Ultrastructure of the Anchoring Villi and Trophoblastic Shell in the Second Week of Placentation

The fine structure of the anchoring villi and the trophoblastic shell of a human embryo with four somites is described. The ultrastructure of the cytotro‐phoblast of the proximal part of the anchoring villi was similar to that of the Langhans' cells of the free villi. The distal cytotrophoblasts had irregular nuclei and contained more rough endoplasmic reticulum. The cytotrophoblasts of the base of the villi were large and contained large amounts of glycogen. The basal plate comprised a variety of trophoblastic cells. Most commonly seen was the mononuclear giant cell, which was rich in rough endoplasmic reticulum, glycogen and fibrils. In the periphery of the placental site, the trophoblasts were scattered among the maternal cells. Foetal and maternal cells were often found side by side without any reaction. However, the cell membranes of trophoblastic giant cells and those of the uterine gland cells were closely apposed and desmosomes were observed between the foetal and maternal cells. In other places cell membranes between trophoblastic cells and uterine epithelium seemed to disintegrate.