mplantation[edit]
Main article: Implantation (human embryo)
Trophoblast differentiation
After ovulation, the endometrial lining becomes transformed into a secretory lining in preparation of accepting the embryo. It becomes thickened with its secretory glands becoming elongated, and is increasingly vascular. This lining of the uterine cavity (or womb), is now known as the decidua and it produces a great number of large decidual cells in its increased interglandular tissue. The trophoblast then differentiates into an inner layer, the cytotrophoblast and an outer layer, the syncytiotrophoblast. The cytotrophoblast contains cuboidal epithelial cells having cell boundaries and are the source of dividing cells and the syncytiotrophoblast is a layer without cell boundaries.
The syncytiotrophoblast implants the blastocyst in the decidual epithelium, by projections of chorionic villi forming the embryonic part of the placenta. The placenta develops once the blastocyst is implanted, and forms to connect the embryo to the uterine wall. The decidua here is termed the decidua basalis and lies between the blastocyst and the myometrium and forms the maternal part of the placenta. The implantation is assisted by hydrolytic enzymes that erode the epithelium. The syncytiotrophoblast also produces human chorionic gonadotropin (hCG), a hormone that stimulates the release of progesterone from the corpus luteum. Progesterone enriches the uterus with a thick lining of blood vessels and capillaries so that it can sustain the developing embryo. The villi begin to branch and contain blood vessels of the embryo. Arteries in the decidua are remodelled to increase the maternal blood flow into the intervillous spaces of the placenta, allowing gas exchange to take place as well as the transfer of nutrients to the embryo. Waste products from the embryo will diffuse across the placenta.
As the syncytiotrophoblast starts to penetrate the uterine wall, the inner cell mass (embryoblast) also develops. The inner cell mass is the source of embryonic stem cells, which are pluripotent and can develop into any one of the three germ layer cells.