Meiotic DNA synthesis during mouse spermatogenesis

The incorporation of radioactivity into various cells in the sequence of spermatogenesis was measured by preparing highly purified spermatozoan nuclei from the cauda epididymidis of mice at daily intervals after injection of (3H)thymidine. The stages of differentiation of these sperm at the time of thymidine administration were calculated from the kinetics of spermatogenesis. The procedure for purification of sperm nuclei included sonication, mechanical shearing, and treatment with trypsin, DNase, Triton X-100, 2M NaC1, and sodium dodecyl sulfate. DNA was isolated from these nuclei by treatment with dithiothreitol and pronase, followed by phenol extraction and ethanol precipitation. The levels of radioactivity in the epididymal sperm head preparations were low (less than 13 dpm/mouse) for 27 days after injection, and then rose dramatically to over 4 times 104 dpm/mouse. Further experiments demonstrated that the 11 dpm of 3H radioactivity contained in sperm heads at 21 or 26 days after injection of (3H)TdR was significantly above background and contamination levels from other cells or other sources. Most of the radioactivity was in the sperm DNA and represented incorporation of tritium from (3H)TdR into the nuclear DNA of meiotic cells at 0.002 percent of the rate of incorporation into S-phase cells. Little, if any, (3H)TdR was incorporation into the DNA of spermatids. The levels of DNA synthesis during the meiotic prophase in the mouse appear to be much lower than those reported for other organisms.