Chapter 38. The Placenta: General Discussion - Review of Medical Embryology Book

Morphology: the placenta is the most important accessory fetal structure and brings the fetal and maternal circulations into close relationship. Morphologically, it is partly of fetal origin (the trophoblast) and partly of maternal origin (arising from the transformation of the uterine mucosa)

The trophoblast is seen at about day By days 6 to 7, it ensures implantation of the egg in the uterine mucosa due to its proteolytic activity, and at that time consists of an inner cellular layer, the cytotrophoblast, and an outer syncytial layer, the syncytiotrophoblast
By week 7, the trophoblast has proliferated into villi which are visible over the entire surface of the chorion (placenta)
At the end of month 2, the villi begin to group together, forming the villous chorion or the chorion frondosum (bushy). The umbilical cord is very thick at this stage and is moving toward this region
By 2 1/2 months, rarefaction of the villi at one of the poles is seen
A human 3-month-old egg shows differentiation of a clearly defined placent The villi are grouped at 1 pole of the egg and form the placent The rest of the embryonic vesicle is devoid of villi and is smooth (chorion laeve), and through it the outline of the fetus can be seen indistinctly
After 3 months, the placenta grows, thickens, and spreads out, developing along with the uterus
The placenta at term is a disk about 20 cm in diameter, 3 cm thick, and weighs about 500 g (about one-sixth of the fetal weight). A ratio markedly different than this indicates a pathologic condition

From the maternal side, one sees that deep furrows divide the placental mass into a number of lobes or cotyledons
Examination of the placenta provides information about placental dysfunction, fetal growth, retardation, neonatal illness, and infant death
Retention of a cotyledon in the uterus may lead to late puerperal hemorrhage, but, more often, retained placental tissue is the cause

Maternal surface: cobblestone appearance caused by 10 to 38 cotyledons separated by grooves formerly occupied by the placental septa

The cotyledon surface is covered by shreds of the decidua basalis. Most of the latter, however, is temporarily retained in the uterus and shed with subsequent uterine bleeding

The amniotic covering of the cord is continuous, with the amnion adherent to this surface of the placenta
The vessels radiating from the umbilical cord are clearly seen through the transparent amnion