extraembionario

Extraembionario refers to structures or tissues that develop outside of the embryo proper during embryonic development. These structures play crucial roles in supporting and nurturing the developing embryo, and in some cases, are essential for its survival and growth.

The primary extraembryonic membranes in amniotes (reptiles, birds, and mammals) are the amnion, chorion, allantois, and yolk sac. The amnion forms a fluid-filled sac that cushions and protects the embryo from mechanical shock and desiccation. The chorion contributes to the formation of the placenta in mammals and plays a role in gas exchange. The allantois serves as a repository for metabolic waste products and also participates in gas exchange, especially in reptiles and birds. The yolk sac contains the yolk, which is the primary source of nutrients for the developing embryo in many species, particularly birds and reptiles, and in early mammalian development.

In placental mammals, the chorion and the allantois combine to form the chorioallantoic placenta, a vital organ for nutrient and gas exchange between the mother and the fetus, as well as for waste removal. The yolk sac is less prominent in placental mammals after the placenta is established, but it is important in early development for the formation of blood cells.

The development and specific functions of extraembryonic structures can vary significantly across different vertebrate groups, reflecting diverse reproductive strategies and adaptations to various environments. Understanding these tissues is fundamental to comprehending the intricacies of embryonic development and the establishment of a viable pregnancy.