Embryonic Chick Cartilage Produces Its Own Somatomedin-Like Peptide to Stimulate Cartilage Growthin Vitro*

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Abstract
Embryonic chick pelvic cartilages increase in size and weight when incubated in a chemically defined medium in the absence of serum. We addressed the question of whether endogenous production of growth factors by the cartilage was responsible for this growth. We found that conditioned medium, in which pelvic cartilages from 9-day-old chick embryos had been incubated for 3 days, increased cartilage dry weight 32% over weights of cartilages incubated in fresh medium. Increasing concentrations of conditioned medium stimulated cartilage weight and proline incorporation in a dose-dependent manner. To determine the molecular size(s) of potential growth-stimulating factors, conditioned medium was dialyzed at acid pH, lyophilized, and fractionated over HPLC-TSK Spherogel 3000. The collections were pooled into five fractions (> 100K, 30-100K, 20-30K, 12-20K, 1-12K, and < 1K). Each fraction was readded to organ culture, and growth was assessed 3 days later. Only the 1-12K fraction stimulated growth above that of control cartilage. We assayed cartilage and conditioned medium for somatomedin-C (Sm-C) by RIA to determine if Sm-like peptides were present. Although SM-C was not detectable within the cartilage, it was readily measurable in concentrated medium (248 .+-. 35 pg/ml). Since Sm-like peptides might play a functional role in the growth process, we used a monoclonal antibody to Sm-C to determine whether immunoneutralization of the Sm-like peptides would inhibit cartilage growth in vitro. Addition of anti-Sm-C to organ culture of chick cartilage prevented increases in cartilage wet and dry weights (only 16% and 0%, respectively, above preincubation weights). The inhibitory effect of anti-Sm-C could be reversed by the addition of high doses of insulin to the medium. These studies suggest that endogenously produced Sm-like peptides have a functional role in promoting cartilage growth and support the hypothesis that growth factors may regulate growth through autocrine mechanisms.