Early implantation events in the baboon (Papio anubis) with special reference to the establishment of anchoring villi.

The development of the baboon anchoring villus has been studied from day 14 to day 48 of gestation, using light and electron microscopy. At day 14, cords of trophoblast could be seen streaming into the endometrium, invading maternal vessels and forming blood-filled lacunae; by 20 days gestation some of these had differentiated into distinctive anchoring villi, with an outer covering of syncytiotrophoblast and inner cytotrophoblast cells which differed from those of floating villi in that a subpopulation detached from the syncytium to form an interconnecting network of cells within the centre of the villus. Subsequent migration of cytotrophoblast into the endometrium formed the cytotrophoblastic shell while fibrillin-like extracellular matrix biosynthesis within the body of the villus provided a firm mechanical support. At the trophoblast-decidual interface, a zone of necrosis and phagocytosis initially developed, which became less extensive with time, so that by 40 days a stable interface was evident with only residual pockets of necrosis. During this period, there was differentiation of decidual cells which by 28 days developed characteristic pedunculated cell processes, and later became surrounded by a basal lamina. The factors that control detachment of cytotrophoblast from the syncytium and the biosynthesis of the specialized, fibrillar extracellular matrix, features that are not apparent in other placental villi, require further investigation, possibly by carefully controlled in vitro experimentation.