Chapter 39. The Placental Villi - Review of Medical Embryology Book

Structure and development of the placental villi: the human placenta is classified as villous, hemochorial, and chorioallantoic, since the placental villi are bathed directly by maternal blood and are transversed by vessels coming from the allantoic circulation of the fetus

About day 13: the villi begin to appear in the form of syncytial branches separated by lacunae or intervillous spaces
About day 15: a cytotrophoblast core is seen inside each column with its outer covering of syncytiotrophoblast. As the cell columns develop and progress, they open maternal vessels whose blood spreads out in the intervillous spaces - the beginnings of the maternal-placental circulation
About day 18: the villus consists of a mesenchymal core surrounded by a double layer of cyto- and syncytiotrophoblast. In the center of the mesenchyme, vascular islets appear which are the beginnings of the future fetal circulation. In contrast, the lacunae, which are now intervillous spaces, are already sites of intense maternal circulation

The intervillous space is divided into compartments by the placental septa, but, because the septa do not reach the chorionic plate, there is communication between the intervillous spaces of different compartments
The intervillous spaces are drained by veins found over the entire surface of the decidua basalis

About day 21: the intravillous vascular network connects with the umbilical-allantoic vessels establishing the fetal placental circulation (chorioallantoic)

The villus develops a treelike shape and is bordered by a double trophoblast layer, the superficial syncytiotrophoblast, and the deeper or inner cytotrophoblast or the cells of Langerhans
The villi that make contact with the maternal tissue are called stem or anchoring vill The others remain free in the intervillous spaces and are referred to as branch or floating villi. In sections, the villus of 4 months is far more dense than the villus of 2 months
As the villi invade the decidua basalis, they leave several wedge-shaped areas of decidua tissued called placental septa, which appear in month 4 starting out from the maternal (decidua) plate but do not reach the chorionic plat These septa divide the fetal part of the placenta into 10-38 convex areas composed of lobes and lobules, the so-called cotyledons

The placenta of 1 month already reveals some single vessels in the villus. By the second month, the vascular core has develope At term, large vessels follow the villous core up to the basal plate, and these vascular trunks in the anchoring villi give rise to the capillary networks which are involved in all branches of the villi

After 4 months, the villus is richly vascular and has a thin coat as a result of the disappearance of the cytotrophoblast (contrast with the villus of 2 months)

The placenta at term may show a few large, persistent cytotrophoblastic areas (particularly on the maternal plate), but, as term nears, the cytotrophoblast vanishes in this region as well and is replaced by a fibrinoid layer

As a result of numerous branchings, the villus has become comparable to a "bushy tre" Its branches form a tangled mass in whose meshes the maternal blood circulates