Effects of thymulin on in vitro incorporation of 3H-thymidine into gonocytes of newborn rat testes.

3H-Thymidine incorporation into gonocytes was investigated by means of quantitative autoradiography in organ cultures of testes from newborn rats between day 0 (day of birth) and day 7, in the presence or absence of thymulin. The data indicated that thymulin had no effect on gonocyte incorporation on day 0 or on days 1 and 7; in contrast, thymulin had a strong effect on day 2. At day 2 the percentage of labeled gonocytes was 10-fold higher in experimental animals than in controls. Between days 2 and 6, thymulin had moderate activity. Thymulin thus appeared to have an age-dependent effect upon incorporation of 3H-thymidine into germ cells. On the other hand, in spite of a high incorporation rate of 3H-thymidine into gonocytes after incubation with thymulin, the number of mitoses remained low, suggesting that thymulin may affect DNA duplication in gonocytes of newborn rat testes.