Restoration of growth plate function following radiotherapy is driven by increased proliferative and synthetic activity of expansions of chondrocytic clones.

Radiation therapy encompassing an active epiphysis can negatively impact the potential for bone growth by disrupting cell-cycle progression and accelerating apoptosis and terminal differentiation in physeal chondrocytes. Despite functional derangement following radiation exposure, the irradiated growth plate retains a capacity for regeneration and recovery of growth. The purpose of this study was to characterize the initial sequence of events leading to functional growth recovery in irradiated weanling rat growth plates. We hypothesized that growth in an irradiated epiphysis would be partially restored due to the expansion of chondrocytic clones. Stereological histomorphometry was used to compare chondrocytic cell and matrix turnover between the first and second week following irradiation, and to determine the relative contribution of each of the cellular and extracellular matrix (ECM) compartments to growth. We found that restoration of growth in the irradiated limb was strongly associated with the proliferative activity and production of ECM by these chondrocytic clones, as they expand in average volume, but not in numerical density. We conclude that chondrocytes forming expansive clones and exhibiting increased mitotic and matrix synthesis activity initiate the early restoration of function in the irradiated growth plate, and would be a logical target for strategies to restore full growth potential. Copyright (c) 2006 Orthopaedic Research Society.