Blastocoeli

Blastocoeli (singular: blastocoel) is the fluid-filled interior cavity of the blastula, the hollow stage formed during early embryogenesis after fertilization and cleavage. It is bounded by the outer cell layer of the blastula (the blastoderm or trophoblast, depending on species) and contains blastocoelic fluid.

Formation and structure: Following cell divisions that produce a ball of cells from the zygote, the solid morula develops into a hollow sphere—the blastula. Accumulation of fluid into the interior creates the blastocoel. The size and presence of the blastocoel vary among taxa, influenced by yolk content and cleavage pattern. In mammals, the stage with a blastocoel is called the blastocyst; the cavity is the blastocyst cavity and is lined by the trophoblast with the inner cell mass attached at one pole.

Function: The blastocoel provides a physical space for cell movements during gastrulation and serves as a reservoir for ions, nutrients, and signaling molecules that influence patterning.

Developmental significance: The formation of the blastocoel is a key milestone in early development, enabling cellular rearrangements that establish the body axes. It also marks the transition from cleavage to gastrulation, when germ layers form.

Clinical and research relevance: In reproductive technologies, the presence and quality of a blastocoel or blastocyst cavity are used to assess embryo viability for transfer.