| Summary: | 博士 === 國立臺灣大學 === 漁業科學研究所 === 96 === In higher vertebrates, the reproduction of teleost is controlled by the hypothalamus-pituitary-gonadal axis. This study examines the effects of exogenous gonadotropic hormone (salmon pituitary homogenate, SPH) and sex steroids (estradiol-17β and methyltestosterone) on the dynamics of the Japanese eel (Anguilla japonica) ovary during the process of vitellogenesis. This study attempted to obtain the cDNA encoding for Vtg from female Japanese eels, Anguilla japonica. Rapid amplification of cDNA ends (RACE) and polymerase chain reaction (PCR) were used to amplify Vtg cDNA prepared from liver extracts. Obtained PCR products were subcloned and sequenced. The overall sequence of eel Vtg cDNA isolated in this study contained 5395 bp nucleotides. This Vtg sequence encodes 1743 amino acids of the precursor molecule, and is entirely composed of the characteristic N-terminal lipovitellin-I region, an internal polyserine domain region, and a c-terminal lipovitellin-II region. The deduced amino acid sequence from these clones shares 34–61% identity with other teleost Vtgs. Northern blot assays of Vtg gene expression following hormonal treatment demonstrated that this Vtg is synthesized in the liver under stimulation by estradiol injection. However, Vtg synthesis may not be enhanced by salmon pituitary homogenate (SPH) induction for the developing ovarian follicles. Notably, the effect of methyltestosterone, following SPH injection, may be more appropriate for the uptake of Vtg by ovarian follicle maturation during the artificial maturation of Japanese female eels.
On the other hand, in situ TUNEL demonstrated that apoptosis-mediated atresia of follicular cells, in the previtellogenic and early vitellogenic stages, display sensitivity for SPH and sex steroids, respectively, during short-term induction; long-term inductions also indicate that the development and growth of ovarian follicles (i.e., total number and size-frequency distribution) responded differently to the sexual hormone. SPH treatment was effective and indispensable for artificially hastening maturation; however, multiple injections of E2 or MT alone failed to promote ovarian development in these cultivated fish. Both the E2 and SPH plus E2 treatment at the time of vitellogenesis led to the failure of ovarian development (i.e., a significant decrease of total numbers) in the present study. Interestingly, the synergistic effect of SPH combined with MT seems to directly affect the gonadal development of female Japanese eels in the present study. This study demonstrated that androgen is not atretogenic and actually enhances the growth and survival of primary follicles, and has a potential effect on the process of vitellogenesis in SPH treatment.
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