A transient increase in intracellular calcium concentration [Ca2+](i)
occurs throughout the cell as sea urchin embryos enter anaphase of the
first cell cycle. The transient just precedes chromatid disjunction a
nd spindle elongation. Microinjection of calcium chelators or heparin,
an InsP(3), receptor antagonist, blocks chromosome separation. Photor
elease of calcium or InsP, can reverse the block. Nuclear reformation
is merely delayed by calcium antagonists at concentrations that block
chromatid separation. Thus, the calcium signal triggers the separation
of chromatids, while calcium-independent pathways can bring about the
alterations in microtubule dynamics and nuclear events associated wit
h anaphase progression. That calcium triggers chromosome disjunction a
lone is unexpected. It helps explain previous conflicting results and
allows the prediction that calcium plays a similar role at anaphase in
other cell types.