File:Placenta anchoring villi.jpg

Placenta anchoring villi

Histological image showing the junctional region between the trophoblast shell of the conceptus and the maternal decidua.

In week 2, the trophoblast shell cells proliferate and form a syncitiotrophoblast and cytotrophoblast layer around he conceptus. Syncitiotrophoblast cells migrate into the uterine wall, forming maternal blood-filled spaces (lacunae).

Placentation begins once the conceptus begins to implant in the uterine wall and the placenta will have both a fetal and a maternal component. The fetal component begins as villi form. The fetal chorion will form two regions: smooth chorion (chorion laeve) and villous chorion (chorion frondosum).

The maternal component is formed by the decidualization of the endometrium.

Image Source: UNSW Embryology, no reproduction without permission. Placenta Development