learning outcomes 1. describe the formation of the tubular embryo by creation of body folds
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MEMBRANOGENESIS AND PLACENTAL FUNCTION. 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 - PowerPoint PPT PresentationTRANSCRIPT
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 the chorio-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
EctodermNeural tube
Mesoderm
Endoderm
The flat embryo begins to fold downwards at the sides and at the front and back to enclose a primitive gut
FUSION POINTS OF ECTODERM AND ENDODERM
FUSION POINTS WITH EXTRA- EMBRYONIC MEMBRANES
Ectoderm
Neural tubeMesodermEndoderm
Oral plate
Regions of brain
Hindgut
Cloacal plate
Longitudinal view
The folding process not only creates the body form but also the extra-embryonic structures of the yolk sac, allantois, amnion and chorion
Primitive gutEmbryo proper
Somatopleure
Presumptive chorion
Presumptive amnion
Yolk sac Splanchnopleure
Allantois
Transient chorio-vitelline placenta
Longitudinal view
Chorio-allantoic placenta
Yolk sacAllantois
Amnion
Chorion
Longitudinal view
The chorio-vitelline placenta is temporary (or absent) and is replaced by the chorio-allantoic placenta
Folds also undercut the sides of the body
MesodermNeural tube
EctodermEndoderm
Gut tube
Yolk sac
Somatopleure
Splanchnopleure
Transverse view
Video of chick embryo (50h) showing body folds and amnion In the chick, the formation of the head fold
precedes that of the tail and the formation of the body sides progresses caudally
https://www.eevec.vet.ed.ac.uk/vc/node.asp?ID=vcembr02
Histological classification of placentas is based on the degree of removal of the maternal layers
KEY CHARACTERISTICS OF MAMMALIAN PLACENTA
Foetal capillary (from umbilical artery) Endothelial layer Connective tissue layer (may be minimal) Cellular layer (may be trophoectoderm + maternal epithelium or a syncytium of the two, or solely trophoectoderm) Connective tissue layer (may be minimal) Maternal capillary (in haemochorial placenta of primates the endothelium is degraded)
EPITHELIOCHORIALMaternal endometrial epithelium intact(horse,pig)
SYNEPITHELIOCHORIALSyncytium of maternal epithelium and Chorion(ruminants)
ENDOTHELIOCHORIALRemoval of endometrial epitheliumAnd connective tissue(dogs, cats)
HAEMOCHORIALRemoval of maternal endothelium(human, some rodents)
Gross anatomical classification of placentas is based on the pattern of contact between chorion and endometrium
DIFFUSEUniform distribution of chorionic villiover contact surface (horse, pigs)
COTYLEDONARYVilli restricted to defined area(cotyledons) (ruminants)
ZONARYGirdle of chorionic villi around middleof chorionic sac (dogs,cats)
DISCOIDALDisc-shaped area on chorionic sac(humans, rodents)
The haemochorial placenta
Umbilical vein
Umbilical arteries
Maternal blood pool
Maternal venule
Maternal arterioleChorionic
villi
A
A http://instruct1.cit.cornell.edu/courses/biog105/pages/demos/105/unit8/ovaryplacenta.htmlB from Johnson, Essential Reproduction
Foetal capillaries
B
Notice the expansions at the’turnaround’ to allow slower blood flow and better equilibration with maternal blood
The haemochorial placenta shows the intimate juxtaposition of foetal and maternal blood allowing efficient exchange
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 this with a shift to fatty acid oxidation and a shift away from amino acids and glucose)
Glucose is the dominant energy yielding substrate for the foetus with little use of fatty acids
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 programmes maternal metabolism to ensure that it meets the needs of the foetus
FOETAL INFLUENCES ON MATERNAL METABOLISM 1
Maternal tissues
Maternal liver
GLUCOSECSM
MOTHER FOETUS
GLUCOSE
LIPID METABOLISM IN THE FOETUS
Maternal adipose
Maternal liver
LIPOPROTEINS FATTY ACIDS
MOTHER FOETUS
CELL MEMBRANES
TAG SYNTHESIS
OXIDATION
LPL
FATTY ACIDS
4
2
3
1
Although fatty acids are little used by the foetus for energy they are essential for growth 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 (9,12,15)18:2 (9,12)20:4 (5,8,11,14)
As with glucose, the foetal-placental unit programmes mammalian metabolism to ensure that it meets the Nitrogen needs of the foetus
FOETAL INFLUENCES ON MATERNAL METABOLISM 2
Maternal tissues
AMINO ACIDS
MOTHER FOETUS
Maternal liver
PROGESTERONE
AMINO ACIDS
GROWTHOXIDATION
UREA1
Notes: 1. An added benefit of the redirection of amino acids from the maternal liver is that maternal urea production is low thus favouring urea return across the placenta
Several foetal adaptations contribute to the ability of the foetus to deliver sufficient oxygen to its tissues
OXYGEN SUPPLY - FOETAL ADAPTATIONS 1
Cardiac anatomy limits intermixing of oxygenated blood and venous return from the head Foetal haemoglobin has a high affinity for oxygen There is a double Bohr effect acting on the placental transfer of oxygen Cardiac output is high Haemoglobin concentration is 50% higher than maternal
DA
FO
Trunk
HindlimbPlacenta
Liver
From lungs
To lungs
Brachycephalic vessels14
30
14
2525
25
22
19
19
The foetal cardiovascular system is adapted to providing well-oxygenated blood to the brain in spite of intermixing of venous return and an incompletely divided heart
OXYGEN SUPPLY - FOETAL ADAPTATIONS 2
1. Numbers are partial pressures of oxygen in mm Hg
2. Low vascular resistance in placenta takes 45% of cardiac output
3. Blood returning to right atrium is a mixture of oxygenated umbilical blood and venous return from trunk and limbs
4. Crista dividens directs this better oxygenated blood through foramen ovale for preferential delivery to brain via left ventricle
5. The poorly oxygenated blood from the brain is directed to the right ventricle and then via ductus arteriosis to the dorsal aorta
OXYGEN SUPPLY - FOETAL ADAPTATIONS 3
50% saturation at 30 mm
75% saturation at 30 mm
P50 maternal
P50 foetal
Hb as HbO2
100%
50%
pO2 (mm Hg)30
FOETAL
MATERNAL
The haemogobin of foetal red blood cells has a higher affinity for oxygen than that in maternal blood
REFERENCES
Cunningham JGC (2002) Textbook of Veterinary Physiology (Saunders)
Guyton and Hall (2005) Textbook of Medical Physiology (Elsevier)
Johnson MH (2007) Essential Reproduction (Blackwells)