extraembrionário

The term *extraembrionário* refers to structures and tissues that develop outside the embryo itself during early vertebrate embryogenesis, playing crucial roles in nourishment, protection, and support until the fetus becomes fully independent. These structures are essential for the proper development of the embryo within the mother’s uterus, particularly in mammals, though similar formations exist in other vertebrates.

The most prominent extraembrionário structures include the **chorion**, **amnion**, **yolk sac**, and **allantois**. The **chorion** is an outer membrane that facilitates gas exchange and contributes to the formation of the placenta in mammals, enabling nutrient and waste exchange between the mother and embryo. The **amnion** is a fluid-filled sac that cushions the embryo, maintaining a stable environment and preventing desiccation. The **yolk sac** (in mammals, it is vestigial but still involved in early blood cell formation) stores nutrients and aids in early circulation. The **allantois**, primarily found in reptiles, birds, and early mammals, functions in waste storage and, in some species, contributes to gas exchange.

In humans and other placental mammals, the placenta itself is an extraembrionário organ formed by the fusion of chorionic villi with maternal tissues, ensuring the transfer of oxygen, nutrients, and hormones while filtering waste products. These structures collectively ensure the embryo’s survival and development before birth, after which they are typically absorbed or reabsorbed by the maternal body.

The study of extraembrionário structures is fundamental in developmental biology, offering insights into evolutionary adaptations, reproductive strategies, and congenital disorders. Their absence or malformation can lead to complications such as miscarriage or birth defects, highlighting their indispensable role in vertebrate reproduction.