Sources of calcium in egg activation: a review and hypothesis

A careful reanalysis of the literature indicates that the initial mechanism of activation in sea urchin eggs is remarkably similar to the mechanism established in medaka eggs: i.e., sea urchin eggs are activated by a qualitatively and quantitatively similar calcium explosion; one which is propagated in a wave sustained by the calcium-stimulated release of calcium from internal sources. These sources are probably in the endoplasmic reticulum. An exhaustive survey of the literature reveals that a wide variety of other activating eggs in the vertebrate line also exhibit secretory waves which are propagated at about 10 microns/sec, and can thus be assumed to reflect the same basic mechanism. Activating protostome eggs on the other hand do not exhibit such waves. This and other systematic differences from deuterostomes suggest that unlike deuterostome eggs, protostome eggs are primarily activated by calcium ions which enter the cytosol from the medium, and do so in response to depolarization of the egg's plasma membrane.