Chapter 9: Implantation & Placentation

Implantation & Placentation comprehensively details the critical processes of implantation and placentation, which establish the anatomical and physiological basis for maternal-fetal interaction. Implantation commences with the blastocyst's outer layer, the trophoblast, attaching to the endometrial luminal epithelium, initiating the decidual response. The trophoblast differentiates into three key lineages: the syncytiotrophoblast, a multinucleated endocrine layer covering the villi that secretes hormones like human chorionic gonadotrophin (hCG); villous cytotrophoblast cells, which are the proliferative germinative population; and extravillous trophoblast cells, which invade the maternal decidua and remodel the uterine spiral arteries. Successful implantation prompts the endometrium to transform into the decidua, distinguished into the decidua capsularis, decidua parietalis, and the decidua basalis, where the placenta forms. Placental development proceeds through the formation of primary, secondary, and anchoring villi. Crucially, during the first trimester, extravillous trophoblast cells plug the maternal spiral arteries, restricting true maternal blood flow and creating a physiologically low-oxygen environment that supports early development primarily through histiotrophic nutrition provided by uterine gland secretions. By postmenstrual week 12, these plugs dissipate, allowing maternal blood to bathe the villi in the intervillous space, establishing a fully haemochorial circulation. The definitive placenta consists of the fetal chorionic plate and the maternal basal plate. Chorionic villi function as the essential units for exchange, relying on processes like simple diffusion, facilitated diffusion, active transport, and receptor-mediated transcytosis to move gases, nutrients, and waste between the fetal and maternal circulations. Associated fetal membranes include the chorion, the early absorptive secondary yolk sac, the allantois (which gives rise to umbilical vessels), and the amnion, which eventually fuses with the chorion to form the chorio-amnion. The amniotic fluid contained within the amnion provides mechanical support and is derived from multiple sources, including fetal urine and lung secretions. The umbilical cord, which connects the conceptus to the placenta, is composed of Wharton's jelly (mesenchyme) and typically contains one umbilical vein and two umbilical arteries. Failures in trophoblast invasion or utero-placental remodeling are linked to severe complications of pregnancy such as pre-eclampsia and placenta accreta spectrum disorders.