In situ localization of collagen production by chondrocytes and osteoblasts in fracture callus.

An experimental model of fracture-healing was used to study the production of types-I and II collagen by in situ hybridization. The distribution of cartilage matrix in callus was determined by histochemical staining. Messenger RNA (mRNA) for cartilage-specific type-II collagen was detectable as early as the fifth day in a small number of cells that had acquired a chondrocyte phenotype but that also contained type-I collagen mRNA, suggesting an ongoing change in the expression of collagen genes. The location of the first chondrocytes, which were adjacent to cortical bone, suggested that they originated from cells that had derived from the periosteum by differentiation. On the seventh day of callus formation, the presence of both type-I and type-II collagen mRNA in chondrocytes of expanding cartilage suggested that most growth occurred by differentiation of mesenchymal cells and less by proliferation of differentiated chondrocytes. Expansion continued until the tenth to fourteenth day, after which the cartilage was replaced by woven bone. This was characterized by the presence of osteoblasts that were active in the synthesis of type-I collagen.