DEVELOPMENT AND CHARACTERISTICS OF PLACENTATION IN A CARNIVORE, THE DOMESTIC CAT

Among the carnivores, development of the fetal membranes and placentat ion 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 placent a. Following the triphasic process of implantation (which involves app osition, adhesion, and intrusion), the chorioallantoic placenta forms a zonary girdle which separates two paraplacental cupules. These cupul es are subdivided from base to tips into 1) extravasate zones with hem atomal 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 z one 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 endome trial glands. In the lamellar zone the interhemal membrane (placental barrier) is of the endotheliochorial type. As pregnancy progresses, cy totrophoblast in the interhemal barrier contributes to the formation o f syncytiotrophoblast and is gradually reduced from being a continuous layer to only scattered cells. The syncytiotrophoblast is usually sep arated from endothelial cells of the maternal capillaries by a thicken ed basal lamina (the interstitial membrane) or faces persisting endome trial connective tissue containing some enlarged decidual cells. The e fficiency of maternal-fetal physiological exchange depends not only up on the thickness of the interhemal membrane, which in reduced in place s to 1.5 mum, but also upon the materno-fetal blood flow interrelation ship. This is of the simple crosscurrent type in the cat. (C) 1993 Wil ey-Liss, Inc.