Cell proliferation and apoptosis during fracture healing

This study investigated the relation between cell proliferation and apoptosis during fracture healing in a mouse femoral fracture model. Left femoral osteotomies were performed in 30 mature male CFLP mice immobilized with uniplanar external fixators. Six animals were killed on days 2, 4, 8, 16, and 24 postfracture for examination. Localization of cell proliferation was examined using immunohistochemistry with proliferating cell nuclear antigen (PCNA) monoclonal antibody. Apoptotic cells were visualized with the terminal deoxynucleotidyl transferase (TdT)-mediated deoxyuridine triphosphate (dUTP)-biotin nick end-labeling (TUNEL) method. Images of each time-specific specimen were captured. The total callus area, the positively labeled cells by PCNA, and TUNEL per high-power field were quantified. Cell proliferation and apoptosis were found coexisting during the entire period of study. In the early phases of fracture healing (days 2-8), PCNA-positive labeling was predominant and peaked at day 8 and the TUNEL-positive labeling was minimal. In later stages of fracture healing (days 16-24), PCNA expression declined at day 16 as callus ossification and remodeling spread within the fracture site and apoptosis was the dominant cell activity with the TUNEL-positive labeling peaking at day 16 and declining sharply at day 24. These cell activities were reflected by the change of fracture callus, where there was a continuous increase in total callus area to day 16 and subsequent decrease at day 24. This study indicated that cell proliferation and apoptosis are coupled events during fracture repair, cell proliferation is active at the early stages, and apoptosis is active during the phase of callus remodeling.