In humans, placental formation begins with implantation during the second week after fertilization. By the end of the third week the uteroplacental circulatory system has progressed far enough to provide gas and nutrient exchange between the mother and embryo. Embryonic contributions to the placenta 1. The tips of these maternal blood vessels have actually been eroded away by syncytiotrophoblastic proteases so that the blood goes into the lacunae, in which exchange of gases and nutrients takes place by diffusion. The embryonic circulatory system derives from the cytotrophoblast.
Chorionic villi develop as invaginations of the cytotrophoblast.
Next extraembryonic mesoderm fills in these villi. Lastly, embryonic blood vessels and blood cells arise in this mesoderm. The chorion then becomes highly vascularized, initially all around the embryo. Later, vascularization disappears from all regions except the chorion frondosum, which is near the connecting stalk. Maternal contributions to the placenta 1.