Abstract
The role of sodium ions in fertilization and development of sea urchin (Arbacia punctulata and Strongylocentrotus purpuratus) eggs was studied by culturing eggs and embryos in low sodium (choline-substituted) sea water. Hardening of the fertilization envelope was impaired in 19 mM Na+ as indicated by the collapse of this investment 30-60 min after insemination. Cross-linking of the fertilization envelope, assayed in terms of the onset of resistance to dispersal by isotonic urea, was not affected by low sodium. However, impregnation of the fertilization envelope by structural proteins derived from the egg's cortical granules did not take place in low sodium. The "I-T" transition in the configuration of the fertilization envelope in Strongylocentrotus from "igloo" shaped casts of microvilli that were at the surface of the unfertilized egg to sharp "tent" shaped spikes also was prevented in low sodium. Potassium and lithium effectively substituted for sodium in promoting the normal structuralization of the fertilization envelope, while choline and Tris did not. Fertilized eggs divided more slowly in low sodium than in normal (419-425 mM Na+) sea water. Arbacia embryos fertilized and cultured in low sodium sea water showed a reversible developmental arrest at the swimming blastula stage. This finding suggests that a sodium activated switch initiates gastrulation (differentiation) in sea urchins. Taken together, these results show that sodium is essential for several physiologic processes related to fertilization and developmental in sea urchins, in addition to the previously described rapid-electrical block to polyspermy and the coupling of early and late events in egg activation.
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