The placenta is a fetal organ, responsible for nutrient and gas exchange between the mother and fetus throughout pregnancy. At day 6.5, gastrulation begins in the posterior region of the embryo. Pluripotent cells located in the epiblast will produce the three germ layers of the embryo and germ cells, as well as the extraembryonic mesoderm of the yolk sac, amnion, and allantois. Eight days after fertilization, the trophoectoderm differentiate into two layers, cytotrophoblast and syncytiotrophoblast. This specialized cell type attach to the maternal endometrium and invade into it differentiating in villous trophoblast and extravillous trophoblast.
The villous trophoblastic layer consists of the outer syncytiotrophoblast, and a subjacent layer of proliferative cytotrophoblast cells resting on a fine lamina basalis. The ultrastructure of the syncytiotrophoblast shows minor modifications between 6 and 11 weeks of gestation, but there is a progressive decrease in the prominence of the cytotrophoblast cells, which by the end of the 10 week form a discontinuous layer. From 6 weeks of gestation, some capillaries are already in close proximity to the trophoblastic basal lamina.