The structure of the tupaia placenta. II. Ultrastructure.

Both placental discs of the tupaia placenta consist of three large mesometrial and numerous additional smaller antimesometrial lobules. The larger lobules can be subdivided into a central (maternal arterial inflow), intermediate (maternal capillaries, exchange area), and a peripheral zone (maternal veins). The ultrastructure of the intermediate lobular zone has been studied from day 21 until day 41 of pregnancy. The materno-fetal barrier is endotheliochorial in structure. The voluminous maternal endothelium is highly differentiated. The adjacent basement membrane, separating endothelium and trophoblast, is constantly interrupted by endothelial and trophoblastic extensions, forming intense interdigitations between both tissue layers. The trophoblast is a multinuclear continuous layer, syncytial or plasmodial in nature. Cytotrophoblast is missing in all stages studied so far. Towards term, the trophoblast differentiates into three distinct types, arranged beneath each other in a mosaic-type pattern: trophoblast with rough e.r. and trophoblast with smooth e.r. as dominating organelles, as well as thin trophoblastic lamellae, almost devoid of organelles. The connective tissue is composed of very compact, epithelium-like cells, whereas connective tissue fibres and interstitial space are virtually absent. Macrophages, undifferentiated fibroblasts and highly differentiated fibroblasts can be observed. There are structural transitions between connective tissue and neighbouring fetal capillary endothelium.