Regulation of Insulin-Like Growth Factor I and Stage-Specific Levels of Epidermal Growth Factor in Stage Synchronized Rat Testes

Abstract

Stage synchronization of seminiferous epithelium after withdrawal and replenishment of vitamin A provides a valuable and powerful approach to the investigation of paracrine interactions within the testis. However, since the discovery of this model, little attention has been given to the events surrounding the synchronous reinitiation of spermatogenesis after depletion of vitamin A. Synchronization of spermatogenesis was observed in all animals previously deficient in vitamin A. However, the degree of synchrony observed, as assessed by a ratio of synchrony, decreased markedly with time. The possibility that spermatogenic synchrony decreases with time due to variability of the temporal duration of stages of the cycle of the seminiferous epithelium is supported by this observation. However, long-term studies are required to substantiate this point. After initiation of stage synchrony of spermatogenesis, increased testicular concentrations of epidermal growth factor (EGF) were observed in testes synchronized between stages IX-II than at other stages of the cycle of the seminiferous epithelium. This elevation in testicular EGF concentrations correlated well with mitotic division of type A spermatogonia at stages IX, XII, and XIV of the cycle of the seminiferous epithelium. Previous in vitro studies have implicated an EGF-like factor in the stimulation of type A spermatogonial division in the mouse. A significant increase in testicular insulin-like growth factor I (IGF-I) concentrations was observed in control animals 14 days after the injection of retinol acetate. In vitamin A deficient animals, a marked increase in testicular IGF-I concentrations was observed as compared to age-matched controls. Maximal levels of testicular IGF-I concentrations were present 14 and 28 days and again 126 days after re-supplementation with retinol acetate. No stage dependent changes in testicular IGF-I were observed but the data provided suggest the retinol may be one of the factors involved in the regulation of testicular IGF-I.