Practice question

Morula compaction depends on extracellular divalent cation calcium enabling functional conformation of E-cadherin adhesion molecules. Calcium binding rigidifies extracellular cadherin repeats facilitating trans-interaction between neighboring blastomeres, initiating formation of adherens junctions and recruitment of catenins to cortical actin. Chelation by EDTA or EGTA rapidly decompacts embryos, reversibly. Potassium, sodium or magnesium cannot substitute for calcium in this homophilic adhesion. Calcium influx also participates in downstream signaling for polarization including activation of protein kinase C, establishing apical-basal polarity. Thus calcium is irreplaceable ionic cofactor ensuring stable inter-blastomere adhesion during transition from loose cleavage stage to compact morula.