Reinitiation of gonocyte mitosis and movement of gonocytes to the basement membrane in testes of newborn rats in vivo and in vitro.

Movement of postnatal gonocytes to the periphery of the seminiferous cord, where they contact the basement membrane, and resumption of mitosis by these previously quiescent cells are likely to be critically important in establishing spermatogenesis in neonatal rats. We used several approaches both in vivo and in vitro to determine precisely when each of these two events begins, to study their temporal relationship to each other, to determine whether gonocyte division is a prerequisite for relocation or vice versa, and to probe the source of factors initiating and/or regulating these events. Both light and electron microscopy were used to determine that the first gonocytes make contact with the basement membrane on postnatal day 4, while quantitative autoradiography following 3H-thymidine administration in vivo indicated that the first gonocytes to re-initiate cell division do so one day earlier, on day 3, and that the percentage of gonocytes dividing remains at a stable level through day 5. Moreover, we organ-cultured neonatal testes from birth onwards in the presence of defined, serum- and hormone-free medium and determined that both proliferation and relocation of gonocytes begin and continue in vitro as in vivo. This observation argues against involvement of extratesticular factors in stimulating gonocyte relocation and division, and points to the testis itself as the most likely source of agent(s) regulating postnatal maturation of these cells. In other, similar incubations we included 3H-thymidine for varying periods of time to label either those gonocytes that are the first to divide or all gonocytes that divide during the first 48 hr of culture. From these studies, we confirmed that the first gonocytes to divide do so while separated from the basement membrane and found that, although some cells divide before moving peripherally, others do not.