Identification of apoptotic cell death in distraction osteogenesis

The purpose of this experimental work was to investigate whether apoptosis contributes to tissue remodelling during distraction bone healing. In a rabbit model of mandibular distraction osteogenesis, we quantitatively analysed the extent of apoptotic cell death in relation to differently applied mechanical loadings. Apoptotic cells were identified by means of an in situ detection assay for nuclear DNA fragmentation using a modified TUNEL procedure and by electron microscopical examination for typical morphological features of programmed cell death. TUNEL-positive cells were frequently detected in samples distracted at higher strain magnitudes. Ultrastructurally, these apoptotic cells displayed a condensed chromatin and fragmented nuclei, while the continuity of their plasma membranes remained intact. Our results clearly indicated that the discontinuous traction of osteotomized mandibles induced enhanced apoptosis. In contrast to non-distracted samples and mandibles distracted at low strain magnitudes, in which only minimal evidence of apoptotic cell death was detected, the application of hyperphysiological strain magnitudes resulted in an increased apoptosis rate. Thus, mechanical loading seems to be a triggering factor for apoptotic changes in osteoblastic cells. These findings suggest a pathophysiological role of apoptotic cell death in the control of tissue integrity during distraction osteogenesis.