Scheme of electron microscopy of tertiary villus (human placenta, midpregnancy)

(See also POJA-L1227). (A) Survey and (B) detail of a tertiary placental villus lined by the multinucleated syncytiotrophoblast cell (1) (STC). The apex shows protrusions and extensive microvilli of varying sizes (brushborder, 2). Nuclei are sectioned at different levels and the cytoplasm contains abundant organelles, endoplasmic reticulum, vacuoles and dense-stained lysosomal structures. The underlying desmosomes-linked electron-light cytotrophoblast cells (CTCs or Langhans cells, 3) are close adhered to the STC. The epithelial linings are separated by a continuous basal lamina from the loose connective tissue (4) with a fetal fibroblast (5), partly a Hofbauer cell (fetal phagocyte, 6) and two capillaries with erythrocytes (7). The area indicated by (8 ↔) indicates a very close contact between both the epithelial and the capillary basal laminae and is called a vasculosyncytial membrane.