Expression of mRNA encoding IGF-I, IGF-II and type 1 IGF receptor in bovine ovarian follicles

IGFs regulate gonadotrophin-stimulated proliferation and differentiation of granulosa and theca cells in vitro. However, the detailed pattern of mRNA expression of IGFs in bovine follicles remains controversial. The objective s of this study were therefore to describe the temporal and spatial pattern of expression of mRNA encoding IGF-I, IGF-II and the type 1 IGF receptor i n bovine follicles in vivo. The expression of mRNA encoding IGF-II was dete cted in theca tissue from around the time of antrum formation up to and dur ing the development of dominance. No IGF-II mRNA expression was detected in granulosa cells. In the majority of follicles we were unable to detect mRN A encoding IGF-I in either granulosa or theca tissue from follicles at any stage of development. Occasionally low amounts of mRNA encoding IGF-I were detected in the theca externa and connective tissue surrounding some follic les. Type 1 IGF receptor mRNA was detected in both granulosa and theca cell s of preantral and antral follicles. Expression was greater in granulosa ti ssue compared with theca tissue. We also measured IGF-I and -II mRNA in tot al RNA isolated from cultured granulosa and theca cells using reverse trans criptase PCR. In contrast to the in vivo results, IGF-II mRNA was detected in both granulosa and theca tissue. IGF-I mRNA was detected in theca tissue and in very low amounts in granulosa cells. Using a specific IGF-I RIA we were unable to detect IGF-I immunoreactivity in granulosa conditioned cell culture media. Using immunohistochemistry we detected IGF-I immunoreactivit y in some blood vessels within the ovarian stroma. We conclude from these r esults that IGF-II is the principal intrafollicular IGF ligand regulating t he growth of bovine antral follicles. In preantral follicles the expression of mRNA encoding type 1 IGF receptor but absence of endogenous IGF-I or -I I mRNA expression, highlights a probable endocrine mechanism for the IGF re gulation of preantral follicle growth.