Electron microscopy of syncytiotrophoblast cells in tertiary villus (human placenta, midpregnancy)

The left photograph (A) shows the apical cytoplasm of a syncytiotrophoblast cell (STC, nucleus). The free cell surface displays small protrusions and a characteristic pattern of differently shaped microvilli; pinocytotic invaginations and a single macropinocytotic vacuole. Note the apical localized electron-dense lysosomal structures also visible at the lightmicroscopical level. The cytoplasm is stuffed with endoplasmic reticulum and all types of organelles. In the right picture (B) electron-grey lysosomal structures as well as peculiar long, thin loops of endocytotic structures as well as interconnecting clusters of lysosomal and endocytotic elements, illustrating high phagocytotic ability of these STCs.