Calcium signals in and around the nucleus in sea urchin eggs

A transient rise in cytoplasmic Ca2+ activity in the sea urchin egg occurs during fertilization due to calcium release from an intracellular store. Using a combination of the Ca2+ sensitive dye Calcium Green dextran and the Ca2+ insensitive dye tetramethylrhodamine dextran we have obtained confocal ratio images of free cytoplasmic calcium distribution during the fertilization calcium wave. We can also trigger calcium release using calcium-releasing agonists such as InsP3, ryanodine and 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-temporal calcium signal observed after fertilization. In fact, a second calcium wave propagates through the egg as pronuclear migration begins; this wave also originates at the point of sperm entry. A global calcium increase is also recorded during the fusion of the male and female pronuclei. We conclude that calcium concentrations in the nucleus are similar 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 of pronuclear fusion.