Effects of IGF1 and IGF2 on In Vitro Ovarian Follicle Maturation in Rainbow Trout, Oncorhynchus mykiss

Abstract

Ovarian follicle maturation (OFM), including the resumption of meiosis in the oocyte, is under hormonal regulation. Insulin-like growth factors (IGF) have been shown to participate in the regulation of OFM with species-specific actions in teleost fishes. In the present study, in vitro treatment of rainbow trout ovarian follicles with recombinant human insulin-like growth factor-1 (rhIGF1) or rhIGF2 did not induce germinal vesicle breakdown (GVBD), a marker for the resumption of meiosis, in the oocytes. Co-incubation of follicle-enclosed oocytes with rhIGF1 and the maturation-inducing steroid (MIS) in rainbow trout, 17α,20β-dihydroxy-4-pregnen-3-one (17,20βP), also did not induce GVBD in follicles from rainbow trout that were not able to respond to the MIS alone suggesting IGFs cannot induce oocyte maturational competence (OMC), which is the ability of the oocyte to respond to the MIS. Nevertheless, the addition of rhIGF1 with the MIS increased the proportion of oocytes completing GVBD compared with MIS alone, although this potentiation was small and varied greatly among clutches of follicles from fish with oocytes at different stages of germinal vesicle migration. Collectively, these observations suggest IGFs may have synergistic actions with the MIS but cannot induce resumption of meiosis directly at the oocyte and are not potent inducers of OMC in rainbow trout. Rainbow trout are the first teleost fish in which IGFs were found to induce neither OMC nor resumption of meiosis in vitro.