During implantation the trophoblast differentiates itself into two layers:
the syncytiotrophoblast (ST)
the cytotrophoblast (CT)
The syncytiotrophoblast forms an external layer without intercellular boundaries (syncytium) and its cells form cords infiltrating the endometrium. The syncythiothrophoblast derives from the fusion of cytotrophoblast cells. Through apoptosis (programmed cell death) of uterine stromal cells spaces are created through which the blastocyst penetrates further into the endometrium.
The cytotrophoblast, on the other hand, consists of an irregular layer of ovoid, mono-nucleated cells and lies directly below the syncytiotrophoblast.
In the middle of the 2nd week irregular spaces appear in the syncythiothrophoblast. They then come together to form lacunae. These lacunae are originally filled with tissue fluids and uterine secretions.
The ST grows more and more into the uterine wall and erodes the vessel walls (capillaries, venules). The blood that circulates in them is delivered into the lacunae (stage 5c).