Practice question

Blastocyst cavitation requires transepithelial fluid transport generating blastocoel. Outer trophectoderm cells differentiate into polarized epithelium sealed by tight junctions expressing Na/K ATPase pumps on basolateral membranes. Active pumping of sodium ions into nascent intercellular spaces creates electrochemical and osmotic gradient driving water influx via aquaporins 3 and 9, inflating cavity. Potassium is pumped oppositely, calcium important for adhesion not flux, chloride follows passively. Inhibition of Na/K pump with ouabain prevents blastocoel expansion, embryo remains morula. Hence sodium flux provides driving force for mammalian blastulation linking metabolic energy to morphogenetic cavity formation essential for lineage segregation and implantation readiness.