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
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 importance
in 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 and
explain 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
The flat embryo begins to fold downwards at the sides and at the
front and back to enclose a primitive gut
Ectoderm
Neural tube
Mesoderm
Endoderm
Ectoderm
Neural tube
Hindgut
Mesoderm
Endoderm
Cloacal plate
Oral plate
FUSION POINTS
OF ECTODERM
AND ENDODERM
Regions of brain
FUSION POINTS
WITH EXTRAEMBRYONIC
MEMBRANES
Longitudinal view
The folding process not only creates the body form but also the extraembryonic structures of the yolk sac, allantois, amnion and chorion
Embryo proper
Primitive gut
Presumptive amnion
Allantois
Presumptive chorion
Transient chorio-vitelline placenta
Somatopleure
Yolk sac
Splanchnopleure
Longitudinal view
The chorio-vitelline placenta is temporary (or absent)
and is replaced by the chorio-allantoic placenta
Chorio-allantoic placenta
Amnion
Chorion
Allantois
Yolk sac
Longitudinal view
Folds also undercut the sides of the body
Somatopleure
Neural tube
Mesoderm
Gut tube
Ectoderm
Endoderm
Splanchnopleure
Yolk sac
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)
EPITHELIOCHORIAL
Maternal endometrial epithelium intact
(horse,pig)
SYNEPITHELIOCHORIAL
Syncytium of maternal epithelium and
Chorion
(ruminants)
ENDOTHELIOCHORIAL
Removal of endometrial epithelium
And connective tissue
(dogs, cats)
HAEMOCHORIAL
Removal of maternal endothelium
(human, some rodents)
Gross anatomical classification of placentas is based on the pattern
of contact between chorion and endometrium
DIFFUSE
Uniform distribution of chorionic villi
over contact surface (horse, pigs)
COTYLEDONARY
Villi restricted to defined area
(cotyledons) (ruminants)
ZONARY
Girdle of chorionic villi around middle
of chorionic sac (dogs,cats)
DISCOIDAL
Disc-shaped area on chorionic sac
(humans, rodents)
The haemochorial placenta shows the intimate juxtaposition
of foetal and maternal blood allowing efficient exchange
A
Foetal capillaries
The haemochorial placenta
B
Umbilical vein
Umbilical arteries
Chorionic
villi
Maternal
blood pool
Maternal
venule
Maternal
arteriole
Notice the expansions
at the’turnaround’ to
allow slower blood flow
and better equilibration
with maternal blood
A http://instruct1.cit.cornell.edu/courses/biog105/pages/demos/105/unit8/ovaryplacenta.html
B from Johnson, Essential Reproduction
Glucose is the dominant energy yielding substrate
for the foetus with little use of fatty acids
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)
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
CSM
GLUCOSE
GLUCOSE
Maternal liver
MOTHER
FOETUS
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
Although fatty acids are little used by the foetus for energy they are
essential for growth and also for laying down fat reserves
LIPID METABOLISM IN THE FOETUS
TAG
3
SYNTHESIS
Maternal adipose
FATTY
ACIDS
1
2
OXIDATION
FATTY
ACIDS
LIPOPROTEINS
LPL
Maternal liver
MOTHER
4
CELL
MEMBRANES
FOETUS
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 acids
18:3 (D9,12,15)
18:2 (D9,12)
20:4 (D5,8,11,14)
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
Maternal liver
UREA
1
OXIDATION
GROWTH
PROGESTERONE
AMINO
ACIDS
AMINO
ACIDS
MOTHER
FOETUS
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
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
The haemogobin of foetal red blood cells has a higher
affinity for oxygen than that in maternal blood
OXYGEN SUPPLY - FOETAL ADAPTATIONS 3
100%
FOETAL
MATERNAL
75% saturation
at 30 mm
Hb as
HbO 2
50% saturation
at 30 mm
50%
30
pO 2 (mm Hg)
P50 maternal
P50 foetal
REFERENCES
Cunningham JGC (2002) Textbook of Veterinary Physiology (Saunders)
Guyton and Hall (2005) Textbook of Medical Physiology (Elsevier)
Johnson MH (2007) Essential Reproduction (Blackwells)