Enhancement of A spermatogonial proliferation and differentiation in irradiated rats by gonadotropin-releasing hormone antagonist administration.

The initial changes in the numbers, proliferation, and differentiation of A spermatogonia in irradiated rats after the administration of a GnRH antagonist, which is known to induce differentiation in this system, were investigated. LBNF1 rats were given 6 Gy of gamma-irradiation; some were treated with the GnRH antagonist Cetrorelix beginning 15 weeks after irradiation. Although the spermatogonia in the irradiated rats without hormone treatment continue to proliferate (labeling and mitotic indexes of 24% and 18%, respectively), they underwent apoptosis (apoptotic indexes of 21% by the terminal transferase-mediated end labeling assay and 9% by nuclear morphology), resulting in a constant number of A spermatogonia. Whole mount analysis of clones ofA spermatogonia revealed that larger clones were more likely to undergo apoptosis than mitosis. Hormone administration decreased the intratesticular testosterone concentration to 6% of the level in irradiated rats within 1 week. Concomitantly, there was a decrease in spermatogonial apoptotic indexes to 43% of levels in irradiated-only rats, leading to an increases in their numbers by 150%, their diameters by 11%, and their labeling indexes by 31%. The sizes of the mitotic clones gradually increased, and clones of more than eight cells appeared at week 3 of hormone treatment. A spermatogonial differentiation began at week 4, and by week 6.6, differentiation occurred in 30% of the tubules. Thus, suppression of intratesticular testosterone by the GnRH antagonist may be responsible for the immediate changes in spermatogonial numbers and kinetics, but several additional steps are required before differentiation begins, which did not occur until week 4.