Inhibition of canine spermatogonial mitosis with follicle regulatory protein

Evidence was found that the Graafian follicle produces and secretes a hormone-like protein, referred to as follicle regulatory protein (FRP), which affects the responsivity of other follicles to gonadotropin stimulation. Since a similar material was identified in homogenates of bull testes, we evaluated the effects of FRP on the histology of seminiferous epithelium by its systemic injection into normal dogs. Mongrel dogs were injected IM with 5 mg of FRP for 17 consecutive days. The seminiferous epithelium was evaluated by light microscopy and divided into eight stages according to Foote. The relative frequency of stage 4 was significantly less in the FRP-treated than in the control group, but that of stage 7 was significantly greater in the FRP-treated group. The ratios of type B/type A spermatogonia for control and FRP-treated groups for stages 6-8 were 1.5, 5.8, 2.4, and 1.6, 4.3, 1.6, respectively. The tubular diameters of stages 6 and 7 were significantly different between the two groups. The numbers of tubules containing spermatogonial divisions were significantly decreased at stages 2 + 3 and stages 6 + 7 in the FRP-treated group. A clear decrease in the number of tubule profiles with spermatogonial divisions was present at stages 6 and 7 in the FRP-treated testis. Taken together, these findings suggest that FRP inhibits spermatogonial transformation of type A into type B.