Hormonal symphony: The dynamic duo of IGF and EGF in gonadotropin-induced fish ovarian development and egg maturation

Fish oocyte maturation (FOM) is a critical biological process that occurs before ovulation and is influenced by gonadotropins, particularly luteinizing hormone (LH). The release of LH stimulates the ovarian follicle to produce a maturation-inducing hormone (MIH), specifically 17α, 20β-dihydroxy-4-pregnen-3-one (17α, 20β-DP), which initiates the formation of maturation-promoting factor (MPF) through the activation of cyclin B and cdc2 kinase. Insulin-like growth factor I (IGF-I) significantly regulates ovarian functions, including steroidogenesis, by activating its membrane receptors and the tyrosine kinase pathway. IGF-I influences oocyte maturation directly via the PI3 kinase pathway, independent of steroid hormones. Additionally, epidermal growth factor (EGF) promotes cell growth and differentiation by binding to its receptor (EGFR). It is implicated in mediating human chorionic gonadotropin (hCG)-induced DNA synthesis in ovarian follicles while suppressing apoptosis. The presence of EGF in follicle cells and oocytes, along with its higher expression in oocytes, suggests it may act as a paracrine signal regulating somatic cell activity. Recent studies indicate that the activin system in follicle cells could be a target for EGF activity. The EGFR signaling pathway enhances gonadotropin-induced steroidogenesis and governs the transition of oocyte maturation stages, essential for successful fertilization. This review synthesizes current research on the roles of gonadotropins, IGFs, and EGFs in fish oocyte maturation and ovarian steroid production.