What is the name of the avascular layer of chorion?

The avascular layer of chorion is known as the chorion laeve. This layer is smooth and does not have the distinct structures or projections that are found in other parts of the chorion, such as the chorion frondosum.

Chorion laeve is the part of the chorion that contributes to the membrane surrounding the embryo but does not play a direct role in the attachment to the uterine wall or in placental function. Its avascular nature differentiates it from the chorion frondosum, which contains chorionic villi and is actively involved in nutrient exchange with maternal blood.

Understanding the distinct anatomical roles of these components helps clarify their significance in maternal-fetal health and development. The decidua basalis, which is part of the maternal tissue, and the trophoblast, which is the outer layer of the blastocyst involved in implantation and in forming the placenta, are separate structures that do not describe the avascular layer of the chorion.