Spatiotemporal relationships among early events of fertilization in sea urchin eggs revealed by multiview microscopy

Four early events of egg fertilization, changes in intracellular calcium concentration and intracellular pH, reorientation of the surface membrane, and the elevation of the fertilization envelope, were imaged in real time and in pairs in single sea urchin eggs. The paired imaging allowed the correlation of the four events spatially and temporally. Three of them propagated as waves starting at the sperm entry site. The earliest was the calcium wave, visualized with fluorescent indicator eyes. After a delay of 10 s there followed a large decrease in the fluorescence polarization of membrane-bound dyes, which we interpret as arising from membrane reorientation as a result of cortical granule exocytosis and microvillar elongation. With a further delay of 15 s the fertilization envelope was seen to rise in transmitted light. All three waves propagated with similar velocities of ~10 μm/s, supporting the view that calcium triggers the latter two events. The fluorescence polarization changed in two steps with a clear pause of 10-20 s in between. The second step, which also propagated as a wave, reflects either further elongation of microvilli or straightening of irregular microvilli. This second step was abolished by cytochalasin B and was coincident with an increase in cytoplasmic pH, suggesting that pH- induced actin reorganization may play a role. The cytoplasmic alkalinization, imaged with a fluorescent probe, was quite different from the other events n that it took place homogeneously throughout the egg and slowly (over 100 s). Apparently, the alkalinization is not on a direct downstream pathway of calcium origin. An opposing possibility, that the alkalinization may in fact be triggered by the traveling calcium wave, is also discussed.