ABSTRACT
Morphological changes in the endometrium of species with epitheliochorial and endotheliochorial types of placenta are of great interest, as adaptation to an intimate contact between the uterine epithelium and the conceptus has had to evolve in order to meet the needs of the conceptus. The endometrium passes 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 circulations in the interhemal barrier is also needed in order to obtain the best possible exchange of oxygen and carbon dioxide. In these types of placenta a 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 suppresses undesirable maternal immune reactions, whereas the endometrium appears to regulate and control the degree of invasion. The placenta must develop rapidly to achieve the capacity to act as a respiratory, intestinal, and endocrine system. The interplay between regulatory factors that are important for placentation varies among species. Three epitheliochorial systems are considered here:
• the non-invasive diffuse placenta of the pig • the non-invasive diffuse microcotyledonary pla-
centa of the horse, which exhibits a temporary invasion of the trophoblast into the endometrium for endometrial cup formation
• 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.
