ABSTRACT
Viviparity is a defining characteristic of the eutherian or placental mammals. Evolution of this trait required extensive modification of the ovary,1 as well as the uterus and the fetal membranes, which underwent modifications resulting in the establishment of the placenta.2 The mammalian evolutionary pressure toward placental formation resulted in significant variation among the mechanisms of implantation, embryo invasion, and placentation.3 The simplest mechanism, resulting in the least intimate maternal-fetal interaction, is known as epitheliochorial placentation. It comprises apposition and attachment of the embryonic chorion to the endometrial epithelium and is exemplified by the placentation in the pig, with variations in other hoofed mammals. In equids, for example, there is invasion of the specialized trophoblast cells of the chorionic girdle to form the endometrial cups.4 In ruminants, specialized binucleate trophoblast cells invade the epitheliochorial interface and fuse with the endometrial epithelium.4 A closer association of mother and fetus occurs in the endotheliochorial placental subtype, where the trophoblast erodes or bypasses the endometrial epithelium to associate directly with the maternal capillaries.5 This placentation is found in species and groups as diverse as the mustelids, pinnepeds, shrews, squirrels, and elephants.3
The most intimate of interactions, hemochorial placentation, is present when the trophoblast is in contact with maternal blood, following erosion of both the endometrial epithelium and the vascular endothelium. Versions of this subtype are found in primates, rodents, bats, and rabbits, among others.3 Comparative morphological investigations across mammalian taxa have provided some surprises. The presence of trophoblast invasion, erosion of maternal epithelium, and endotheliochorial placentation in insectivores, believed to be the most primitive mammals, suggests that the least invasive subtype, epitheliochorial placentation, is a secondary specialization.3 Moreover, there are variations in invasiveness of trophoblast
