From the University of Dodoma

1. THE PLACENTA
Prof. Dr. David Ngassapa
Department of Anatomy

2. THE PLACENTA
Shape and Size
• The placenta at term, is flat and circular or
oval in shape with an average volume of
500ml; average weight of 500gm. The
diameter is 250mm while the average
thickness is about 25mm.
• The placenta is thickest in the middle and
thin at the periphery.

3. THE PLACENTA
Appearance and surface
• The placenta has a smooth shiny fetal
surface and the rough and granular
maternal surface
i) Fetal surface
• The fetal surface is covered by amnion.
Beneath the amnion is the chorion.

4. THE PLACENTA
• The umbilical cord attaches near the
centre (i.e. it is a bit eccentric) of the fetal
surface and branches of the umbilical
vessels spread out under the amnion from
this area.

5. THE PLACENTA

6. THE PLACENTA
ii) Maternal surface
• The roughness of the maternal surface is
due to the presence of 10 to 15 lobes
separated by a series of fissures or
grooves.
• The lobes are termed cotyledons while
the fissures or grooves correspond to the
bases of incomplete placental septae.

7. THE PLACENTA

8. THE PLACENTA
Structure
• A full thickness section of the placenta
shows that the placenta consists of a
chorionic plate (fetal component of the
placenta) and a basal plate (maternal
component of the placenta). The basal
plate has no villous stems and their
branches.

9. THE PLACENTA
• The intervillous space which is filled with
maternal blood intervenes between the
two plates .

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11. THE PLACENTA
Chorionic plate
• The chorionic plate (also known as the
fetal plate) consists, from the amnion-
covered fetal surface outward, of (i)
extraembryonic mesoderm (ii)
cytotrophoblast and (iii)
syncytiotrophoblast.

12. THE PLACENTA
• From the chorionic plate arise villous
stems which give rise to side branches
and terminal villi. Each villous stem and
its branches form a villus tree. The
chorionic plate gives attachment to about
800 – 1000 such villous trees in early
pregnancy.

13. THE PLACENTA
• Anchoring villi connect the chorionic plate
to the deciduas basalis, in this way is
formed the feto-maternal junction.
• The branch villi or terminal villi of the stem
villi and anchoring villi are the functional
units of the placenta.

14. THE PLACENTA
• Each has a central core of extraembryonic
mesoderm covered by the two epithelial
layers of the trophoblast – cytotrophoblast
and syncytiotrophoblast
• Within the central core lie one to six
capillaries which form a paravascular
capillary plexus.

15. THE PLACENTA
• This plexus of capillaries increases the
vascular bed of the branch villi; it also
slows the flow of blood so that circulation
time through the villus is increased
ensuring more complete exchange with
maternal blood.

17. THE PLACENTA
• The capillaries drain into adjacent venules
which lead to veins. Frequent direct
communications between paravascular
capillaries and veins occur. These
arteriovenous shunts provide a by-pass
protecting the terminal capillary loops from
overload.

18. THE PLACENTA
Placental membrane or barrier
• It consists of tissues which separate fetal
blood in the capillaries (paravascular
capillary plexus) from maternal blood
occupying the intervillous space.
• It ensures that the two systems of blood
never mix. Up to the third month of
gestation the membrane consists four
layers.

19. THE PLACENTA
• These layers, from the fetus to mother are:
i) Fetal capillary endothelium
ii) Primary (extraembryonic) mesoderm
iii) Cytotrophoblast
iv) Syncytiotrophoblast

20. THE PLACENTA
• As pregnancy advances (fourth month
onwards) the barrier becomes gradually
reduced so that the fetal and maternal
circulations become more intimately
related.
• Ultimately it is formed of capillary
endothelium and syncytiotrophoblast
with a thin layer of delicate mesodermal
connective tissue between them.

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22. THE PLACENTA
Basal plate
• The basal plate consists from outside
inward of:
i) stratum spongiosum of deciduas basalis
ii) outer layer of syncytiotrophoblast
iii) cytotrophoblastic shell or its remains
iv) inner layer of syncytiotrophoblast which
undergoes fibrinoid degeneration in its
outer zone

23. THE PLACENTA

24. THE PLACENTA
Intervillous space
• About 500 ml of maternal blood passes
through the intervillous space each
minute. The blood spurts into the
intervillous space reaching chorionic plate
before spreading into the rest of space.

25. THE PLACENTA
• It then slowly percolates through the
sponge-like meshwork of villi to the
venous exits where it is drawn away by
tributaries of the uterine vein.
• Mild myometrial contractions (Braxton –
Hicks contractions) are thought to aid in
emptying the intervillous blood into the
draining veins.

26. THE PLACENTA
Functions of the placenta
1. It allows respiratory gases to reach and
leave the embryo. The uptake of oxygen
from the intervillous space and the
release of carbon dioxide into the
intervillous space is by simple diffusion
along a gaseous tension gradient across
the placental barrier.

27. THE PLACENTA
2. It provides a route for excretion of the end
products of fetal metabolism. These
include urea, uric and creatinine and
they diffuse into the intervillous space.
3. It provides protection in two ways:
a) The placenta barrier prevents the passage of
most pathogenic organisms and toxins.
This barrier is breached by viruses e.g.
rubella virus, HIV/Aids virus. measles

28. THE PLACENTA
b) It allows the passage of maternal
antibodies into the fetus. Gamma
globulins (especially 1gG) in maternal
blood are taken up by the
syncytiotrophoblast by the process of
pinocytosis. In the fetus, gamma
globulins confer passive immunity to viral
infections e.g. measles, whooping cough.

29. THE PLACENTA
4. It acts as an immunologic barrier. In
pregnancy the genetically dissimilar feto-
placental unit develops in intimate
association with maternal tissues without
eliciting any apparent allograft rejection
reactions.
The trophoblast acts as on immunologic
barrier between the fetus and the mother

30. THE PLACENTA
5. The chorionic plate of the placenta is an
endocrine organ responsible for the
production of the following hormones:
a) Oestrogens – Three different oestrogens
are produced by the placenta. These are
esteron (E1), Estradiol (E2) and Estriol
(E3). Estriol accounts for 95% of the
placental oestrogens.

31. THE PLACENTA
Oestrogens are synthesized by the feto-
placental unit – a term indicating that
both the fetus and placenta are essential
for the production of oestrogens during
pregnancy.

32. THE PLACENTA
c) Human chorionic gonadotropin (HCG)
• This hormone is detectable in maternal urine 1
day after implantation of the fertilized ovum i.e.
8 days after ovulation.
• It production increases rapidly in the early part
of pregnancy reaching a peak about the tenth
week. There after hormone production
gradually declines. HCG is synthesized by the
syncytiotrophoblast