Oogenesis in the Japanese Eel, Anguilla japonica

Abstract

Oogenesis in the teleost is generally divided into oocyte growth and maturation phases. The oocyte growth phase is further divided into primary and secondary growth phases, and the secondary growth phase consists of previtellogenic and vitellogenic phases. Both vitellogenic oocyte growth (vitellogenesis) and oocyte maturation, and possibly other phases as well, are under the control of pituitary gonadotropic hormone (GTH). However, in most cases, GTH action is not direct, but exerts its influence through the production of sex steroid hormones. During the period of vitellogenesis, ovarian follicle cells produce estrogen, mainly 1713estradiol (E2), to stimulate hepatic synthesis of a yolk precursor protein, vitellogenin (VTG). VTG is incorporated into the oocyte and reconstructed as a yolk protein (Wallace 1985). After the completion of oocyte growth, ovarian follicle cells produce maturation-inducing hormone (MIH) to induce oocyte maturation directly (Nagahama 1987). Oocyte maturation is one of the most important steps in acquiring the competency to undergo fertilization and consists of the breakdown of the germinal vesicle (GVBD), chromosome condensation, and extrusion of the first polar body. Oocyte meiosis is reinitiated by stimulation with MIH and is then arrested at metaphase in the second meiotic division. Subsequently, the oocyte is ovulated as a fertilizable egg. In teleosts, either 17α,20β-dihydroxy-4-pregnen-3one (DHP) or 17α,20β, 21-trihydroxy-4-pregnen-3-one (20β-S) have been known as MIH (Nagahama and Adachi 1985; Trant and Thomas 1989).