|Budget Amount *help
¥45,760,000 (Direct Cost : ¥35,200,000、Indirect Cost : ¥10,560,000)
Fiscal Year 2004 : ¥11,180,000 (Direct Cost : ¥8,600,000、Indirect Cost : ¥2,580,000)
Fiscal Year 2003 : ¥15,210,000 (Direct Cost : ¥11,700,000、Indirect Cost : ¥3,510,000)
Fiscal Year 2002 : ¥19,370,000 (Direct Cost : ¥14,900,000、Indirect Cost : ¥4,470,000)
Ovulation, which is triggered by a preovulatory surge of luteinizing hormone (LH) released from the pituitary gland, is a dynamic process that results in the liberation of a mature fertilizable ovum from the ovarian follicle. This event, which is known as follicle rupture, has been the subject of intensive investigation over the past century. Previous studies, which have been carried out mainly in mammalian species, including humans, have established that follicle rupture is accomplished by dissolution of the granulosa cell basement membrane and fragmentation of the collagenous matrix at the apex of the follicular wall, thereby implicating proteolytic enzymes. Indeed, a variety of proteases have been proposed as candidates for rupturing the follicle. However, the proteases that are essential for follicle rupture in ovulation have not yet been identified, despite much effort. In this study, we investigated vertebrate ovulatory processes in the medaka, with the particular aim of identifying the enzymes responsible for the proteolytic degradation of the follicle walls. We found that follicle rupture in the medaka ovary involves the cooperation of at least three matrix metalloproteinases (MMPs), together with the tissue inhibitor of metalloproteinase-2b (TIMP-2b) protein. We determined the discrete roles of each of these proteins during follicle rupture. Our results indicate that gelatinase A induces the hydrolysis of type IV collagen constituting the basement membrane, membrane-type 2 (MT2) MMP degrades type I collagen present in the theca cell layer, and MT1-MMP and TIMP-2b are involved in the production and regulation of gelatinase A. The in vitro ovulation system made it possible to identify proteases that are critical for ovulation in the medaka, and the use of similar experimental systems should help to identify ovulatory enzymes in mammals.