Oocyte hydration in the Japanese eel (Anguilla japonica) during meiosis resumption and ovulation

In this study, to elucidate the mechanisms of oocyte hydration in the Japanese eel (Anguilla japonica), we examined the in vivo and in vitro morphological changes and hydration process occurring during oocyte maturation and ovulation. We also investigated the effects of the presence of ovarian follicles, aquaporin water permeability (HgCl2), and yolk proteolysis (bafilomycin A1) inhibitors, gap junction uncouplers (carbenoxolone and 1‐octanol), a 3β‐hydroxysteroid dehydrogenase inhibitor (trilostane), and a P450 antagonist (aminoglutethimide) on oocyte hydration during in vitro oocyte maturation and ovulation. The oocytes underwent more than threefold increase in volume during maturation and ovulation, which was artificially induced by injecting salmon pituitary extracts and 17,20β‐dihydroxy‐4‐pregnen‐3‐one (DHP). Wet and dry weight measurements indicated that water accumulation during oocyte maturation is the major factor contributing to the follicular diameter increase, suggesting that follicular diameter measurements can be used as a hydration index. In the in vitro experiments, human chorionic gonadotropin (HCG) and DHP caused an increase in the diameter of follicle‐enclosed oocytes but not defolliculated oocytes. Addition of HgCl2 and bafilomycin A1 to the incubation media inhibited the HCG‐ and DHP‐induced increase in the follicular diameter in a dose‐dependent manner. Neither carbenoxolone nor 1‐octanol influenced the HCG‐induced increase in the follicular diameter. Trilostane and aminoglutethimide slightly but significantly inhibited HCG‐induced oocyte hydration. Consequently, we concluded that ovarian follicles are essential for HCG‐ and DHP‐induced oocyte hydration. Furthermore, aquaporin facilitates water uptake by acting as a water channel, and yolk proteolysis is essential for water influx into oocytes via osmotic mechanisms. J. Exp. Zool. 311A:752–762, 2009. © 2009 Wiley‐Liss, Inc.