Difference Between Amnion and Chorion

Both amnion and chorion are extra embryonic membranes that protect the embryo and provide it with nutrients for the growth and development throughout the intrauterine life. Amnion is the inner layer that surrounds the amniotic cavity while chorion is the outer layer that covers amnion, yolk sac and the allantois. This article points out the differences between amnion and chorion with regard to their development, location and the functions.

Amnion

As mentioned above, amnion is an extra embryonic membrane that lines the amniotic cavity. It consists of two layers, where the outer most layer is formed from the mesoderm, and the innermost layer is formed from the ectoderm. Once it is formed at the early pregnancy, it is in contact with the body of the embryo, but 4-5 weeks later amniotic fluid begins to accumulate in between the two layers forming amniotic sac. Amnion does not contain any vessels or nerves but do contain a significant amount of phospholipids as well as enzymes involved in phospholipid hydrolysis.

Initially amniotic fluid is mainly secreted from the amnion, but by about 10th week of gestation, it is mainly a transudate of the fetal serum via the skin and the umbilical cord. Amniotic fluid volume increases progressively, but towards the end of pregnancy, there is a rapid fall in the volume. The main functions of the amniotic fluid are to protect the fetus from mechanical injury, permit movement of the fetus and prevent contractures, aid in the development of the fetal lung and prevent adhesion formation between fetus and amnion.

Amnion is present in birds, reptiles and in mammals.

Chorion

Chorion is an extra embryonic membrane that covers the embryo and the other membranes. It is formed from extra embryonic mesoderm with two layers of trophoblasts. As in amnion it does not contain any vessels or nerves but contain a significant amount of phospholipids and enzymes involved in phospholipid hydrolysis.

The chorionic villi, which are finger like processes that emerge from the chorion, invade the endometrium and entrusted with the task of transferring nutrients from mother to the fetus. Chorionic villi consist of two layers, where the outer layer is formed from the trophoblasts, and the inner layer is formed from the somatic mesoderm. These chorionic villi get vascularized from the mesoderm that carrying branches of the umbilical vessels. Until the end of the second trimester, villi covering the chorion are uniform in size but later they develop unequally.

It contributes the formation of the placenta.