Recent advances in the understanding of teleost medaka ovulation: the roles of proteases and prostaglandins.

Ovulation is the process of liberating oocytes from the preovulatory follicles, and is observed in the ovaries of virtually all female vertebrate animals. Compared with mammalian species, there have been far fewer studies that address the ovulatory mechanisms of non-mammalian species. We have examined the molecular mechanism of follicle rupture during ovulation using the teleost model, medaka, or Oryzias latipes. Follicle rupture in medaka ovulation involves the cooperation of the tissue inhibitor of metalloproteinase-2b protein with at least three matrix metalloproteinases (MMP): membrane type-1 MMP (MT1-MMP), MT2-MMP, and gelatinase A. Our studies also indicate that the serine protease, i.e., plasmin, participates in the rupture for only a few hours prior to the activation of MMP-mediated hydrolysis at ovulation. The involvement of prostaglandin E2 (PGE2) in medaka ovulation was also demonstrated. Cyclooxygenase-2 and PGE2 receptor subtype EP4b were respectively shown to be an enzyme responsible for PGE2 synthesis and a receptor for the generated ligand in the preovulatory follicles. Based on the results obtained from our studies of fish, we discuss the similarities and differences in vertebrate ovulation compared with mammalian species.