Practice question

Blastocyst formation follows cleavage divisions that increase cell number without growth. Key intermediate step is compaction at eight to sixteen-cell stage where blastomeres maximize contacts via E-cadherin, forming compact morula. Compaction establishes polarity and positions outer cells as trophectoderm precursors and inner cells as pluripotent inner cell mass. Subsequent activation of Na/K ATPase on basolateral membranes pumps sodium into intercellular spaces, water follows osmotically creating blastocoel cavity. Though cleavage without growth and germ layer formation occur, compaction uniquely drives epithelialization essential for blastocyst architecture and implantation competence.