Influence of Exogenous Gonadotropin and Sexual Steroids on Ovary Development in Japanese Eel Anguilla japonica

Abstract

This study examines Japanese eel ovary responded to the administration of exogenous sexual hormone throughout the reproductive process. In short-term experiment, ovarian follicle exhibited sensitivity to salmon pituitary homogenate (SPH) and sexual steroids (E2 and MT) in the early vitellogenic stages. The apoptotic signals were exclusively localized in the outer theca layer which was detected from in situ TUNEL, furthermore, the appearance of the apoptotic signal in the early vitellogenic stage seems to correlate with a reduction of oocyte numbers in long-term experiment. In long-term experiment, the mean GSI and total oocyte numbers indicated that multiple injections of E2 or MT were not of much benefit to promote the gonadal development of those fish. Despite SPH treatment was an effective and indispensable method to hasten artificially sexual maturation, and the size-frequency distribution indicated that the most developed oocytes of those were growing from the vitellogenic stage to the final maturational phase. However, the treatment of SPH plus E2 failed to promote the ovarian development and exhibited a remarkable reduction of total oocyte numbers. Of interest is the synergistic effect of SPH plus MT revealed that MT, a potent, aromatizable androgen, provides a potential usefulness to participate the process of vitellogenesis when it combines with SPH treatments, furthermore, the synergistic effect actually enhances the survival and synchronous development of ovarian follicle throughout the reproductive process.