Some of the histological changes that accompany placental growth and maturation improve transport and exchange to meet advancing fetal metabolic requirements. Among these changes are a thinner syncytiotrophoblst, signiicantly reduced cytotrophoblast number, decreased stroma, and increased number of capillaries with close approximation to the syncytial surface. By 16 weeks' gestation, the apparent continuity of the cytotrophoblasts is lost. At term, villi may be focally reduced to a thin layer of syncytium covering minimal villous connective tissue in which thinwalled fetal capillaries abut the trophoblast and dominate the villi.
There are some changes in placental architecture that can cause decreased placental exchange eiciency if they are substantive. These include thickening of the basal lamina of trophoblast or capillaries, obliteration of certain fetal vessels, greater villous stroma, and ibrin deposition on the villous surface.
FIGURE 5-1 3 Uterus showing a normal placenta and its membranes in situ.
Because the placenta is functionally an intimate approximation of the fetal capillary bed to maternal blood, its gross anatomy primarily concerns vascular relations. he fetal surface is covered by the transparent amnion, beneath which chorionic vessels course. A section through the placenta includes amnion, chorion, chorionic villi and intervillous space, decidual (basal) plate, and myometrium (Figs. 5-13 and 5-14).
FIGURE 5-14 Photomicrograph of early implanted blastocyst. Trophoblasts are seen invading the decidua basalis. eNS central nervous system. (Used with permission from Dr. Kurt Benirschke.)
Deoxygenated venous-like fetal blood flows to the placenta through the two umbilical arteries. As the cord joins the placenta, these umbilical vessels branch repeatedly beneath the amnion as they run across the chorionic plate. Branching continues within the villi to ultimately form capillary networks in the terminal villous branches. Blood with signiicantly higher oxygen content returns from the placenta via a single umbilical vein to the fetus.
The branches of the umbilical vessels that traverse along the fetal surface of the placenta in the chorionic plate are referred to as the placental surface or chorionic vessels. These vessels are responsive to vasoactive substances, but anatomically, morphologically, histologically, and functionally, they are unique. Chorionic arteries always cross over chorionic veins. Vessels are most readily recognized by this interesting relationship, but they are diicult to distinguish by histological criteria.
Truncal arteries are perforating branches of the surface arteries that pass through the chorionic plate. Each truncal artery supplies one main stem villus and thus one cotyledon. As the artery penetrates the chorionic plate, its wall loses smooth muscle, and its caliber increases. The loss of muscle continues as the truncal arteries and veins branch into their smaller rami.
Before 10 weeks' gestation, there is no end-diastolic low pattern within the umbilical artery at the end of the fetal cardiac cycle (Fisk, 1988; Loquet, 1988). After 10 weeks, however, end-diastolic low appears and is maintained throughout normal pregnancy. Clinically, these flow patterns are studied with Doppler sonography to assess fetal well-being (Chap. 10, p. 213).