Practice question

Blastocyst cavitation depends on active transepithelial transport by trophoblast. Outer cells establish basolateral Na/K ATPase that pumps sodium into intercellular spaces, chloride follows via channels, creating osmotic gradient driving water influx through aquaporins AQP3 and AQP9. Tight junctions between outer cells sealed by ZO-1 prevent back leak, allowing fluid accumulation coalescing into single blastocoel cavity. Inhibitors of Na/K ATPase like ouabain block cavitation and collapse blastocyst. Sodium-driven osmosis therefore provides biophysical force creating blastocyst architecture essential for implantation preparation, lineage separation, nutrient distribution and embryonic patterning events.