Sperm Receptivity in Sea Urchin Oocytes and Eggs

An electrophysiological method for studying the receptivity to spermatozoa of sea urchin eggs and oocytes is described. Pairs of adjacent oocytes and eggs, with intact jelly layers, were impaled and simultaneously exposed to a known concentration of spermatozoa. Two parameters were studied. The time from insemination to the first successful collision (as indicated by a step depolarization across the egg plasma membrane) and the total number of successful collisions. Sperm densities of 10ml~ were used, which ensured almost immediate interaction between several thousand spermatozoa and each female gamete. In all cases, under control conditions, the oocyte was more receptive to spermatozoa than was the egg, giving rise to the first electrical event at about 5 s post-insemination, compared to about 13 s for the egg. In addition, whereas 10-15 spermatozoa usually entered the oocyte, only one entered the egg. The low receptivity of eggs appears to be independent of resting potential. Cytoplasmically immature eggs or mature eggs briefly exposed to nicotine, strychnine, choline or Tris tend to be polyspermic and have comparable receptivity to oocytes. The data suggest that there are a limited number of viable interaction sites on the oocyte surface and that during cytoplasmic maturation these sites are rendered less receptive. In the mature egg there may be one preferential sperm entry site. This hypothesis is consistent with the experimental data and would explain why sea urchin eggs are usually monospermic at high sperm densities.