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H.M.V.Udayantha*, Hyung-Bok Jeong, Thanthrige Thiwnuwan Priyathilaka, Jehee Lee, J.D.H.E. Jayasinghe,  Bongsoo Lim.
Department of Marine Life Science, School of Marine Biomedical Sciences, Jeju National University, Jeju Special Self Governing Province 690-756, Republic of Korea. .

Liver plays a vital role by providing egg yolk precursor vitellogenin for gonadal development through stimulation of 17b-estradol(E2) in oviparous vertebrates. Vitellogenin gene (Vtg) expression is regulated by the binding of estrogen receptors (ERs)- E2 complex to the estrogen responsive elements at the promoter site of the Vtg. Normally Vtg is undetectable in male and immature female fish. However, catadromous Japanese eels are not become mature and spawn in culturing conditions. Therefore, so far induced breeding is not achieved for mass production of glass eels. Therefore, in this study we have investigated the mRNA expression of liver VTg, ER alpha (ERa) and ER beta (ERb) fluctuation with the serum E2 during different gonadal development stages in Japanese eels to observe their vitellogenesis pattern.

Feminized Japanese eels were weekly injected with salmon pituitary extract (20mg/Kg) period of 13weeks. During the period of experiment, un-injected and after the 2nd, 4th, 6th, 13th injections and after ovulation (AO) five eels from each step were sacrificed to isolate liver, gonad and serum. Isolated liver and serum were stored in -80 ֯C for mRNA extraction and ELISA assay respectively. A piece of gonad samples in each eel were preserved in Buin's fluid for histological observation.

In the current study, Vtg expression was extremely lower in perinucleoular (PN) and nucleolar (N) stages. After that, it was started to increase and robustly increased in late vitellogenic (LV)stage and after that significantly decreased (Fig.1). As similar to the Vtg transcript levels, ERa, ERb and E2 were also significantly increased in LV stage but not significantly decreased after the LV stage. According to above investigations we can suggest upregulated ERs and E2 stimulate the Vtg expression in liver during vitellogenesis in artificially matured Japanese eels. However, further experiments are required to investigate the reason for continuous elevation of ERs and E2 in migratory nucleus (MN) stage.

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