Symposium: reproduction in baboons. From blastocyst to placenta: the morphology of implantation in the baboon

Implantation and placentation in the baboons share many morphologicalfeatures with other primates, as well as having some specificdistinctions. The ability to use deturgescence of the sex skinas a method of timing ovulation and the ease with which theuterine lumen can be flushed have been used to examine morphologicalaspects of blastocyst differentiation and implantation in thisspecies. Preimplantation blastocysts were obtained by non-surgicalflushing of the uterus 68 days after ovulation, and implantationsites were excised from uteri removed on days 10-16 of gestation.All tissues were prepared for electron microscopy by aldehydefixation and plastic embedding. Maturation of trophoblast fromthe compacted morula stage to the expanded blastocyst stageincludes increase in numbers of polyribosomes, changes in conformationof mitochondria, and development of an effective endocytic apparatus.An endodermal layer forms beneath the inner cell mass priorto loss of the zona pellucida, and parietal endodermal cellsextend beyond the inner cell mass. Azonal blastocysts have regionsof syncytial trophoblast adjacent to the inner cell mass, andthey may represent adhesion stages of early implantation. Inearly postimplantation stages, trophoblast replaces the uterineepithelium and processes of syncytial trophoblast invade dilatedsuperficial maternal vessels. In subsequent lacunar stages thereis rapid elevation of the developing conceptus above the uterinesurface as the lacunae enlarge. Cytotrophoblast rapidly entersmaternal vessels, and arterioles are partially or completelyoccluded by migrating cytotrophoblast. The early access to controlledmaternal blood flow apparently allows trophoblastic lacunaeto expand superficially as opposed to more extensive endometrialinvasion.