The cytotrophoblast layer
The cytotrophoblast of the anchoring villus expands until a further layer outside the syncytiotrophoblast arises, forming the cytotrophoblast layer. It slips in between the syncytiotrophoblast and the uterine endometrium.
Over the course of the 4th month the cytotrophoblast cells slowly disappear out of the villus wall and the chorionic plate. They persist, however, in the cytotrophoblast layer. The cytotrophoblast cells penetrate into the decidua and the myometrium and also colonize the wall of the spiral arteries close to their openings.
This invasion of the maternal vessels by the cytotrophoblast leads to the destruction of the smooth muscle layer and to a partial replacement of the endothelial cells. It is responsible for the change in elasticity of the spiral arteries, whereby the blood circulation of this fetoplacental unit is adapted to the rapid growth of the fetus. This phenomenon of cell exchange is absent in preeclampsia or an intra-uterine growth retardation.
An excessive proliferation of the cytotrophoblast can lead to tumor formation, especially to a .