The present study demonstrates that activin A and in-hibin A stimulate oocyte maturation in the zebrafish. Both activin A and inhibin A increased GVBD in a dose-dependent manner, with significant stimulation at 10-100 ng/ml. Although the physiological concentrations of activin A and inhibin A have not been determined in fish, the doses used in this study are within the concentrations of activins and inhibins found in mammalian follicular fluid and have been widely used to study the role of activins and inhibins in mammals. The observation that activin A induces final oocyte maturation is consistent with several other studies. Activin A was found to increase GVBD in the rat, rhesus monkey, human, and cow. Recently, Pang and Ge also reported that recombinant goldfish activin B induced zebrafish oocyte maturation. Thus, both activin A and activin B have a similar function in inducing final oocyte maturation. In contrast, earlier studies in the rat, pig, and seabream showed that activin A was ineffective in inducing GVBD. Similarly, previous studies on inhibin in oocyte maturation have also yielded somewhat conflicting results.
For example, O et al. reported that inhibin A inhibited spontaneous maturation of rat oocytes while Tsafriri et al. found no effects of inhibin A during in vitro maturation of oocytes taken from eCG-treated rats. In seabream, inhibin A was also found to be ineffective in inducing GVBD. However, inhibin A was shown recently to stimulate rhesus monkey oocyte maturation. Although the exact causes of such discrepancies are not known, species variation, developmental stages of the oocytes used, and culture conditions (whether or not serum is present), as well as forms of activin and inhibin used, may contribute to the different results reported.