Ekstraembryonisia

Ekstraembryonisia refers to tissues and membranes that arise from the developing conceptus but are not part of the embryo proper. The term is used in some linguistic or regional contexts as a variant or translation of the standard English term extraembryonic. In English, the usual term is extraembryonic, and it describes structures that support the developing embryo without becoming part of the body of the embryo itself.

In vertebrates, the principal extraembryonic structures include the amnion, chorion, allantois, yolk sac, and, in mammals, the placenta. These structures support development by providing protection, nutrition, gas exchange, and waste management while the embryo grows. Formation begins early in development when cells associated with the embryo differentiate to form the membranes and related tissues. In amniotes, the amnion and chorion arise from extraembryonic ectoderm and mesoderm, while the yolk sac and allantois contribute to nutrient transfer and waste handling.

In mammals, the placenta evolves as a specialized organ that interfaces maternal and fetal tissues for nutrient uptake and gas exchange, while in birds and reptiles the yolk sac and allantois coordinate similar functions with the other membranes. The study of these tissues is central to developmental biology and reproductive medicine, as abnormalities of extraembryonic structures can lead to pregnancy complications and affect fetal development.