Blastocoel formation depends on:
Blastocoel is fluid-filled cavity created within morula to form blastocyst. Its generation depends on establishment of functional epithelium by outer trophectoderm cells sealed by tight junctions. Basolateral Na/K ATPases actively transport sodium ions into intercellular space, creating osmotic gradient. Water follows via aquaporins, expanding microcavities into single blastocoel pushing inner cell mass eccentrically. Calcium is needed earlier for compaction and tight junctions, but sodium flux is direct driver of cavitation. Inhibition of Na/K pump with ouabain or removal of extracellular sodium blocks blastocoel formation, proving sodium gradient central to mammalian blastulation and embryonic fluid homeostasis.
Ref: Kim et al., Nat Comm 1998: Sodium pump mediated blastocoel formation in mammalian preimplantation embryos.