extraembryonic

Extraembryonic refers to tissues and membranes that develop from the embryo but extend outside the body of the developing embryo. These structures support growth and development before and after implantation, and they vary across vertebrate groups.

In most amniotes, the embryo forms two extraembryonic lineages: the trophoblast, which gives rise to the chorion and contributes to the placenta, and the hypoblast/primitive endoderm, which contributes to the yolk sac. Through development, several membranes form: the amnion surrounds the embryo in a fluid-filled cavity; the chorion envelopes the embryo and interacts with maternal tissues; the allantois arises as an outpocketing of the gut and participates in gas exchange and waste storage; the yolk sac provides nutrient transfer early in development before placental blood flow is established.

In the eggs of birds, reptiles, and monotremes, these membranes manage nutrients, gas exchange, and waste. In marsupials and placental mammals, the placenta forms from chorionic tissue and maternal decidua, enabling sustained fetal support; the yolk sac remains during early gestation, and the allantois contributes to placental structures and later forms part of the urinary system via the urachus.

Extraembryonic tissues are essential for proper development; abnormalities can lead to pregnancy complications or embryonic lethality in some species. They are a central topic in developmental biology, comparative anatomy, and medicine.