The placental barrier
The placental barrier is composed of structures that separate the maternal and the fetal blood .The makeup of thechanges over the course of the pregnancy.
In the first trimester it consists of the syncytiotrophoblast, the cytotrophoblast (Langhans' cells), the villus mesenchyma (in which numerousthat exhibit macrophage properties are found) and the fetal capillary walls.
During the 4th month the cytotrophoblast disappears from the villus wall and the thickness of the barrier decreases while the surface area increases (roughly 12 m2 towards the end of the pregnancy). In the 5th month the fetal vessels have multiplied their branches and gotten closer to the villus surface.
During the 6th month the nuclei of the syncytiotrophoblast group together in the so-called proliferation knots. The other zones of the syncythiothrophoblast lack nuclei and are adjacent to the capillaries (exchange zones).