Apoptosis in the regenerate produced by mandibular osteodistraction in the mature rat

AUTHORS: Liu ZJ, Anderson MW, Gu GM, King GJ.

Little is known about the contribution of apoptosis to the formation of the regenerate in distraction osteogenesis. This study investigated the role of apoptosis in the regulation of osteogenesis in relation to mandibular distraction rate and recovery time.

One hundred and twenty-nine 3-month rats received unilateral mandibular ramus osteotomy and distraction device placement. After 3-day latency, these were distracted for 5 days. The slow group was distracted a total of 1 mm (0.2 mm/day), the moderate 2 mm (0.4 mm/day), the rapid 3 mm (0.6 mm/day), and the sham was not distracted (0.0 mm/day). Eight to nine rats from each group were killed at 6 (mid-distraction), 10 (early-consolidation), 24 (mid-consolidation) and 38 (late-consolidation) days following device placement. Baseline data were obtained from an additional eight rats killed at day 3 (end of latency). Sagittal sections (7 mum) of the harvested hemi-mandibles were embedded in paraffin, double-stained with the DeadEnd(TM) Colorimetric TUNEL system for apoptotic cells and tartrate-resistant acid phosphatase for osteoclasts. Cell counts of apoptotic cells and osteoclasts (apoptotic or otherwise) were preformed at 40x magnification using a calibrated grid at the middle regions of the caudal, central and rostral parts of the distraction gap. Counting reproducibility was verified as <13%.

Counts from all three regions were combined because no significant regional difference was found. The majority of the fibrous matrix regenerate was seen at days 6 and 10 while osseous regenerate occurred mainly at days 24 and 38. Significantly higher levels of apoptotic activity were seen at day 24. Apoptotic osteoclasts also peaked at day 24. Apoptotic cell numbers in the slow and moderate groups most closely followed the pattern of the sham in which the apoptosis activity peaked at days 10 or 24. However, those in the rapid group showed delayed peaks at days 24 or 38.

Chalmers Street, Surry Hill, NSW 2010, Australia. gshe6437@mail.usyd.edu.au The transition from fibrous to osseous regenerate during mandibular distraction osteogenesis is accompanied by an increase in cell clearance via apoptosis. A slow to moderate distraction rate allows for the most typical pattern of bone healing while a rapid rate prolongs the healing process.