Increased free Ca2+ levels delay the onset of mitosis in fertilized and artificially activated eggs of the sea urchin

Increasing free Ca2+ in the cytoplasm of fertilized and NH4OH- or A23187-activated eggs of Strongylocentrotus purpuratus and Lytechinus variegatus resulted in significant delays in the onset of mitosis. The increased Ca2+ levels were induced by addition of the calcium ionophore A23187 to suspensions of fertilized and activated eggs, or by fertilizing NH4OH-activated eggs. The delays may represent a temporary inhibition of the activation and activities of the mitotic proteins. It is postulated that relatively high free Ca2+ levels in the cytoplasm prevents activation of the mitotic proteins. Active Ca2+ sequestration is therefore suggested as an essential factor in the onset and progression of early mitosis.