Temporal and spatial CGRP innervation in recombinant human bone morphogenetic protein induced spinal fusion in rabbits

Study Design. Animal experiment using a rabbit posterolateral intertransverse process fusion model. Objective. To explore the temporal and spatial distribution of sensory nerve fibers expressing calcitonin-gene related peptide (CGRP) during spinal fusion induced by recombinant human bone morphogenetic protein-4 and the role of the CGRP innervation in ectopic bone formation and remodeling. Summary of Background Data. Sensory neuropeptide CGRP involved in local bone turnover has been evidenced but its underlying mechanism is poorly understood. Knowledge in the CGRP innervation in ectopic bone induced by bone morphogenetic proteins can help us to understand its role in bone turnover. Methods. Twenty-seven New Zealand white rabbits underwent single level posterolateral intertransverse process fusion of the lumbar vertebrae with implantation of porous poly-d,l-lactic acid blocks loaded with 1.25 μg recombinant human bone morphogenetic protein-4 solution. Animals were killed and the operated lumbar vertebrae were harvested for histomorphological evaluation at 3 days (n = 3), 1 week (n = 6), 3 weeks (n = 6), 7 weeks (n = 6), and 12 weeks (n = 6) following surgery, respectively. Results. New cartilage presented at 1 week postimplantation adjacent to the implant, reached a peak volume at week 3 followed by a drop till week 12 after its ossification. Trabeculae-like woven bone structure presented at week 3. CGRP-positive nerve fibers regenerated already at 3 days postimplantation, reached its peak density at week 3. The CGRP-positive fibers presented both in fibrous tissues adjacent to proliferating cartilages and in bone marrow of newly formed trabecular bone. Conclusions. The observed spatial and temporal regeneration of CGRP-positive nerve fibers in ectopic bone formation suggested CGRP innervation is associated with ectopic osteogenesis.