Gonadal estrogen profile and immunohistochemical localization of steroidogenic enzymes in the oyster and scallop during sexual maturation

Estrogen levels in the gonads of marine bivalves, the Pacific oyster Crassostrea gigas and scallop Patinopecten yessoensis were determined by high performance liquid chromatography (HPLC) using an electrochemical detector. Estrone (E₁), estradiol-17β (E₂), and a small amount of estriol (E₃) were identified in the ovary, while only E₂ was found in the testis. The level of E₂ in the ovary was consistently higher than E₁ and it increased with sexual maturation. These results indicate that E₂ may play a role in the reproductive events of the oyster and scallop. In vitro experiments demonstrated the presence of 17β-hydroxysteroid dehydrogenase (17β-HSD) in the ovaries of both bivalves. The activity of 17β-HSD in the ovary was lower in the postspawning stage than in the early differentiating stage. The evidence for the presence of aromatase activity in the scallop ovary was obtained by ³H-water assay. The immunoreactivity against 3β-hydroxysteroid dehydrogenase (3βHSD), P450 aromatase and E₂ was detected in the cells along the outside of germinal acini of the scallop ovary. It is concluded that estrogens can be synthesized in the gonad, that their levels vary with the reproductive cycle, and that they have a role in the development of gametes.