In this study, we examined the in vitro effects of insulin-like growth factor I (IGF-I) in the presence or absence of 11-ketotestosterone (11-KT: the spermatogenesis-inducing hormone) on the proliferation of Japanese eel (Anguilla japonica) testicular germ cells. Initially, a short-term culture (15 days) of testicular tissue with only type A and early type B spermatogonia (preproliferated spermatogonia) was carried out in Leibovitz-15 growth medium supplemented with different concentrations of recombinant human IGF (rhIGF)-I or -II in the presence or absence of 10 ng/ml of 11-KT. Late type B spermatogonia (proliferated spermatogonia) were observed in treatments of 100 ng/ml of both rhIGF-I and -II in combination with 11-KT, indicating the onset and progression of spermatogenesis. In all tested rhIGF-I concentrations (except 0.1 ng/ml) supplemented with 11-KT, late type B spermatogonia were detected in at least one individual. Then, we proceeded with an in vitro 45-day culture of testicular tissue with 100 ng/ml of rhIGF-I in the presence or absence of 10 ng/ml of 11-KT to test the long-term effects of rhIGF-I on the spermatogenetic cycle. The presence of all types of germ cells, including spermatozoa, in the testis cultured with the admixture of the two hormones indicated that the germ cells underwent complete spermatogenesis whereas no germ cell proliferation was observed when the rhIGF-I was applied alone. These results suggest that IGF-I in the presence of 11-KT plays an essential role in the onset, progress, and regulation of spermatogenesis in the testis of the Japanese eel.
SUMMARY:
To assess the number of mitotic divisions in fish spermatogonia, an easy method was devised which can estimate the number of mitotic divisions by only counting the cells in the cross‐section of a cyst at its largest diameter. Using clay to simulate spermatogonial cysts, we obtained results which related the number of cells in a cross‐section of a cyst at its largest diameter to the total number of cells in the same cyst. The results were then compared to the number of mitotic divisions observed in seven species. The number of divisions estimated from the simulation experiment coincided exactly with the observed values in all species with spherical shaped cysts. However, the counts determined in fish with elliptical shaped cysts differed from the expected values. As a result, it is clear that the obtained values from the simulation experiment are valid for the estimation of the number of mitotic divisions.
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