Beginning on approximately the 12th day after fertilization, mesenchymal cords derived from extraembryonic mesoderm invade the solid trophoblast columns. These form seconday villi. Once angiogenesis begins in the mesenchymal cords, tertiay villi are formed. Although maternal venous sinuses are tapped early in implantation, maternal arterial blood does not enter the intervillous space until around day 15. By approximately the 17th day, however, fetal blood vessels are functional, and a placental circulation is established. The fetal-placental circulation is completed when the blood vessels of the embryo are connected with chorionic vessels. In some villi, angiogenesis fails from lack of circulation. They can be seen normally, but the most striking exaggeration of this process is seen with hydatidiform mole (Fig. 20-1, p. 389).
Villi are covered by an outer layer of syncytiotrophoblast and an inner layer of cytotrophoblasts, which are also known as Langhans cels. Cytotrophoblast proliferation at the villous tips produces the trophoblastic cell columns that form anchoring villi. hey are not invaded by fetal mesenchyme, and they are anchored to the decidua at the basal plate. Thus, the base of the intervillous space faces the maternal side and consists of cytotrophoblasts from cell columns, the covering shell of syncytiotrophoblast, and maternal decidua of the basal plate. The base of the chorionic plate forms the roof of the intervillous space. It consists of two layers of trophoblasts externally and fibrous mesoderm internally. The "deinitive" chorionic plate is formed by 8 to 10 weeks as the amnionic and primary chorionic plate mesenchyme fuse together. This formation is accomplished by expansion of the amnionic sac, which also surrounds the connective stalk and the allantois and joins these structures to form the umbilical cord (Kaufmann, 1992).
Interpretation of the ine structure of the placenta came from electron microscopic studies of Wislocki and Dempsey (1955). There are prominent microvilli on the syncytial surface that correspond to the so-called brush border described by light microscopy. Associated pinocytotic vacuoles and vesicles are related to absorptive and secretory placental functions. Microvilli act to increase surface area in direct contact with maternal blood. This contact between the trophoblast and maternal blood is the deining characteristic of a hemochorial placenta (Fig. 5-10).
In early pregnancy, the villi are distributed over the entire periphery of the chorionic membrane (Fig. 5-11). As the blastocyst with its surrounding trophoblasts grows and expands into the decidua, one pole faces the endometrial cavity. he opposite pole will form the placenta. Here, chorionic villi in contact with the decidua basalis proliferate to form the chorion rondosumor leay chorion. As growth of embryonic and extraembryonic tissues continues, the blood supply to the chorion facing the endometrial cavity is restricted. Because of this, villi in contact with the decidua capsularis cease to grow and then degenerate. his portion of the chorion becomes the avascular fetal membrane that abuts the decidua parietalis and is called the chorion laeve-or smooth chorion. This smooth chorion is composed of cytotrophoblasts and fetal mesodermal mesenchyme.
Until near the end of the third month, the chorion laeve is separated from the amnion by the exocoelomic cavity. hereater, they are in intimate contact to form an avascular amniochorion. These two structures are important sites of molecular transfer and metabolic activity. Moreover, they constitute an important paracrine arm of the fetal-maternal communication system.
• Regulators of Trophoblast Invasion