Stress fractures (SFxs) are debilitating
injuries and exact mechanisms that initiate their repair incompletely understood. We hypothesised that osteocyte apoptosis and expression of
cytokines and
proteins such as sclerostin,
VEGF, TGF-β, COX-2 and
IL-6 were early signalling events to facilitate the formation of periosteal woven bone and recruitment of osteoclast precursors to the site of remodelling. A SFx was created in the right ulna of mature female wistar rats using cyclic end loading. Rats were killed 1, 4 and 7 days after loading (n=5 per group). Standard histological staining was used to examine SFx morphology and immunohistochemistry to detect the localisation of these
proteins and in situ hybridisation to detect
mRNA along the SFx line or gene expression to quantify the target genes. Unloaded ulnae served as controls. The labelling index of
caspase-3, COX-2 and
IL-6 was significantly elevated in the region of SFxs at all time points compared with controls (P<0.001). In addition, the labelling index of sclerostin
protein was significantly reduced in osteocytes adjacent to the SFx region when compared with controls at all three time points (P<0.001). Both
VEGF and TGF-β expressions were only localised in the woven bone. These data reinforce the involvement of osteocyte apoptosis in the healing of
fatigue damage in bone, and demonstrate that local regulation of sclerostin, COX-2 and
IL-6 are important signalling events associated with new bone formation and SFx remodelling.