CALCIUM SIGNALS IN AND AROUND THE NUCLEUS IN SEA-URCHIN EGGS

A transient rise in cytoplasmic Ca2+ activity in the sea urchin egg oc curs during fertilization due to calcium release from an intracellular store. Using a combination of the Ca2+ sensitive dye Calcium Green de xtran and the Ca2+ insensitive dye tetramethylrhodamine dextran we hav e obtained confocal ratio images of free cytoplasmic calcium distribut ion during the fertilization calcium wave. We can also trigger calcium release using calcium-releasing agonists such as InsP(3), ryanodine a nd cADP-ribose. Calcium levels are in all cases similar within nucleus and in the cytoplasm. A striking result from confocal calcium imaging is that the fertilization calcium wave is not the only spatio-tempora l calcium signal observed after fertilization. In fact, a second calci um wave propagates through the egg as pronuclear migration begins; thi s wave also originates at the point of sperm entry. A global calcium i ncrease is also recorded during the fusion of the male and female pron uclei. We conclude that calcium concentrations in the nucleus are simi lar to those in the cytoplasm during these calcium transients, that a remnant at the point of sperm entry can originate a second propagating calcium wave and that a global calcium transient occurs at the time o f pronuclear fusion.