FRACTIONATION OF GROWTH-PLATE CHONDROCYTES - DIFFERENTIAL EXPRESSION OF IGF-I AND GROWTH-HORMONE AND IGF-I RECEPTOR MESSENGER-RNA IN PURIFIED POPULATIONS

In vitro studies of growth plate cell kinetics have been hindered by t he spatial arrangement and heterogeneity of cells within the plate. In this study, we describe a fractionation method that consistently gene rated five relatively pure populations of growth plate chondrocytes. E ach fraction exhibited morphology, proliferative rates, and marker mRN A expression consistent with in vivo positional phenotypes. In charact erizing the fractional response, fibroblast growth factor was most eff ective in stimulating resting cells to proliferate and least effective on cells actively dividing (fraction 3). Insulin-like growth factor-I (IGF-I) was most active on fraction 3 while epidermal growth factor's mitogenic induction was equivalent across ail fractions. Growth hormo ne receptor (GHR) mRNA was most abundant in mature hypertrophic cells and undetectable in resting cells; IGF-I receptor (IGF-IR) mRNA was de tectable in resting cells but two-fold higher in the fraction adjacent to cells possessing high GHR mRNA, while proliferating and resting ch ondrocytes had elevated IGF-I mRNA levels when compared to that for hy pertrophic chondrocytes. The growth plate distribution of IGF-IR and G HR mRNA implies distinct roles for circulating IGF-I vs. paracrine pro duced IGF-I.