Estradiol Upregulates the Expression of the TGF-β Receptors and during the Gonadal Development of .

TGF-β receptors play important roles in mediating TGF-β signals during gonadal development. To identify the functions of TGF-β receptors, including the type I receptor (activin receptor-like kinase 5, ) and type II receptor (bone morphogenetic protein receptor 2, BMPR2), during the gonadal development of , the full-length cDNA sequences of and were isolated and characterized. Their expression patterns in developing gonads and in the gonads of exogenous estradiol (E) -fed fish were analyzed. The cDNAs of and were 1925 bp and 3704 bp in length and encoded 501 and 1070 amino acid residues, respectively. and were mostly expressed in gonads, particularly in cortical alveoli stage ovaries and mid-spermatogenic stage testes; however, the overall level of mRNA was higher than that of during gonadal development. Furthermore, immunohistochemical signals of ALK5 and BMPR2 were mostly detected at chromatin nucleolar oocytes and perinuclear oocytes in ovaries and at spermatocytes and spermatogonia in testes. Exogenous E induces the gonadal expression of and , and is more responsive to E than . These results suggest that ALK5 and BMPR2 might play a potentially vital role in both folliculogenesis and spermatogenesis in .