Physiology of Implantation in Primates

Embryo implantation in primates relies on endocrine mechanisms that are distinct from those in nonprimate species. In all primate species studied to date, the embryo enters the uterus as a morula on day 3–4 after ovulation, hatches from the zona pellucida on day 6–8 and attaches to the maternal endometrial epithelium on day 8–9 in Old World species (1,2); day 11–12 in the marmoset monkey (3, 4) and probably day 6 in the squirrel monkey (5). The morphology of implantation is now well described for a few primate species, including the rhesus monkey (1, 6), baboon (7), and marmoset (8, 9). Comparisons with human implantation still depend largely on the classic studies of Hertig and Rock (10).