DNA Synthesis in the Fertilizing Hamster Sperm Nucleus: Sperm Template Availability and Egg Cytoplasmic Control1

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Abstract
To assess the role of the availability of sperm nuclear templates in the regulation of DNA synthesis, we correlated the morphological status of the fertilizing hamster sperm nucleus with its ability to synthesize DNA after in vivo and in vitro fertilization. Fertilized hamster eggs were incubated in 3H-thymidine for varying periods before autoradiography. None of the decondensed sperm nuclei nor early (Stage I) male pronuclei present after in vivo or in vitro fertilization showed incorporation of label, even in polyspermic eggs in which more advanced pronuclei were labeled. In contrast, medium-to-large pronuclei (mature Stage II pronuclei) consistently incorporated 3H-thymidine. To investigate the contribution of egg cytoplasmic factors to the regulation of DNA synthesis, we examined the timing of DNA synthesis by microinjected sperm nuclei in eggs in which sperm nuclear decondensation and male pronucleus formation were accelerated experimentally by manipulation of sperm nuclear disulfide bond content. Although sperm nuclei with few or no disulfide bonds decondense and form male pronuclei faster than nuclei rich in disulfide bonds, the onset of DNA synthesis was not advanced. We conclude the the fertilizing sperm nucleus does not become available to serve as a template for DNA synthesis until it has developed into a mature Stage II pronucleus, and that, as with decondensation and pronucleus formation, DNA synthesis also depends upon egg cytoplasmic factors.