Circulating growth factor studies in growth plate versus resting cartilage in vitro: tissue responsiveness

We attempted to develop a system in which cartilage growth-plate responses could be examined in vitro. In vivo, rat osteochondral growth-plate width was decreased by hypophysectomy and restored toward normal by GH. Subsequently, segments of resting costal cartilage and osteochondral growth plate from hypophysectomized rats were incubated in vitro with [35S]sulfate and [3H]thymidine. Within ribs, growth plate generally had basal uptake greater than resting cartilage and a greater increase in activity on addition of normal rat serum. Across ribs 3-9, there was little change in growth plate or resting cartilage sulfate uptake, and only a modest increase in thymidine uptake caudally. Increasing concentrations of normal rat serum provided parallel stimulation for both growth plate and resting cartilage, with significant responses to as little as 1 microliter (sulfate uptake) or 5 microliters (thymidine uptake). Stimulation for both growth plate and resting cartilage by hypophysectomized rat serum was significantly less than by normal rat serum; activity was restored by GH in vivo, establishing growth plate responsiveness to GH- and pituitary-dependent factors. Growth plate and resting cartilage were generally unaffected by exposure in vitro to supraphysiological concentrations of GH, cortisol, testosterone, estradiol, or T3. Use of osteochondral junction tissue appears to yield an assay system that is sensitive and sufficiently reproducible to allow study of growth factor action at the growth plate in vitro.