Sperm nuclear decondensation in Barnea candida (mollusca, pelecypoda) oocytes does not require germinal vesicle breakdown

Intracellular pH change does not appear as a prerequisite for triggering activation of Barnea candida (Mollusca, Pelecypoda) oocytes

Barnea candida oocytes, exposed to excess KCl, ammonia, or digitonin, exhibit germinal vesicle breakdown (GVBD) and reinitiate meiosis, at least up to first polar body extrusion. While we confirm that KCl- but not ammonia-induced activation required external calcium, our findings that digitonin is effective at any pH from 6 to 8, in the presence of calcium, while the phorbol ester TPA and diacylglycerol fail to reinitiate meiosis, strongly suggests calcium as the main trigger for this process. Preliminary experiments using the fluorescent probes fluorescein diacetate and Quin 2/AM show, moreover, that KCl and ammonia produce both an intracellular calcium surge (30 nM) and a slight alkalinization of the intracellular cytoplasm from 7.84 to 8.05.

The germinal vesicle material required for sperm pronuclear formation is located in the soluble fraction of egg cytoplasm

The chromatin of Xenopus laevis sperm nuclei was induced to decondense, swell and form mitotic chromosomes following its injection into mature Rana pipiens oocytes. In contrast, the sperm chromatin did not decondense or form mitotic chromosomes when injected into oocytes from which the germinal vesicle (GV) was removed prior to the initiation of maturation. Injection into enucleated oocytes of the material extracted from manually-isolated GVs restored their ability to decondense sperm nuclei. This soluble GV material was stable at 18 degrees C for 16 h but was inactivated by heating to 80 degrees C for 10 min. We examined the distribution of this GV material in a cytoplasmic preparation from activated eggs which can induce sperm pronuclear formation in vitro. The cytoplasmic preparation was separated into soluble and particulate fractions by centrifugation and then each fraction was injected into enucleated eggs to determine whether or not it restored the ability to decondense sperm nuclei. We found that the soluble, but not the particulate fraction could restore the ability to decondense sperm nuclei to enucleated oocytes. This result clearly indicates that the soluble fraction contains most of the GV material required for chromatin decondensation. However, since the soluble fraction fails to decondense sperm chromatin in vitro in the absence of material from the particulate fraction, sperm pronuclear formation appears to require both the soluble material derived from the GV and particulate material which can develop in the oocyte cytoplasm in the absence of the GV.

Possible involvement of calmodulin in maturation and activation of Chaetopterus eggs

We report the isolation of calmodulin from oocytes of Chaetopterus pergamentaceus. The identification of this protein is based on (1) activation of beef heart cAMP phosphodiesterase, (2) heat stability, (3) sensitivity to chlorpromazine, and (4) electrophoretic mobility identical to that of porcine brain calmodulin after sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the presence of either Ca2+ or EGTA. We treated oocytes with chlorpromazine and W-7 to investigate the involvement of calmodulin in meiosis initiation and egg activation. Very low concentrations of chlorpromazine inhibited germinal vesicle breakdown (GVBD). This effect was shown to be dependent upon bright indirect light, since the drug was much less effective at GVBD inhibition under conditions of very low illumination. Higher concentrations of chlorpromazine and W-7 (100 microM) inhibited GVBD and activated eggs with intact germinal vesicles as determined by fertilization envelope formation and the onset of ameboid activity. Neither egg activation nor inhibition of calmodulin stimulation of phosphodiesterase activity in vitro was affected by light. These results are consistent with a role for calmodulin in egg activation and GVBD, but suggest that chlorpromazine in bright light may prevent GVBD by some mechanism other than calmodulin inhibition.

Sperm nuclear dispersion coordinate with meiotic maturation in fertilized Spisula solidissima eggs

Investigations were carried out with fertilized Spisula solidissima eggs, in which changes in incorporated sperm nuclei were determined by measurement of the diameter of dispersing paternal chromatin. Results of such an analysis demonstrated that sperm nuclear dispersion does not proceed at a constant rate and consists of four phases (1-4), coordinate with major changes in the status of the maternal chromatin. (1) The first phase was a short lag period prior to germinal vesicle breakdown in which the size of the sperm nucleus increased only slightly. (2) This was followed by a rapid dispersion of the sperm nucleus coordinate with germinal vesicle breakdown. With the development of the first meiotic spindle, sperm chromatin dispersion slowed dramatically; this phase (3) lasted until the completion of the meiotic divisions at which time the sperm chromatin underwent a second rapid increase in size (4) that was correlated with development of the female pronucleus. When zygotes were treated with agents that inhibited germinal vesicle breakdown (verapamil, sodium-free seawater, and chloroquine), sperm nuclear dispersion did not occur. Evidence is presented indicating that nucleocytoplasmic interactions coincident with germinal vesicle breakdown induce sperm nuclear dispersion in Spisula zygotes.