By the beginning of the second month, the trophoblast is characterized by a great number of secondary and tertiary villi that give it a radial appearance (Fig. 6.7). The villi are anchored in the mesoderm of the chorionic plate and are attached peripherally to the maternal decidua by way of the outer cytotro-phoblast shell. The surface of the villi is formed by the syncytium, resting on a layer of cytotrophoblastic cells that in turn cover a core of vascular mesoderm (Fig. 6.8, A and C). The capillary system developing in the core of the villous stems soon comes in contact with capillaries of the chorionic plate and connecting stalk, thus giving rise to the extraembryonic vascular system (see Fig. 5.15).
During the following months, numerous small extensions sprout from existing villous stems into the surrounding lacunar or intervillous spaces. Initially these newly formed villi are primitive (Fig. 6.8C), but by the beginning of the fourth month, cytotrophoblastic cells and some connective tissue cells disappear. The syncytium and endothelial wall of the blood vessels are then the only layers that separate the maternal and fetal circulations (Fig. 6.8, B and D). Frequently the syncytium becomes very thin, and large pieces containing several nuclei may break off and drop into the intervillous blood lakes. These pieces, known as syncytial knots, enter the maternal circulation and usually degenerate without causing any symptoms. Disappearance of cytotrophoblastic cells progresses from the smaller to larger villi, and although some always persist in large villi, they do not participate in the exchange between the two circulations.
Figure 6.7 Human embryo at the beginning of the second month of development. At the embryonic pole, villi are numerous and well formed; at the abembryonic pole they are few in number and poorly developed.
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