Morphological Aspects of Human Placentation

The complex anatomy of the placenta has made studies of the entire organ difficult. Commonly the placenta is depicted as a simple, pancake-shaped sponge connecting the embryo/fetus to the uterus. This portion of the placenta, which is expelled from the uterus during delivery, is easy to obtain and, consequently, frequently studied. But the most interesting part of the placenta is rarely seen. This portion, which lies buried within the uterine wall, separates from the rest of the placenta during pregnancy termination or delivery. As a result, the only way to obtain this tissue is by uterine biopsy of the site where the placenta attached. Thus, special procedures that are similar to the methods used to obtain any other surgical specimen are required to collect this portion of the placenta.

A full understanding of the placenta can be obtained only by studying both its parts. Figure 1 diagrams these two parts and joins them into a single unit to show how they function in conjunction with modified uterine structures during human pregnancy. The placenta is made up of individual units termed chorionic villi. Each villus has a connective tissue core that contains fetal blood vessels and numerous macrophages, termed Hofbauer cells. The macrophages often lie adjacent to a thick basement membrane, which underlies a layer of CTB progenitors that give rise to all the trophoblast lineages.

The differentiation pathway that CTB progenitors take depends on their location. In floating villi, the CTBs fuse to form a multinucleate syncytium, the syncytiotrophoblast, that covers the villus surface. These villi are attached at only one end to the treelike fetal portion of the placenta. The rest of the villus floats in a stream of maternal blood, which optimizes exchange, across the syncytium, of substances between the mother and fetus. In anchoring villi, CTB progenitors detach from the basement membrane and form a column of nonpolarized mononuclear cells that invade the uterus. As a result, these villi are attached at one end to the fetal portion of the placenta and at the other end to the

Figure 1 Diagram of a longitudinal section of the maternal-fetal interface. Fetal cytotrophoblasts from the placenta form cell columns that attach to the uterine wall. In turn, the columns give rise to a specialized subpopulation of cytotrophoblasts that invade the uterine wall (decidua and myometrium), thereby anchoring the fetus to the mother. Cytotrophoblasts also invade uterine blood vessels. In this location they replace the resident maternal endothelial and vascular smooth muscle cells. As a result they form vascular channels that carry blood to and from the placenta. (Reproduced with permission from

Biochemistry.)

Figure 1 Diagram of a longitudinal section of the maternal-fetal interface. Fetal cytotrophoblasts from the placenta form cell columns that attach to the uterine wall. In turn, the columns give rise to a specialized subpopulation of cytotrophoblasts that invade the uterine wall (decidua and myometrium), thereby anchoring the fetus to the mother. Cytotrophoblasts also invade uterine blood vessels. In this location they replace the resident maternal endothelial and vascular smooth muscle cells. As a result they form vascular channels that carry blood to and from the placenta. (Reproduced with permission from

Biochemistry.)

uterus. This arrangement anchors the villus to the uterine wall. Invasive CTBs rapidly traverse most of the uterine parenchyma (interstitial invasion). They also breach the uterine veins and arteries they encounter (endovascular invasion). Their interactions with veins are confined to the portions of the vessels that lie near the inner surface of the uterus, but CTBs migrate in a retrograde direction along much of the intrauterine course of the arterioles. Eventually, these fetal cells completely replace the maternal endothelial lining and partially replace the muscular wall of these vessels. This unusual process diverts uterine blood flow to the floating villi.

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