Gene expression in mineralizing chick epiphyseal cartilage

To map transcriptional events associated with mineralization in developing long bones, we have established protocols for preparing RNA from regions of chick epiphyseal cartilage. Using these RNA preparations, we have probed for appearance of mRNA coding for type I, II, and X collagen, as well as osteonectin and calmodulin. Type II collagen mRNA was found in proliferating cartilage and, in lower amounts, in hypertrophic/calcifying cartilage. Type X mRNA was absent from proliferating cartilage and present in hypertrophic/calcifying cartilage at steady state levels slightly lower than that of type II mRNA. Type I mRNA was the major collagen mRNA species in endochondral bone; however, significant amounts of type X mRNA were also found. Examination of type X/type II ratios suggest that the cells producing type X mRNA in bone are different from those in the hypertrophic/calcifying cartilage region. Osteonectin mRNA was present in endochondral bone; however, significant amounts were also detected in precalcified cartilage. Indeed, the level of osteonectin mRNA was significantly higher in the resting/proliferating region than in the hypertrophic/calcifying region of the cartilage. No correlation was observed between calmodulin mRNA and the development of mineralization; levels of this message were slightly lower in endochondral bone, embryonic sterna, and calvaria than they were in chick liver and considerably lower than the calmodulin mRNA levels in chick brain.