Programmed cell death in post-traumatic bone callus

Some osteoblasts in the expanded population of periosteal cells that occurs following bone injury are removed from the callus by apoptosis. Our objective was to study whether the consequences of activation of the death program could include feedback control of the healing response. Transforming growth factor beta and interleukin-1beta were delivered together continuously to a standardized tibial defect in rats for 3 days using implanted micro-osmotic pumps. The bones were recovered at 1, 2, 3, 5, 7, 10 and 14 days after injury (n = 6 in each treated and control group) and concentrations of proliferating cells, osteoblasts and apoptotic bodies were determined. The injury-induced apoptotic component of the healing response was shifted in time due to the combined cytokines, compared with vehicle only, with the result that the peak in the concentration of apoptotic bodies occurred 2-3 days earlier in the treated animals. Neither osteoprogenitor proliferation nor osteoblast concentration was affected by addition of the cytokines. The results suggested that activation of apoptosis during injury repair was not necessarily a passive consequence of the cellular response to injury. Programmed cell death could therefore have an active role in modulating bone repair.