Effects of Ca2+ ions on the formation of metaphase chromosomes and sperm pronuclei in cell-free preparations from unactivated Rana pipiens eggs.

Nuclei transplanted into unactivated amphibian eggs are known to condense into metaphase chromosomes whereas those transplanted into activated eggs decondense and enlarge. We have made cell-free cytoplasmic preparations from Rana pipiens eggs which can induce demembranated Xenopus laevis sperm to undergo changes similar to those seen in intact eggs. Sperm chromatin which is incubated for 3 hr in unactivated egg preparations made using a buffer containing 3 mM EGTA is induced to form metaphase chromosomes. However, decondensed interphase nuclei are formed when chromatin is incubated in unactivated egg preparations made without EGTA as well as in activated egg preparations. When Ca2+ ions are added to unactivated egg preparations made with EGTA, the preparations lose the ability to induce metaphase chromosome formation and become capable of decondensing sperm chromatin. Once the ability to decondense chromatin has developed, either in unactivated or activated egg preparations, it cannot be suppressed by the addition of EGTA. However, decondensation of sperm chromatin in activated egg preparations can be suppressed by the addition of unactivated egg preparations made with EGTA. In this case, the incubated sperm chromatin is induced to form metaphase chromosomes. These results may indicate that the chromosome condensation activity of unactivated egg cytoplasm can be sustained in cell-free preparations when Ca2+ ion levels are kept low, but when Ca2+ ion levels increase this activity is lost and replaced by a new activity which can decondense chromatin. Since this change in cytoplasmic activities is comparable to that occurring in the intact egg following fertilization, these results suggest that Ca2+ ions play a crucial role during activation in altering the cytoplasmic activities which control nuclear behavior.