ABSTRACT
Those primate species in which implantation has been studied extensively share many common features that differ widely from features of implantation in other species. Orientation of the inner cell mass (ICM) towards the uterine epithelium, formation of a trophoblastic plate after epithelial penetration, differentiation of the initial invasive syncytial trophoblast into microvillous absorptive syncytium-lining clefts, and establishment of a lacunar stage followed by formation of primary villi are all common features of implantation in the baboon, macaque, and human. Formation of the amnion by cavitation, formation of a secondary yolk sac, and formation of the exocelom and extraembryonic mesoderm from the primary yolk sac hypoblast are also common features of these species. There are, however, differences among species
involving the amount of the sphere of trophoblast involved in invasion of the endometrium, the rapidity with which superficial vessels are breached, and the distance of invasion into the endometrium before establishing the lacunar stage. In the baboon, trophoblast rapidly establishes communication with enlarged maternal vessels, and consequently forms lacunae by growth above the level of the endometrial surface. The macaque blastocyst, which is the largest of the three, establishes a secondary placenta on the opposite endometrial surface after the initial invasion. In the human, trophoblast appears to invade into the endometrium more deeply before penetrating maternal vessels, all of the sphere of trophoblast is involved in syncytium and cytotrophoblast formation, and individual cytotrophoblast cells invade in great abundance into the endometrial stroma. The human endometrium decidualizes more rapidly
than the other species but lacks the plaque response, which is well developed in the macaque and less well developed in the baboon. Understanding both the similarities and dissimilarities is essential in considering whether or not data obtained from one species may be relevant to similar species.
