Scanning electron microscopy of maternal blood cells and their surface relationship with the placenta

. With the aid of the scanning electron microscope the surface of full‐term human placenta and the maternal blood cells displayed on that surface were examined. The possible site(s) of materno‐fetal transfer of gases were further investigated. Small, fresh pieces of tissue were obtained and processed from at least six regions of central and peripheral parts of the maternal surface. It seemed that there is a firm association between the presence of the maternal red blood cells, the main vehicles for gas transport, and that of the microvilli. Our results clearly confirm the absence of maternal blood cells from the smooth apices of the dome‐shaped protrusions that occur on the tips of some floating villi. However, maternal red blood cells were consistently seen in close contact only with areas covered with microvilli. Direct evidence of the close contact was demonstrated by either red cell adherence to the microvilli or by their lying in recesses on the trophoblastic surfaces. Impressions (“foot prints”) on some surfaces, and the presence of red blood cells with detached microvilli adhering to their surfaces were interpreted as indirect evidence for the adherence. The lack of such evidence regarding the white blood cells or platelets is remarkable. Different types of white blood cells were identified in various shapes, ranging from rounded to flattened. Some of them possessed surface microvilli, while others demonstrated projections and surface folds. Our findings indicate, therefore, that the red cells have a unique and intimate relationship with the trophoblastic surfaces ‐more so than the other blood corpuscles. Thus this would appear to support our previous view that one of the functions of the microvilli is transfer of the gas across the placenta.