ABSTRACT
Morphological changes in the endometrium of epitheliochorial and endotheliochorial types of placentae are of great interest, as adaptation to an intimate contact between the uterine epithelium and the conceptus has to be developed in order to meet the needs of the conceptus. The endometrium has to pass through a phase of growth and remodeling in order to achieve an increase in the surface area of well-vascularized tissue. A decrease in the physical distance between maternal and fetal blood circulation in the interhemal barrier is also needed in order to obtain the best possible exchange of oxygen and carbon dioxide. In these types of placentae a well regulated cellular transfer of nutrients takes place over the maternal epithelium and or endothelium to be taken up by the trophoblast. The genetically foreign conceptus has, in addition, the capability to suppress undesirable maternal immune reactions, whereas it seems to be the endometrium that regulates and controls the degree of invasion. The placenta has to develop rapidly to achieve the capacity to act as respiratory, intestinal and endocrine system. The interplay between regulatory factors of importance for placentation varies among species. Three epitheliochorial systems are emphasized: the noninvasive diffuse placenta of the pig, the noninvasive diffuse microcotyledonary placenta of the horse, which exhibits a temporary invasion of the trophoblast into the endometrium for endometrial cup formation and the cotyledonary synepitheliochorial placenta, where binucleate trophoblast cells invade and fuse with uterine epithelial cells or syncytia, as seen in cow and small ruminants respectively. The description of the endotheliochorial placenta is mainly confined to the mink and the cat, which have an incomplete and a complete zonary placenta respectively.
