membra no genesis

7/28/2019 Membra No Genesis

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LEARNING OUTCOMES

1. describe the formation of the tubular embryo by creation of body folds

2. note the juxtaposition of ectoderm and endoderm at the oral plate and cloacal membrane

3. describe the formation and fusion of the amnion to create a protective bubble around the embryo

4. be aware of the vestigial nature of the yolk sac in mammals with regard to nutrition but its importancein terms of haematopoiesis and its transient significance in contributing to the choriovitelline placenta

5. show the development of the allantois as a bud of the gut tube and its importance in thechorio-allantoic placenta

6. understand the different histological forms that the foetal/maternal placenta interface can take

7. understand the different anatomical forms that the foetal/maternal placenta interface can take

8. Note the emphasis on glucose and amino acids as energy and growth resources in the foetus andexplain how the placenta has an endocrine function in ensuring these resources are directed to the foetus

9. Explain the special foetal adaptations that ensure adequate perfusion of foetal tissues with oxygen

MEMBRANOGENESIS AND PLACENTAL FUNCTION


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EctodermNeural tube

MesodermEndoderm

The f lat embryo begins to fo ld downwards at the sides and at the

front and back to enclose a pr imi t ive gut

FUSION POINTSOF ECTODERMAND ENDODERM

FUSION POINTSWITH EXTRA-

EMBRYONICMEMBRANES

EctodermNeural tube

MesodermEndodermOral plate

Regions of brain

Hindgut

Cloacal plate

Longitudinal view


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The fo ld ing pro cess not o nly creates the body form but also the extra-

embryon ic structu res of the yolk sac, allanto is, amnion and chor ion

Primitive gut

Embryo proper

Somatopleure

Presumptive chorion

Presumptive amnion

Yolk sacSplanchnopleure

Allantois

Transient chorio-vitelline placenta

Longitudinal view


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Chorio-allantoic placenta

Yolk sac

Allantois

Amnion

Chorion

Longitudinal view

The cho r io-vi te l line placenta is temporary (or absent)

and is replaced by the cho r io-al lantoic p lacenta


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Folds also undercut the sides of the body

Mesoderm Neural tubeEctoderm

Endoderm

Gut tube

Yolk sac

Somatopleure

Splanchnopleure

Transverse view


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Video of chick embryo (50h)

showing body folds and amnionIn the chick , the formation o f the head fold

precedes that of the tai l and the form ation

of the body sides progresses caudally

https://www.eevec.vet.ed.ac.uk/vc/node.asp?ID=vcembr02
https://www.eevec.vet.ed.ac.uk/vc/node.asp?ID=vcembr02https://www.eevec.vet.ed.ac.uk/vc/node.asp?ID=vcembr02


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Histolog ical classi f icat ion o f placentas is based on the degree

of removal of the maternal layers

KEY CHARACTERISTICS OF MAMMALIAN PLACENTA

Foetal capill ary(from umbilical artery)

Endothelial layer

Connective tissue layer(may be minimal)

Cellular layer(may be trophoectoderm + maternalepithelium or a syncytium of the two,or solely trophoectoderm)

Connective tissue layer

(may be minimal)

Maternal capillary(in haemochorial placenta of primatesthe endothelium is degraded)

EPITHELIOCHORIAL

Maternal endometrial epithelium intact(horse,pig)

SYNEPITHELIOCHORIALSyncytium of maternal epithelium andChorion(ruminants)

ENDOTHELIOCHORIALRemoval of endometrial epitheliumAnd connective tissue(dogs, cats)

HAEMOCHORIALRemoval of maternal endothelium

(human, some rodents)


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Gross anatomical classi f icat ion of p lacentas is based on the pattern

of co ntact between chor ion and endometr ium

DIFFUSEUniform distribution of chorionic villiover contact surface (horse, pigs)

COTYLEDONARYVilli restricted to defined area(cotyledons) (ruminants)


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ZONARYGirdle of chorionic villi around middle

of chorionic sac (dogs,cats)

DISCOIDALDisc-shaped area on chorionic sac

(humans, rodents)


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The haemochorial placenta

Umbilical vein

Umbilical arteries

Maternal

blood pool

MaternalvenuleMaternal

arterioleChorionic

villi

A

A http://instruct1.cit.cornell.edu/courses/biog105/pages/demos/105/unit8/ovaryplacenta.html

