In the early stage of development the trophoblastic villi form all around the embryo and give it the appearance of a «».
During the 3rd month most placental villi disappear but remain present at the basal plate:
- The chorion gets here to be a villus-rich chorion (chorion frondosum), a major component of the placenta.
- At other locations, where the villi degenerate, the chorion becomes smooth (chorion laeve). At these locations, no exchange between the maternal and fetal blood circulation systems takes place.
The chorion laeve is formed from a chorion layer (consisting of extra-embryonic mesenchyma and cytotrophoblast).
In this stage all placental villi are tertiary villi. After the 9th week the tertiary villi lengthen through the proliferation of the terminal villus mesenchyma (see more info).