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 tube

Mesoderm

Endoderm

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 gut

Embryo proper

Somatopleure

Presumptive chorion

Presumptive amnion

Yolk sac Splanchnopleure

Allantois

Transient chorio-vitelline placenta

Longitudinal view

Chorio-allantoic placenta

Yolk sac

Allantois

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

GLUCOSE

CSM

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

GROWTH

OXIDATION

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 vessels

14

30

14

25

25

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)