B from Johnson, Essential Reproduction

Foetal capillaries

B

Notice the expansions

at theturnaround to

allow slower blood flowand better equilibration

with maternal blood

The haemochor ial placenta show s the int imate juxtapos i t ion

of foetal and m aternal blood allow ing eff ic ient exchange
http://instruct1.cit.cornell.edu/courses/biog105/pages/demos/105/unit8/ovaryplacenta.htmlhttp://instruct1.cit.cornell.edu/courses/biog105/pages/demos/105/unit8/ovaryplacenta.html


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ENERGY SUBSTRATES

Glucose oxidation accounts from 50% oxygen use

Most of the rest is due to amino acid and lactate oxidation

Rather little is from fatty acid oxidation

(Notice that maternal energy metabolism is almost the mirror of thiswith a shift to fatty acid oxidation and a shift away from amino acidsand glucose)

Glucose is the dom inant energy y ie ld ing sub strate

for the foetus w ith l i t t le use of fat ty acids


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CSM = CHORIONIC SOMATOMAMMOTROPHIN (also know as placental lactogen)

CSM secreted in increasing amounts during gestation

CSM suppresses insulin action

Therefore depresses glucose use by the mother

'Directs' glucose to the foetus

Maternal insulin resistance can precipitate maternal type 2 Diabetes mellitus

To some extent the foetal-placental unit programm es maternal

metabo l ism to ensure that i t meets the needs o f the foetus

FOETAL INFLUENCES ON MATERNAL METABOLISM 1

Maternal tissues

Maternal liver

GLUCOSE

CSM

MO TH E R FOE TUS

GLUCOSE


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LIPID METABOLISM IN THE FOETUS

Maternal adipose

Maternal liver

LIPOPROTEINS FATTYACIDS

MO TH E R F OE T US

CELL

MEMBRANES

TAG

SYNTHESIS

OXIDATION

LPL

FATTYACIDS

4

2

3

1

Altho ugh fatty acids are l i t t le used by the foetus for energy th ey are

essent ial for grow th and also for laying down fat reserves

1. Fatty acids transported via maternal (or foetal) serum albumin

2. (a) Triacylglcyerols contain mostly palmitate(b) Palmitate will also be formed from excess glucose

(c) Epitheliochorial placentas have poor rates of diffusion of fatty acids and neonates (eg calf and piglet)have little body fat compared to the haemochorial model (human)

3. TAG deposits in both white and brown adipose tissue. Brown fat essential for thermogenesis in neonate

4. Crucial here are the essential fatty acids18:3 (D9,12,15)

18:2 (D

9,12)20:4 (D5,8,11,14)


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Several foetal adaptat ions con tr ibute to the abi l i ty of

the foetus to del iver suff ic ient oxy gen to its t issues

OXYGEN SUPPLY - FOETAL ADAPTATIONS 1

Cardiac anatomy limits intermixing of oxygenated bloodand venous return from the head

Foetal haemoglobin has a high affinity for oxygen

There is a double Bohr effect acting on the placentaltransfer of oxygen

Cardiac output is high

Haemoglobin concentration is 50% higher than maternal


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The foetal cardiovascu lar system is adapted to providing wel l-oxyg enated b lood to th e

brain in spi te of intermix ing of venous return and an incom pletely div ided heart


1. Numbers are partial pressures of oxygen inmm Hg

2. Low vascular resistance in placenta takes45% of cardiac output

3. Blood returning to right atrium is a mixture

of oxygenated umbilical blood andvenous return from trunk and limbs

4. Crista dividens directs this betteroxygenated blood through foramenovale for preferential delivery to brain vialeft ventricle

5. The poorly oxygenated blood from the brainis directed to the right ventricle and thenvia ductus arteriosis to the dorsal aorta


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50% saturationat 30 mm

75% saturationat 30 mm

P50 maternal

P50 foetal

Hb asHbO2

100%

50%

pO2 (mm Hg)30

F O E T A L

M A T E R N A L

The haemog ob in of foetal red blood cel ls has a hig her

aff in i ty for ox ygen than that in maternal blood


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REFERENCES

Cunningham JGC (2002) Textbook of Veterinary Physiology (Saunders)

Guyton and Hall (2005) Textbook of Medical Physiology (Elsevier)

Johnson MH (2007) Essential Reproduction (Blackwells)

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