o Name the structures that form the placenta
o Distinguish between the foetal and maternal contributions to the placenta
o List the main functions of the placenta
o Distinguish between identical and fraternal twins and how they arise
o Explain how identical twins can share a common amnion and chorion
When the blastocyst embeds itself in the maternal endometrium it is covered with chorionic villi derived from the syncitiotrophoblast and cytotrophoblast. The distribution and size of the villi are not uniform throughout the surface of the chorion.
The chorion frondosum consists of numerous villi over the embryonic pole. This will contribute to the formation of the placenta .
The chorion laeve contains very sparse villi over the abembryonic pole. The villi will eventually disappear, and here the chorionic membranes are formed.
• The foetal chorion
• The maternal deciduas
At 9 -16 weeks they are termed mesenchymal villi
At 16 -25 weeks they are immature intermediate villi. At this age the cytotrophoblast persist only in patches.
At 25-32 weeks they form mature intermediate villi.
After 32 weeks they sprout out terminal villi (consisting of a thin syncitiotrophoblast and foetal capillaries with minimal intervening mesoderm)
The Placenta consists of branched chorionic villi bathed in lacunae of maternal blood.
On the foetal surface of the placenta the chorion forms a continuous surface from which the villi arise. The cytotrophoblast proliferates from the tips of the villi and forms a cytotrophoblastic shell on the maternal surface of the placenta. Elsewhere the villi are lined by syncitiotrophoblast, while the cytotrophoblast becomes restricted to small patches.
The decidua is derived from the secretory endometrium, which continues to proliferate and secrete under the influence of persistent high levels of progesterone, which in turn is stimulated by increasing levels of HCG, secreted by the growing syncitiotrophoblast. As the decidual cells continue to proliferate, they accumulate lipids and glycogen, and, the whole decidua becomes more vascular.
Septa grow from the decidua and project into the intervillous spaces dividing the placenta into 15 to 20 cotyledons.
At 4 weeks (2nd month) the amniotic cavity grows and obliterates the chorionic cavity (extra-embryonic coelom).
The amnion fuses with the chorion to form the chorio-amniotic plate.
Functions of the placenta
1. Exchange of gases mainly by diffusion
2. Exchange of metabolites:
b. Amino acids
c. Fatty acids
Waste products: urea, creatinine, bilirubin etc
3. Transport of maternal substances:
a. Maternal antibodies IgG conveys passive immunity to the new-born infant
b. Transferrin - for iron transport
c. Maternal hormones - may affect foetus
d. Chemical, drugs, viruses - all potentially teratogenic
4. Secretion of hormones - into maternal circulation:
a. Human chorionic gonadotrophin
b. Progesterone and oestrogen
Passage of cells into the maternal circulation. This is acquiring increasing importance because new methods are being developed to isolate the foetal cells from the maternal blood, and use them for prenatal diagnosis instead of the invasive methods of amniocentesis and chorionic villus biopsy.
The umbilical cord consists of:
o Two umbilical arteries
o One umbilical vein
o Wharton’s jelly
o And, only its most proximal part,
o The yolk sac and vitello-intestinal duct
o The allantoic diverticulum
o A canal connecting the extra-and intra-embryonic coeloms, which is subsequently obliterated.
The amniotic fluid
o 800 – 1000 ml
o functions as a “shock absorber” for the foetus
o Allows foetal movements
o Is replaced every 3 hours
o Circulates continuosly:
o It is ingested through mouth
o Excreted as urine
Abnormalities of amniotic fluid
There are two types of twins:
a. Fraternal twins are the result of fertilization of two oocytes. The two zygotes develop and implant themselves separately. They have separate placentae and amniotic cavities. Like all sibs born to the same couple, they have half their genes in common. They may be of different sex or of the same sex.
b. Identical twins are derived from a single zygote, which during early development divides into two groups of cells that continue to develop independently. Identical twins are always of the same sex, and have identical genes. Identical twins are in fact clones as the two individuals are derived from the same cell.
Identical twins may share the same placenta or amniotic cavity, or they may have different placentae and amniotic cavities, depending on the developmental stage at which the separation of the conceptus into two twins occurs:
1. Separation of the blastomeres
into two groups which develop and implant separately. They result in
two separate placentae and gestational sacs (dichorionic, diamniotic)
2. Separation of the inner cell mass or embryoblast into two groups forms two amniotic cavities but one chorion and placenta (form monochorionic, diamniotic twins)
3. Separation of the bilaminar disc into two groups of pluripotent cells forms two embryos sharing a single amniotic cavity, chorion and placenta (form monamniotic, monochorionic twins)
4. Incomplete separation of the inner cell mass gives rise
to conjoined twins.
5. Anastomosis between the circulations of monoamniotic twins may cause failure of normal growth and development in one embryo