Surface ultrastructure of the human placental villi and sites of contact with maternal red blood cells.

With the aid of the scanning electron microscope the possible sites of materno-foetal oxygen transfer were investigated. Fresh small tissue pieces were obtained and processed from at least six regions of central and peripheral parts of the maternal surface of human full-term placentas. The surface ultrastructural features of the syncytiotrophoblast lining the stem and floating villi were surveyed. The microvilli projecting from the apical portion of the syncytiotrophoblast appeared to be highly pleomorphic and showed regional variation in their distribution. On the other hand, our results confirmed the occurrence of non-microvillous areas on the apices of some floating villi. When present, these areas were always free from contact with maternal red blood cells. Maternal red blood cells, however, were seen in close contact only with areas covered with microvilli. Occasionally, impressions ("footmarks") were apparent on some surfaces and detached microvilli were seen adherent to the surface of other maternal red cells which had separated from the villous surface. Our results indicate, therefore, that the microvillous areas of the chorionic villi are the most likely sites for oxygen transfer and that one of the functions of the microvilli is gas transfer across the placenta.