New insights of inhibins in ovarian physiology of fish

Egg quality and early embryonic development are important aspects for successful artificial propagation of fish species of ecological and economical values, and therefore have been intensively researched. It has been demonstrated that several maternal factors including TGF-β, cyclin B, follicle cell transcripts and gonadal peptides like inhibins influence fish egg quality. Inhibins are considered gonadal hormones that act on the anterior pituitary cells to regulate gonadotropin hormone secretion and as a paracrine substance that affects steroidogenesis and folliculogenesis. There are several intra-ovarian growth factors identified and characterized in fish that are important components of the brain-pituitary-gonadal (BPG) axis communication network. The role of inhibins on the reproductive cycle and its regulation by the hormonal negative feedback mechanism from gonads and how this inhibits activin signaling and suppresses the pituitary FSH production has been well established in higher vertebrates but is poorly understood in fish. Many advances have been made to elucidate the endocrine regulations involved during oogenesis in higher vertebrates. Neutralization of endogenous inhibins helps in many reproductive processes, including programed production of high-quality oocytes in large quantity, improved embryonic development and increased survival of offsprings. In fish, little is known about inhibin structure and endocrine regulation despite its immense importance in reproduction of female mammals. This review focuses on recent advances in the field of inhibins in fish with special emphasis on its structure, receptor interaction, mechanisms of action and its physiological role of in the oogenesis and reproductive cycle.