Loss of osteocyte integrity in association with microdamage and bone remodeling after fatigue in vivo

As a result of fatigue, bone sustains microdamage, which is then repaired b y bone-remodeling processes. How osteoclastic activity is targeted at the r emoval of microdamaged regions of bone matrix is unknown. In the current st udies, we tested the hypothesis that changes in osteocyte integrity, throug h the initiation of regulated cell death (apoptosis), are associated with f atigue-related microdamage and bone resorption, Ulnae of adult rats mere fa tigue-loaded to produce a known degree of matrix damage. Osteocyte integrit y was then assessed histomorphometrically from terminal deoxynucleotidyl tr ansferase-mediated deoxyuridine triphosphate-nick end labeling (TUNEL)-stai ned sections to detect cells undergoing DNA fragmentation associated with a poptosis; toluidine blue-stained sections were used for secondary morpholog ical confirmation. Ten days after loading, large numbers of TUNEL-positive osteocytes were found in bone surrounding microcracks and in bone surroundi ng: intracortical resorption spaces (similar to 300% increases over control s, p < 0.005). TUNEL labeling in loaded ulnae at sites distant from microcr acks or resorption foci did not differ from that in control bone. Osteocyte s in toluidine blue-stained sections showed equivalent trends to TUNEL-stai ned sections, with significant increases in pyknotic nuclei and empty lacun ae associated with microcracks and intracortical resorption spaces, TUNEL-p ositive osteocytes were observed around bone microdamage by 1 day after loa ding (p < 0.01 relative to baseline), and their number remained elevated th roughout the entire experimental period. Increases in empty lacunae and dec reases in normal osteocyte numbers were observed over time as well. These s tudies show that (1) osteocyte apoptosis is induced by bane fatigue, (2) th is apoptosis is localized to regions of bone that contain microcracks, and (3) osteoclastic resorption after fatigue also coincides with regions of os teocyte apoptosis. The strong associations between microdamage,osteocyte ap optosis, and subsequent bone remodeling support the hypothesis that osteocy te apoptosis provides a key part of the activation or signaling mechanisms by which osteoclasts target bone for removal after fatigue-induced matrix i njury.