Chrorionic Villi
Chrorionic Villi

Chrorionic Villi

At term, the chorionic villli are filled with capillaries. The syncytiotrophoblast layer is still present but the cytotrophoblasts are mostly gone. Note the close apposition between the capillaries and syncytiotrophoblasts. How many layers does a molecule of oxygen have to traverse between maternal blood and a capillary in a villus?
Answer: The upper capillary would involve traversing 3 layers, because the basement membranes of the syncytiotrophoblast and capillary are fused. The layers are syncytiotrophoblast, basement membrane, and endothelial cell. The lower capillaries would involve 5 layers: the syncytiotrophoblast layer, its basement membrane, cytotrophoblast cells, the endothelial cell basement membrane, and the endothelial cells lining the fetal capillaries.