Mediation of seasonal testicular regression by apoptosis.

Animals restrict breeding to specific times of the year when reproductive success is most likely. In males, the most reliable method of reproductive inhibition is gonadal retrogression to an immature state. Depending on the species, the testes decrease in size by between 10 and 95% in response to environmental cues associated with the non-breeding season. An increased rate of apoptosis can occur during testicular regression, whereas little testicular apoptosis is observed during testicular recrudescence or during the breeding season. The Fas system is implicated as a potential mechanism to transmit the apoptotic signal to germ cells during regression, although many apoptotic pathways probably contribute to testicular regression. Finally, differences in both the contribution of testosterone and in the types of cell that undergo apoptosis differ between rodents and birds, indicating that different cellular mechanisms and possibly signals have evolved at the testis to suspend reproduction on a seasonal basis.