Proteoglycans synthesized in calluses at various stages of fracture healing in rats.

The sequential cartilage and bone formation was observed in fracture calluses which were formed at fibula diaphysis of Sprague-Dawley rats. The proteoglycans synthesized in the calluses at different stages during fracture healing were labeled in vitro with [35S]sulfate and then analyzed by sucrose density gradient centrifugation. A heavy proteoglycan, in which chondroitin 4-sulfate accounted for approx. 90% of total radioactivity, was predominantly synthesized in addition to light, dermatan sulfate-containing proteoglycans in day-10 and day-20 calluses, when cartilaginous areas were predominant in these calluses. The synthesis of the heavy proteoglycan started around day 7, when chondrogenesis started locally in the callus, and ceased by day 30, when cartilage had been replaced by newly formed bone. The heavy proteoglycan had chemical and physical properties similar to those of the major proteoglycan synthesized by bone matrix-induced cartilages, but different from those of the major one synthesized by the epiphyseal cartilage of neonatal rats. These findings suggest that the sequential molecular transitions observed in fracture healing differ from those in the endochondral ossification of embryonic skeletal tissues but resemble those in bone matrix-induced bone formation.