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Spatiotemporal expression of extracellular matrix components during the chondrogenic and osteogenic phases of bone healing.

In this study, we investigated the cascade of events involved in the early phases of bone healing in rats, especially the transition from chondrogenesis to osteogenesis, which involves cellular and extracellular matrix (ECM) components. We used a standardized closed tibial fracture model in Wistar rats, which was divided into nine groups of five animals each, and the fracture area was evaluated at 0, 12, 24, 48, 72, 96, 144, 192, and 240 hours post-injury. Histological, histochemical, immunohistochemical and morphometric techniques were used to evaluate the proliferating cell nuclear antigen (PCNA), transforming growth factor-beta (TGF-β), vascular endothelial growth factor (VEGF), type I procollagen (procoll-I), type I collagen (coll-I), and type II collagen (coll-II) expression at every time point. TGF-β expression peaked after 144 hours, in the initial chondrogenic phase. VEGF expression reached the first peak at 96-144 hours post-injury, in the initial chondrogenic phase and the second peak at 240 hours, in the osteogenic phase. Except at 48 hours, PCNA expression increased gradually from 12 hours and peaked at 96 hours in the prechondrogenic phase, and then decreased gradually until 240 hours in the osteogenic phase. Total collagen (T-coll) and coll-II reached an expression peak at 144 hours, in the chondrogenic phase. No differences were observed between their expression from 12 hours to 72 hours and at 240 hours post-injury. The results suggest that spatiotemporal expression of ECM components during the chondrogenic and osteogenic phases of bone healing depends on several combined and orchestrated factors. A better understanding of the coordinated participation of cells and ECM components in the early bone healing process may provide new insights into the etiology of abnormal or delayed fracture healing.