Temporal sequence and spatial distribution of early events of fertilization in single sea urchin eggs.

Abstract

Measurements and observations of 5 early events of fertilization, singly and in pairs, from single sea urchin eggs have revealed the precise temporal sequence and spatial distribution of these events. In the Arbacia punctulata egg, a wave of surface contraction occurs coincident with membrane depolarization (t = 0). These 2 earliest events are followed by the onset of a rapid, propagated increase in cytoplasmic-free Ca at .apprx. 23 s as measured by Ca-aequorin luminescence. The luminescence reaches its peak value by 40 s after the membrane depolarization. The luminescence remains uniformly elevated for some time before its decay over several minutes. The onset of an increase in the pyridine nucleotide (NAD(P)H) fluorescence follows the membrane depolarization at .apprx. 51 s. The fertilization membrane begins its elevation in a wave-like fashion coincidentally with the increase in NAD(P)H fluorescence. Similar results are observed in the Lytechinus variegatus egg. While the increase in cytoplasmic-free Ca may be important for many changes occurring in the egg, the elevated-free Ca is not directly responsible for the propagated wave of cortical granule exocytosis.