Mooney, M.P., Burdi, A.R.,Fabrizio, P.A., Clemente, F.R., 1998. Searching for thevomeronasal organ of adult humans: preliminary findings onlocation, structure, and size. Microsc. Res. Tech. 41, 483–491.

Smith, T.D., Bhatnagar, K.P., Burrows, A.M., Shimp, K.L., Dennis,J.C., Smith, M.A., Maico-Tan, L., Morrison, E.E., 2005. Thevomeronasal organ of greater bushbabies (Otolemur spp.):species, sex, and age differences. J. Neurocytol. 34, 135–147.

Thompson, R.N., Robertson, B.K., Napier, A., Wekesa, K.S., 2004.Sex-specific responses to urinary chemicals by the mousevomeronasal organ. Chem. Senses 29, 749–754.

Weiler, E., McCulloch, M.A., Farbman, A.I., 1999. Proliferation in thevomeronasal organ of the rat during postnatal development.Eur. J. Neurosci. 11, 700–711.

Wilson, K.C., Raisman, G., 1980. Age-related changes in theneurosensory epithelium of the mouse vomeronasal organ:extended period of postnatal growth in size and evidence forrapid cell turnover in the adult. Brain Res. 185, 103–113.

Wysocki, C.J., 1979. Neurobehavioral evidence for the involvementof the vomeronasal system in mammalian reproduction.Neurosci. Biobehav. Rev. 3, 301–341.

Wysocki, C.J., Lepri, J.J., 1991. Consequences of removing thevomeronasal organ. J. Steroid Biochem. Mol. Biol. 39, 661–669.

Zuri, I., Fishelson, L., Terkel, J., 1998. The morphology and cytologyof the nasal cavity and vomeronasal organ of the blindmole ratSpalax ehrenbergi. Anat. Rec. 251, 460–471.