During the development of a fetus, there are two membranes that separate it from the mother. One of these is called the chorion, which surrounds both the fetus itself and all other membranes within the womb. Chorionic villi originate from the chorion and are used to increase the surface area available to the fetus for the transfer of nutrients between it and the mother. The chorion is made up of an inner and outer layer and also aids in creating the placenta.
The membrane's outer layer is made up of two parts — the ectoderm and the trophoblast. Primarily, the trophoblast’s purpose is to provide the fetus with nutrients during its growth. The ectoderm, on the other hand, later develops into various parts of the body including the teeth and nervous system. During pregnancy, most of the chorion is not in contact with the fetus and hence the villi in this area will degrade. The parts that are in contact, however, will retain the villi.
In between the outer layer and the amnion is the inner layer made up of the somatic mesoderm. During the development of the fetus, the mesoderm plays an important role as it changes to create parts of the body that include the bones and muscles. It also later becomes a layer of the skin and can retain some of its abilities to change purpose.
There are several stages of chorion development. The current stage is determined by the composition of the tissue. During the primary stage, only the trophoblast is present. The secondary stage includes the mesoderm while by the time the tertiary stage begins, blood vessels have penetrated the tissue.
The amnion is the other membrane in the womb during pregnancy. Its purpose is to provide protection to the developing embryo and is found in many other animals including all mammals and birds. The amnion is in contact with the somatic mesoderm and is used to transfer nutrients to the developing fetus.
Chorionic villi are used to provide the maximum area for the fetus to exchange nutrients and other substances with the mother. Blood is carried through the villi where it’s replenished before returning to the fetus. For this reason the villi provide a vital purpose during the development of the baby. There is an even covering of villi across the fetal chorion, but after a few months of pregnancy it starts to develop unevenly.