Site‐specific CGRP innervation coincides with bone formation during fracture healing and modeling: A study in rat angulated tibia

Abstract

Sensory neuropeptide involved in local bone turnover is known, but poorly understood. In the present study, we analyze the occurrence of neuronal CGRP during healing and modeling of straight and angular tibial fractures in 74 rats. Bone healing and modeling was assessed by radiography and reinnervation by semi-quantitative immunohistochemistry method at fracture site between 1-12 weeks postfracture. The regenerating nerve fibers containing CGRP were observed in fracture callus as well as in close proximity to chondrocytes, with woven bone in both fractures already at week 1. Notably, it located predominantly on the concave side of angulated fracture in the manner of sprouting into bone from weeks 3 to 5 postfracture. In both fractures, fracture calluses peaked radiographically at week 3 postfracture. In angulated fracture, a reduction of 11% in callus thickness on convex side and an increase of 365% on concave side were noted from weeks 3 to 12. A 27-fold increase in total neuronal CGRP in straight fracture and 38-fold increases in angular fracture compared to intact bone was observed at week 3. In both types of fracture, neuronal CGRP was greater on the concave side than the convex; this difference was more pronounced in the angulated fracture. CGRP immunoreactivity clearly coincides with amount of new bone formation especially on the concave side of angulated fracture. The combined results suggest that fracture evokes an intense, localized in-growth of new nerve fibers containing CGRP, which may prove to be a prerequisite of fracture healing and modeling. ß

Exuberant sprouting of sensory and sympathetic nerve fibers in nonhealed bone fractures and the generation and maintenance of chronic skeletal pain

“…Following injury to the young healthy bone there is a small but noticeable sprouting of sensory and sympathetic nerve fibers around the site of bone fracture [35,58,59,101]. As simple fractures rapidly heal in the young, healthy skeleton, these newly sprouted nerve fibers are pruned back to normal levels as the cartilaginous callus undergoes mineralization and then resorption.…”

“…Studies including osteoarthritis, temporomandibular joint disorder, and degenerative intervertebral disc (IVD) have also reported the sprouting of sensory and sympathetic nerve fibers into normally avascular, cartilaginous tissue [15,23,24,39,40,42,50,89,96]. Studies have also reported sprouting of CGRP + , Substance P + , and GAP43 + nerve fibers into the callus that forms around the bone fracture in rats [35,58,59] and in the synovium of arthritic joints of humans and mammals [13,94,100]. Furthermore, it is well documented in sarcoma, prostate, and breast cancers in bone that there is ectopic sprouting of sensory and sympathetic nerve fibers in the bone marrow, mineralized bone, and periosteum [10,27,38].…”

Treating skeletal pain: limitations of conventional anti-inflammatory drugs, and anti-neurotrophic factor as a possible alternative

“…Localized, intense ingrowth of new sensory nerve fibers containing the neuropeptide CGRP (calcitonin gene-related peptide) was also observed at the fracture callus in rats. 39 As CGRP immunoreactivity coincides with the amount of new bone formation, this finding suggests an association between CGRP-positive innervation and fracture healing.…”

Synovial Neuronal Changes in Knee Joint Osteoarthritis

“…The immunoreactivity was measured as area (in mm 2 ), using Easy Analysis software (Bergström Instruments, Sweden), as previously described [15]. A standard lower and upper threshold of fluorescence intensity was consistently applied for positively stained nerve fibers.…”