Exocoelomic

Exocoelomic refers to structures associated with the exocoelom, an early body cavity of the developing embryo. In humans and other mammals, the exocoelomic membrane, formed by hypoblast cells, lines the inner surface of the cytotrophoblast and encloses the exocoelomic cavity. This cavity lies between the exocoelomic membrane and the cytotrophoblast and is commonly described as the primitive yolk sac or exocoelomic sac in early development.

Formation and development: During the early stages of implantation, hypoblast cells migrate to cover the inner aspect of the cytotrophoblast, creating the exocoelomic (Heuser’s) membrane. The space beneath this membrane constitutes the exocoelomic cavity. As development proceeds, extraembryonic mesoderm differentiates and splits to form the extraembryonic coelom, surrounding the amnion and chorion. The exocoelomic membrane continues to line the exocoelomic cavity, which functions as an initial yolk-sac structure. Over time, the primitive yolk sac evolves into the definitive yolk sac, and the exocoelomic cavity diminishes as placental circulation becomes established.

Function and significance: The exocoelomic structures provide an early source of nutrition before placental circulation is established and participate in the initial formation of the yolk sac, which is important for primitive hematopoiesis in the early embryo. In many modern texts, the term exocoelomic is largely historical, with emphasis placed on the Heuser’s membrane, the primitive yolk sac, and the progressive development of the definitive yolk sac.

See also: Heuser's membrane; primitive yolk sac; extraembryonic coelom; yolk sac development.