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Placenta

Objectives

Learn the gross and histological structure of the placenta and correlate structure with function.

The placenta is the organ of exchange between maternal and fetal tissues, providing nutrition, respiration, and excretion for the fetus. It also produces hormones and serves as an immunological barrier between the genetically disparate maternal and fetal tissues. At term, the placenta is usually a circular shaped structure measuring approximately 20 cm. in diameter and 2.5 cm. in thickness. It is primarily of fetal origin but has a small maternal component as well. The fetal portion of the placenta consists of the chorionic plate and its villi and the peripheral trophoblastic shell which surrounds the intervillous space and covers the maternal tissue. Projecting toward the decidua basalis from the chorionic plate are numerous highly branched villi. The villi are composed of fetal connective tissue and are covered by trophoblasts. The trophoblast is the parenchyma of the placenta. It is the tissue through which exchanges must occur, and it is involved in the production of placental hormones, both steroids and polypeptides. Most villi are free in the intervillous space and are bathed in blood from the maternal vessels. Anchoring villi contact the decidua basalis. The decidua basalis (maternal portion of the placenta) rests on the stratum basalis and is characterized by large, polyhedral pale blue stromal (decidua) cells. Flattened slit-like glands lie at the boundary between decidua and stratum basalis. Multinucleated giant cells of fetal origin are present in the endometrium. Between the maternal and fetal tissues is an irregular layer of fibrinoid.

A. Maternal Side

Slide #90 odd, the maternal side, has myometrium, stratum basalis, decidua basalis, junctional zone, anchoring villi and cross-sections of villi branches. Slide #90 even, the fetal side, has the amnion, chorionic plate and stem villi and cross-sections of villi branches. Examine both slides.

Initially examine slide #90 odd by holding the slide up to the light and examining it with the reversed ocular. Note the maternal and fetal tissue. Using the microscope, find the thin stratum basalis adjacent to the thick muscle layer (myometrium). This melts into the decidua basalis. In some slides, it is possible to separate the stratum and decidua basalis in the region of the slit-like glands. The decidua basalis contains many enlarged glycogen-containing stromal (decidual) cells, a few fetal giant cells, and blood vessels. Near the surface of the decidua basalis is the junctional zone, which includes decidual cells, fibrinoid, and the surface trophoblastic layer (the trophoblastic layer is lost in many areas). Locate an anchoring villus touching the maternal surface, and note the basophilic cells of the peripheral cytotrophoblast (cell columns). In sections through some villi observe the central core of fetal loose connective tissue containing small blood vessels. The outermost covering is the syncytiotrophoblast, a syncytium with many dark-staining nuclei. During the latter half of pregnancy, as in this slide, groups of aggregated nuclei may project at the surface forming syncytial knots. Examine the electron micrograph of the syncytiotrophoblast. NOTE: the abundant microvilli, rough endoplasmic reticulum and multiple nuclei. The inner trophoblastic layer, the cytotrophoblast (Langhan's layer), is made up of distinctly separate cells which form a continuous layer in early pregnancy. Later in pregnancy, as in this slide, this layer is reduced to a few scattered large pale cells with pale nuclei, located between the syncytiotrophoblast and underlying connective tissue. The mitotic cytotrophoblast provide new cells which fuse with the syncytiotrophoblast and contribute to its growth. Examine the electron micrograph of the cytotrophoblast and compare its ultrastructure to that of the syncytiotrophoblast.

B. Fetal Side

In slide #90 even, under very low power, locate the chorionic plate, a solid tissue mass to one side of the section. Note that the connective tissue of the chorionic plate is artificially separated from the connective tissue of the amnion. Examine the cuboidal epithelium of the amnion. The chorionic plate contains large blood vessels surrounded by connective tissue. The surface of the chorionic plate bordering the intervillous space is covered with trophoblast. Find examples of main stem villi coming from the chorionic plate. Examine the small branches of the villi and note the characteristics of a late placenta: large capillaries, often near the surface, syncytial knots, thinned syncytial cytoplasm of the syncytiotrophoblast, few cytotrophoblast cells and fibrin deposits in the intervillous space. The stroma of the villi contains fetal fibroblasts and Hofbauer cells. Hofbauer cells, thought to be a type of macrophage, are oval cells with an eosinophilic cytoplasm and a round nucleus. They can be found in round vacuolated areas of the stroma.