Essential ions for compaction:
Compaction transforms loosely adherent eight-cell mammalian morula into tightly apposed epithelium-like structure, prerequisite for blastocyst formation. Process depends critically on extracellular calcium enabling homophilic interaction of E-cadherin (uvomorulin) between blastomeres. Calcium stabilizes cadherin extracellular domains, allowing adherens junction formation and recruitment of catenins and actin cytoskeleton. This strengthens intercellular adhesion, induces cell polarization, peripheral microvilli loss and basolateral sealing. Chelation of calcium with EGTA reverses compaction, demonstrating calcium-cadherin-catenin axis indispensability for morula polarization and subsequent lineage specification via Hippo pathway.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Morula compaction - E-cadherin, calcium dependence and Hippo signaling polarization.