Differentiation and development of Leydig cells,and changes of testosterone levels during testicular differentiation in tilapiaOreochromis niloticus

Initial appearance and development of Leydig cells (LCs) during testicular differentiation in tilapia,Oreochromis niloticus, were investigated histologically. In addition, changes of testosterone levels in gonadal tissue and serum were examined by radioimmunoassay. In the gonads of fry at 23–26 days after hatching, initial testicular differentiation was confirmed by the observation of the differentiation of connective tissues into tissues which are characteristic of the adult testis. LCs, which were identified by the ultrastructural features (a moderate number of mitochondria with tubular cristae, well developed smooth endoplasmic reticulum and many free ribosomes) appeared initially at the time of testicular differentiation. LCs increased in number rapidly in the testes of fish at 70 days after hatching. Concomitant with this increase, spermatogonia increased in number. Testosterone was detectable in the fish at 40–50 days after hatching, but levels in tissue and serum were low. Testosterone levels increased gradually in the fish beginning at 70 days after hatching and increased still more at 100–150 days accompanying active spermatogenesis.