| Abstract: | The relationship between the timing of both sperm nuclear decondensation and male pronucleus formation in the oocyte and the relative level of disulfide bonds within the sperm nucleus was evaluated. Since reduction of sperm nuclear disulfide (S-S) bonds is a prerequisite for sperm nuclear decondensation in vitro and in vivo, we hypothesized that sperm nuclei with relatively few S-S bonds would require less time to decondense in the oocyte than sperm nuclei with higher numbers of S-S bonds, and that male pronucleus formation would occur more rapidly as well. Four types of hamster sperm nuclei, in which the extent of S-S bonding differed, were microinjected into hamster oocytes, and the time course of sperm nuclear decondensation and male pronucleus formation was charted. Cauda epididymal sperm nuclei, which are rich in S-S bonds, required 45-60 min to decondense. In contrast, nuclei containing few S-S bonds (namely sonication-resistant spermatid nuclei and cauda epididymal sperm nuclei treated in vitro with the S-S bond-reducing agent dithiothreitol) decondensed within 5-10 min of microinjection. Caput epididymal sperm nuclei, with intermediate S-S bond content, decondensed in 10-20 min. Regardless of when decondensation occurred, formation of the male pronucleus never preceded that of the female pronucleus, which occurred 1.25-1.5 h after microinjection. However, sperm nuclei with few S-S bonds were more likely than S-S rich nuclei to transform into male pronuclei in synchrony with the formation of the female pronucleus. We conclude that the timing sperm nuclear decondensation and pronucleus formation depends in part upon the S-S bond content of the sperm nucleus. |
