Although certain villi of the chorion frondosum extend from the chorionic plate to the decidua to serve as anchoring villi, most villi arborize and end freely within the intervillous space. s gestation proceeds, the short, thick, early stem villi branch to form progressively iner subdivisions and greater numbers of increasingly smaller villi (Fig. 5-12). Each of the truncal or main stem villi and their ramiications constitutes a placental lobule, or cotyledon. Each lobule is supplied with a single chorionic artery. And each lobule has a single vein, so that lobules constitute the functional units of placental architecture.
FIGURE 5-12 Electron micrographs (A, C) and photomicrographs (B, D) of early and late human placentas. A and B. Limited branching of villi is seen in this early placenta. C and D. With placental maturation, increasing villous arborization is seen, and villous capillaries lie closer to the surface of each villus. (Photomicrographs used with permission from Dr. Kurt Benirschke. Electron micrographs reproduced with permission from King BF, Menton DN: Scanning electron microscopy of human placental villi from early and late in gestation. Am J Obstet Gynecol 122:824, 1975.)
In the irst trimester, placental growth is more rapid than that of the fetus. But by approximately 17 weeks' gestation, placen tal and fetal weights are approximately equal. By term, placen tal weight is approximately one sixth of fetal weight.
he mature placenta and its variant forms are discussed in detail in Chapter 6 (p. 112). Briely, viewed from the mater nal surface, the number of slightly elevated convex areas, called lobes, varies from 10 to 38. Lobes are incompletely separated by grooves of variable depth that overlie placental septa, which arise as upward projections of decidua. he total number of placental lobes remains the same throughout gestation, and individual lobes continue to grow-although less actively in the inal weeks (Crawford, 1959). Although grossly visible lobes are commonly referred to as cotyledons, this is not accurate.
Correctly used, lobules or cotyledons are the functional units supplied by each main stem villus.
As villi continue to branch and the terminal ramiications become more numerous and smaller, the volume and promi nence of cytotrophoblasts decrease. As the syncytium thins, the fetal vessels become more prominent and lie closer to the sur face (see Fig. 5-10). he villous stroma also exhibits changes as gestation progresses. In early pregnancy, the branching connec tive-tissue cells are separated by an abundant loose intercellular matrix. Later, the villous stroma becomes denser, and the cells are more spindly and closely packed.
Another change in the stroma involves the iniltration of Hobauer cels, which are fetal macrophages. hese are nearly round with vesicular, often eccentric nuclei and very granular or vacuolated cytoplasm. hey grow in number and maturational state throughout pregnancy and appear to be important mediators of protection at the maternal-fetal interface Qohnson, 2012). These macrophages are phagocytic, have an immunosuppressive phenotype, can produce various cytokines, and are capable of paracrine regulation of trophoblastic functions (Cervar, 1999; Reyes, 2017). As discussed further in Chapter 64 (p. 1219), recent studies suggest that Zika virus can infect Hofbauer cells to allow fetal transmission (Simoni, 2017).