Growth and the initiation of steroidogenesis in porcine follicles are associated with unique patterns of gene expression for individual components ofthe ovarian insulin-like growth factor system

Ovarian follicular growth and steroidogenesis are controlled by the interac tion of insulin-like growth factors (IGFs) and gonadotropins. The objective was to determine the temporal and spatial relationships for gonadotropin r eceptor, steroidogenic enzyme, and IGF system gene expression during the de velopment of preovulatory porcine follicles. Sows (n = 18) were weaned and follicles were monitored by transrectal ultrasonography. Ovaries were colle cted from sows when the mean diameter of the preovulatory follicular cohort was approximately 2, 4, 6, or 8 mm. mRNA were measured by in situ hybridiz ation for individual follicles within the preovulatory cohort (3 to 5 folli cles per sow). Patterns of gene expression detected by in situ hybridizatio n were confirmed by RNase protection analyses of pooled RNA samples. The am ount of LH receptor mRNA and steroidogenic enzyme mRNA (17 alpha-hydroxylas e and aromatase) increased as the mean diameter of the follicular cohort in creased from 2 to 6 mm, but then decreased abruptly for 8-mm follicles. Est radiol concentrations in follicular fluid closely followed the expression p atterns of steroidogenic enzymes and LH receptor mRNA. FSH receptor mRNA wa s present in cohorts of 2-mm follicles but declined in 4-mm follicles and w as undetectable in 6- and 8-mm follicles. The expression of ICF-I and type I IGF receptor mRNA were similar for follicles of 2, 4, 6, and 8 mm. In con trast, IGF-II mRNA progressively increased in follicles collected from 2-, 4-, and 6-mm cohorts, and then decreased slightly at 8 mm. Type II IGF rece ptor mRNA was greatest in 8-mm follicles. ICF binding protein-2 (BP-2) mRNA decreased as follicles achieved progressively larger sizes during the preo vulatory period (2 to 8 mm), whereas the IGFBP-4 mRNA remained relatively l ow for follicles in 2- to 6-mm cohorts but then increased markedly in 8-mm follicles. In summary, temporal and spatial patterns of gene expression for gonadotropin receptor, steroidogenic enzyme, and ICF system genes were cha racterized in preovulatory porcine follicles by using in situ hybridization and RNase protection analyses. The unique patterns of gene expression sugg est interdependence among specific genes that may be essential for preovula tory follicular development.