. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 284 H. SCHUEL when the oolemma that had initially depolarized to +20 mV is gradually repolarizing to the level (-70 mV) characteristic of the fertilized egg (Fig. 8). During normal fertilization in Arbacia elevation of the fertilization envelope (cortical reaction) is completed by 60 s after insemination, long before the rapid electrical (sodium de- pendent) block decays to 50% of its effectiveness at 90 s. Thus sea urchin eggs have adapted two typical cellular responses to stimulation, electrical depolarization of the plasm
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 284 H. SCHUEL when the oolemma that had initially depolarized to +20 mV is gradually repolarizing to the level (-70 mV) characteristic of the fertilized egg (Fig. 8). During normal fertilization in Arbacia elevation of the fertilization envelope (cortical reaction) is completed by 60 s after insemination, long before the rapid electrical (sodium de- pendent) block decays to 50% of its effectiveness at 90 s. Thus sea urchin eggs have adapted two typical cellular responses to stimulation, electrical depolarization of the plasma membrane and exocytotic release of stored secretory products, to operate in tandem to assure monospermic fertilization (Schuel and Schuel, 1981). Presently available data suggests that the egg's fertilization potential regulates sperm-egg membrane fusion by blocking insertion/translocation into the oolemma of potential-sensitive components of the sperm's plasma membrane (Gould-Somero and Jaffe, 1984). Studies of cross species fertilization using voltage clamped sea urchin {Strongylocentrotus purpuratus) and Urechis caupo (a echiuroid worm) eggs showed that the blocking voltage for fertilization in these crosses is determined by the sperm species (Jaffe et ai, 1982). The electrical block to fertilization in Lytechinus eggs clamped at +15 mV can be overcome by brief repolarization to —60 mV for 35-45 ms (Shen and Steinhardt, 1984). These findings show that a sperm attached to the surface of an unfertilized egg can complete its electrically sensitive insertion/transfer process into the egg membrane within milliseconds. Finally, sperm can activate but will not penetrate Lytechinus eggs whose membranes are clamped at potentials more negative than —30 mV (Lynn and Chambers, 1984). The activating sperm is detached from the surface of these eggs as the fertilization envelope elevates. Hence, the oolemma T>20 100 r- O O X E ^ »- o o p CL f- O CL. c 0) c o E -90 -240 Time (sec)
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