See development lecture notes from Gross Anatomy. The placenta mediates exchange between the mother and the fetus, and it is an endocrine organ . The blastocyst consists of the inner cell mass , which will form the embryo, and the trophoblast , which will contribute to the chorion, the fetal part of the placenta. It usually implants on the posterior superior wall of the uterus. Clinical problems can result if implantation is elsewhere.
Trophoblast cells attach to the uterine epithelium.
Spaces develop in the syncytial trophoblast which are termed lacunae . Erosion of maternal blood vessels in the surrounding endometrium by the growing trophoblast results in filling of the lacunae with maternal blood . Histological structure and function of the mature placenta. The villus is the unit of the placenta. The core of the villus is mesenchyme with fetal blood vessels, and the villus is covered by layers of cytotrophoblast and syncytiotrophoblast.