From blastocyst to placenta: the morphology of implantation in the baboon

Implantation and placentation in the baboon share many morphological features with other primates, as well as having some specific distinctions. The ability to use deturgescence of the sex skin as a method of timing ovulation and the ease with which the uterine lumen can be flushed have been used to examine morphological aspects of blastocyst differentiation and implantation in this species. Preimplantation blastocysts were obtained by non-surgical flushing of the uterus 6-8 days after ovulation, and implantation sites were excised from uteri removed on days 10-16 of gestation. All tissues were prepared for electron microscopy by aldehyde fixation and plastic embedding. Maturation of trophoblast from the compacted morula stage to the expanded blastocyst stage includes increase in numbers of polyribosomes, changes in conformation of mitochondria, and development of an effective endocytic apparatus. An endodermal layer forms beneath the inner cell mass prior to loss of the zona pellucida, and parietal endodermal cells extend beyond the inner cell mass. Azonal blastocysts have regions of syncytial trophoblast adjacent to the inner cell mass, and they may represent adhesion stages of early implantation. In early postimplantation stages, trophoblast replaces the uterine epithelium and processes of syncytial trophoblast invade dilated superficial maternal vessels. In subsequent lacunar stages there is rapid elevation of the developing conceptus above the uterine surface as the lacunae enlarge. Cytotrophoblast rapidly enters maternal vessels, and arterioles are partially or completely occluded by migrating cytotrophoblast. The early access to controlled maternal blood flow apparently allows trophoblastic lacunae to expand superficially as opposed to more extensive endometrial invasion.