This H&E–stained specimen shows the amniotic surface (A), the chorionic plate (CP), and, below, the various-sized profiles of the chorionic villi (CV ). These villi emerge from the chorionic plate as large stem villi and branch into the increasingly smaller villi. Blood vessels (BV ) are evident in the larger villi. The smallest villi contain capillaries where exchange takes place. 60. Upper inset. This higher magnification shows the simple cuboidal epithelium of the amnion and the underlying connective tissue. 200. Lower inset. This higher magnification shows a cross-sectioned villus containing several larger blood vessels and its thin surface syncytiotrophoblast layer. 200. b. This H&E–stained specimen shows the maternal side of the placenta. The stratum basale (SB), the part of the uterus to which some of the chorionic villi (CV ) anchor, is seen at the bottom of the micrograph. Also evident is a stromal connective tissue (CT ) component, part of the stratum basale, to which many of the chorionic villi are also attached. Within the stratum basale and the connective tissue stroma are clusters of cells, the decidual cells (arrows), which arose from connective tissue cells. 60. Inset. Decidual cells seen at higher magnification. 200.
FIGURE 23.25 • Human placental barrier in a third trimester of the pregnancy. This high magnification electron micrograph shows the thinnest layer of a fully developed placental barrier (section does not include cytotrophoblast cells that form a discontinuous layer in the human placenta). The lumen of intervillous space containing maternal erythroblasts (ME) (to the left ) is separated from the fetal capillary space containing fetal erythroblasts (FE) (to the right ). The intervillous space is lined by the multinucleated syncytiotrophoblast (Syn). Its surface contains microvilli projecting into maternal blood space. The cytoplasm of syncytiotrophoblast contains multiple nuclei (N) and has an abundance of transport vesicles, rER, and sER, mitochondria, and occasional lipid droplets. The syncytiotrophoblast rests on the basal lamina (TBL), which is separated by the thin layer of the connective tissue (CT ) from the basal lamina (EBL) of the fetal endothelial cells (FEn). 11,000. (Courtesy of Dr. Holger Jastrow.)
The placenta is the site of exchange of gases and metabolites between the maternal and fetal circulation.
Fetal blood enters the placenta through a pair of umbilical arteries (Fig. 23.26). As they pass into the placenta, these arteries branch into several radially disposed vessels that give numerous branches in the chorionic plate. Branches from these vessels pass into the villi, forming extensive capillary networks in close association with the intervillous spaces. Gases and metabolic products are exchanged across the thin fetal layers that separate the two bloodstreams at this level. Antibodies can also cross this layer and enter the fetal circulation to provide passive immunity against a variety of infectious agents—for example, those of diphtheria, smallpox, and measles. Fetal blood returns through a system of veins that parallel the arteries except that they converge on a single umbilical vein.
Maternal blood is supplied to the placenta through 80 to 100 spiral endometrial arteries that penetrate the basal plate. Blood from these spiral arteries flows into the base of the intervillous spaces, which contain about 150 ml of maternal blood that is exchanged three to four times per minute. The blood pressure in the spiral arteries is much higher than that in the intervillous spaces. As blood is injected into these spaces at each pulse, it is directed deep into the spaces. As the pressure decreases, the blood flows back over the surfaces of the villi and eventually enters endometrial veins also located in the base of the spaces.