Absorptive structures of the mouse yolk sac placenta and associated placental surfaces. A scanning electron microscope study.

Membranes of the visceral and parietal yolk sacs and their adjacent placental surfaces were examined at 13, 18 and 21 days of gestation in the mouse by scanning electron microscopy. The exterior or endodermal surface of the visceral yolk sac was characterized by a villous area attached to the placental disc, a central transition zone, and a smooth distal area. At all days, the abundant microvilli of the villous region were either elongated or bleb-shaped. At 13 days microvilli of cells in the transitional zone and smooth area were uniformly rod-shaped in contrast to the longer microvilli seen at 18 and 21 days; moreover, denuded cells were frequently seen at 21 days. Large squamous endodermal cells on the surface of the placental disc outside the visceral yolk sac were separated by wide intercellular spaces bridged by cytoplasmic processes at all days. Microvilli of these cells were sparse at day 13 but became abundant at day 18 then decreased by day 21. Endodermal cells of the adjacent parietal yolk sac changed from a rounded to a flattened shape from 18 to 21 days, with microvilli being more abundant at 18 days. In contrast to the outer endodermal surface of the visceral yolk sac, the inner mesothelial surface was devoid of villi. At 13 days contiguous edges of cells interdigitated giving the lateral cell borders a microvillous-like appearance. The surface of these cells had a sparse covering of microvilli which became more abundant as the surface was drawn into folds at 18 and 21 days. The adjacent fetal placental surface was covered by flattened cells with few surface specializations. Our observations support an absorptive role for the endodermal surfaces of the visceral and parietal yolk sacs and the associated surfaces of the mouse placenta.