Development and characteristics of placentation in a carnivore, the domestic cat.

Among the carnivores, development of the fetal membranes and placentation have now been particularly well studied in the domestic cat (Felis catus). Initially, the cat conceptus is bordered by a primitive and a precontact chorion. This becomes part of a temporary choriovitelline placenta which is subsequently supplanted by a chorioallantoic placenta. Following the triphasic process of implantation (which involves apposition, adhesion, and intrusion), the chorioallantoic placenta forms a zonary girdle which separates two paraplacental cupules. These cupules are subdivided from base to tips into 1) extravasate zones with hematomal areas and solid junctional areas containing intermingled fetal and maternal tissue, 2) free polar zones with almost no feto-maternal contact, and 3) interplacental polar zones freely projecting into the uterine lumen or, near the end of pregnancy, facing comparable regions of neighboring fetuses. The placental girdle consists of a lamellar zone characterized by elongate, parallel fetal (chorionic) and maternal (septal) lamellae, a junctional zone where fetal and maternal tissues face each other and intimately intermingle, and a zone of pure endometrial glands. In the lamellar zone the interhemal membrane (placental barrier) is of the endotheliochorial type. As pregnancy progresses, cytotrophoblast in the interhemal barrier contributes to the formation of syncytiotrophoblast and is gradually reduced from being a continuous layer to only scattered cells. The syncytiotrophoblast is usually separated from endothelial cells of the maternal capillaries by a thickened basal lamina (the interstitial membrane) or faces persisting endometrial connective tissue containing some enlarged decidual cells. The efficiency of maternal-fetal physiological exchange depends not only upon the thickness of the interhemal membrane, which in reduced in places to 1.5 microns, but also upon the materno-fetal blood flow interrelationship. This is of the simple crosscurrent type in the cat.