SELECTIVE EXPRESSION OF INSULIN-LIKE GROWTH-FACTOR SYSTEM COMPONENTS DURING PORCINE OVARY FOLLICULAR SELECTION

In situ hybridization was used to map anatomical patterns of insulin-l ike growth factor (IGF) system gene expression in the gilt ovary throu gh the estrous cycle. IGF-I, IGF-II, and IGF-I receptor messenger RNAs (mRNAs) were coexpressed in granulosa cells of developing and dominan t follicles through the follicular phase. IGF-I and IGF-I receptor mRN As were selectively concentrated in healthy follicles, whereas IGF-II mRNA was found in all follicles regardless of incipient or overt atres ia. IGF-binding protein-1 (IGFBP-1) was not detected in the gilt ovary at any stage. IGFBP-2 mRNA was most abundant in granulosa cells of sm all follicles and in the ovarian vasculature regardless of cycle stage . IGFBP-2 mRNA persisted in granulosa cells of prevulatory follicles a nd corpora lutea. IGFBP-3 mRNA was not detected in developing follicle s, but was detected in all luteal phase corpora lutea, apparently expr essed by theca lutein cells. IGFBP-4 demonstrated a highly dynamic pat tern of gene expression, closely tracking LH receptor gene expression throughout the follicular and luteal phases of the estrous cycle. IGFB P-4 mRNA was concentrated in the theca of medium to large growing foll icles, but was not detected in granulosa cells until the emergence of dominant follicles in the late follicular stage, when the granulosa ce lls showed morphological evidence of luteinization as well as LH recep tor gene expression. IGFBP-4 mRNA expression continued in granulosa ce lls of corpora lutea during the luteal phase and was not detected in a tretic follicles. IGFBP-5 mRNA was concentrated in the surface or germ inal epithelium and in capillary endothelium, particularly in capillar ies of corpora lutea. In summary, there is selective expression of IGF -I, IGF-I receptor, and IGFBP-2 and -4 mRNAs during the process of fol licular selection in the gilt ovary, with IGFBP-4 expression being clo sely associated with follicular selection and luteinization. These obs ervations support an important role for autocrine/paracrine IGF action modulated by IGFBP-2 and 4 in both follicular growth and differentiat ion in the porcine ovary.