Insulin-like growth factor I gene expression in the rat ovary is confined to the granulosa cells of developing follicles.

The in vivo intraovarian synthesis of insulin-like growth factor-I (IGF-I) has been studied in the rat by Northern blot, dot blot hybridization, and in situ hybridization histochemistry. Ovarian IGF-I mRNA transcript sizes (7.0 kilobases (kb), 1.6 kb, and a group from 0.4-0.9 kb) were similar to those in liver and other tissues. The proportion of ovarian IGF-I to tubulin messenger RNA (mRNA) was increased to 176% of control values by treatment with diethylstilbestrol, while the ratio in liver was decreased to 64.4%. In situ hybridization identifies the major in vivo site of IGF-I synthesis in the ovary as the granulosa cells of developing follicles. IGF-I mRNA was present in the granulosa cells of developing preantral and antral follicles, but was not seen in atretic follicles or corpora lutea. In preovulatory follicles high levels of IGF-I mRNA were confined to the antral cell layers and to the cells of the cumulus oophorus. High levels of tubulin gene expression within follicles were seen in a similar distribution to that for IGF-I but 80-90% of corpora lutea also strongly expressed the tubulin gene. Interstitial cells, including thecal cells, express the tubulin gene at low levels but do not express the IGF-I gene. The distribution of IGF-I mRNA is the same as that previously observed for mitotically active granulosa cells, and therefore offers strong support for the view that IGF-I in the ovary acts by an autocrine-paracrine mechanism to promote granulosa cell replication.