ONTOGENY AND CELLULAR-LOCALIZATION OF I-125 LABELED INSULIN-LIKE GROWTH FACTOR-I, I-125 LABELED FOLLICLE-STIMULATING-HORMONE, AND I-125 LABELED HUMAN CHORIONIC-GONADOTROPIN BINDING-SITES IN OVARIES FROM BOVINE FETUSES AND NEONATAL CALVES

In a previous study we reported that ovaries from bovine fetuses, which consist mainly of preantral follicles with few antral follicles, are weakly responsive to gonadotropins (FSH and LH). Insulin-like growth factor-I (IGF-I) is known to enhance gonadotropin responsiveness in vitro, but there is a lack of consistent data on the involvement of IGF-I, FSH, and LH during early stages of folliculogenesis in cattle. In the study reported here, we assessed autoradiographically the ontogeny of I-125-gonadotropin and I-125-IGF-I binding activities during preantral and early antral stages in cattle. Follicular growth was initiated around Day 180 of gestation in fetuses. The density of I-125-FSH binding was high in granulosa cells from primary (mean +/- SEM 10.5 +/- 0.7 grains/cell, 0.05-mm diam.) and secondary follicles (10.8 +/- 0.8 to 13.6 +/- 1.2 grains/cell, 0.06-0.15 mm) but increased significantly (p < 0.05) in early antral follicles (18.2 +/- 1.1 grains/cell, 0.16-3.0 mm). Specific I-125-IGF-I binding levels were low in granulosa cells from preantral follicles, averaging 2.5 +/- 0.6-3.1 +/- 0.9 grains/cell. However, after antrum formation, the density of I-125-IGF-I binding increased significantly (p < 0.05) with follicular diameter in granulosa cells and was 5.7 +/- 0.7 and 9.1 +/- 0.6 grains/cell for antral I (0.16-0.5 mm) and antral II (0.6-3.0 mm) follicles, respectively. I-125-FSH and I-125-IGF-I binding densities were low in theca cells from preantral and early antral follicles as well as in the interstitial tissue and granulosa cells from atretic follicles . I-125-hCG binding values were low in theca cells from secondary follicles (3.2 +/- 0.6 grains/cell) but increased significantly in the theca interna from early antral follicles (11.5 +/- 0.7 grains/cell, p < 0.05). In addition, I-125-hCG binding activity in granulosa cells (8.9 +/- 0.9 grains/cell) consistently later (1.1-3.0 mm) than I-125-IGF-I binding and was restricted to the outermost layers of some follicles. Our results suggest that although FSH may be involved in folliculogenesis as early as the preantral stage, possible amplification of gonadotropin action by IGF-I in granulosa cells may be relevant after antrum formation in fetal and calf ovaries. In addition, granulosa but not theca cells are a major site of IGF-I reception during early antral development of ovarian follicles.