The development of the primate placenta.

The top figure depicts rhesus monkey embryo development from fertilization through escape of the blastocyst stage embryo from the zona pellucida. Two weeks later, the basic structure of the placenta, the chorionic villi, have formed at the attachment (implantation) site of the placenta. The placenta continues to grow with increasing elaboration of the chorionic villi throughout pregnancy.

Below, left, is a histological section of this interface. The empty spaces will be filled with maternal blood in vivo. The components of the placenta include the chorionic plate (Chp), BV = branch villi, SV = stem villi, FV = floating villi, CC = cell column, and Tsh= trophoblastic shell. Dec = maternal decidua (specialized endometrium of pregnancy).