Cytotrophoblastic specializations: An ultrastructural study of the human placenta

Ultrastructural examination of the specialized peripheral cytotrophoblast distinguishes the complex elements of the cell islands, septa, shell, and chorion laeve from the typical villous cytotrophoblast (Langhans' cell). Whereas syncytium, typical decidua, and poorly differentiated cytotrophoblast are easily recognized, many well-differentiated mononuclear elements in the basal plate, septa, and islands cannot be classified as maternal or fetal on strictly morphologic criteria. On the basis of transitional forms and specialized intercellular connections, we conclude that the junctional areas of the human placenta comprise an admixture of fetal and maternal elements, some of which are ultrastructurally similar to each other. The evolution of the syncytium through a series of transitional cells is well demonstrated in the anchoring villi, cell columns, and islands. The ultrastructural differentiation of the trophoblast is correlated with its elaboration of protein hormones. Cytochemical evidence suggests that the syncytium is the tissue most active in the formation of chorionic gonadotropin, with varying contributions from the specialized cytotrophoblast. Mitochondria with tubular cristae have been demonstrated in the specialized cytotrophoblast, which presumably participates in steroidogenesis. In junctional zones of the placenta, fibrinoid and glycocalyces of the trophoblast are probably ancillary though not primary factors in permitting the prolonged retention of the placental homograft.