Endotheliochorial Placentation : Cat, Dog, Bat

Abstract

In endotheliochorial placentation no uterine epithelium survives implantation (Figs. 1.5 and 7.1). The cellular chorioallantoic trophectodermal cells produce, by division and fusion, a fetal syncytium apposed to an amorphous layer around the persisting maternal endothelium. The amorphous layer (also referred to as the interstitial layer or, misleadingly, `membrane') is probably produced by the syncytium and is equivalent, ultrastructurally and in molecular constituents, to the basement membrane of the maternal endothelium. It is neither uniformly thick nor continuous; processes of the fetal syncytium penetrate through it to reach a close relationship with the maternal endothelium in places, and it includes occasional large residual endometrial cells. This structure is characteristic of the early and definitive endotheliochorial placentas which are zonary (Figs. 1.4 and 7.1).