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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=vcembr027/28/2019 Membra No Genesis
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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.html7/28/2019 Membra No Genesis
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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
OXYGEN SUPPLY - FOETAL ADAPTATIONS 2
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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OXYGEN SUPPLY - FOETAL ADAPTATIONS 3
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)