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Epidermal Growth Factor and TGF Promote Zebrafish Oocyte Maturation in Vitro: Potential Role of the Ovarian Activin Regulatory System

Department of Biology, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China

Address all correspondence and requests for reprints to: Dr. Wei Ge, Department of Biology, Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China. E-mail: weige{at}cuhk.edu.hk

Epidermal growth factor (EGF) and TGF are well known for their activities in the ovary. Both factors initiate signal transduction by binding to the common EGF receptor that has been demonstrated in the ovary across vertebrates from fish to humans. Using zebrafish as the model, we demonstrated in the present study that recombinant human EGF and TGF significantly enhanced final maturation of the fully grown, follicle-enclosed oocytes (0.58–0.65 mm) in vitro in a clear time- and dose-dependent manner. The effect of EGF/TGF was additive to that of hCG at low concentrations, but the additivity diminished when the concentration increased. Both actinomycin D and cycloheximide completely blocked the effect of EGF/TGF, indicating that the promotion of oocyte maturation by EGF/TGF requires de novo mRNA transcription and protein synthesis. Interestingly, the effect of EGF/TGF could be blocked by cotreatment with follistatin, a potent binding protein for activin, an ovarian growth factor belonging to the TGFß superfamily. Semiquantitative RT-PCR assays showed that both EGF and TGF significantly stimulated the expression of activin ßA and activin type II receptor in the cultured zebrafish ovarian follicle cells in a clear time- and dose-dependent manner. This together with our previous report that activin had a potent stimulatory effect on zebrafish oocyte maturation strongly suggests that the intrinsic ovarian activin system is probably a downstream mediator of EGF/TGF actions in the zebrafish ovary.

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