Organization of a unique net-like meshwork of CGRP+ sensory fibers in the mouse periosteum: Implications for the generation and maintenance of bone fracture pain

Abstract

Although bone fracture frequently results in significant pain and can lead to increased morbidity and mortality, it is still not clearly understood how sensory neurons are organized to detect fracture pain. In the present report we focused on the periosteum, as this thin tissue is highly innervated and tightly adherent to the outer surface of bone. To define the organization and distribution of the sensory and sympathetic fibers in the mouse femoral periosteum, we used whole mount preparations, transverse sections, immunofluoresence, and laser scanning confocal microscopy. While both the outer fibrous layer and the inner more cellular cambium layer of the periosteum receive an extensive innervation by calcitonin gene related peptide (CGRP) and 200-kDa neurofilament (NF200) positive sensory fibers as well as tyrosine hydroxylase (TH) positive sympathetic fibers, there is a differential organization of sensory vs. sympathetic fibers within the periosteum. In both layers, the great majority of TH+ fibers are closely associated with CD31+ blood vessels and wind around the larger vessels in a corkscrew pattern. In contrast, the majority of CGRP+ and NF200+ sensory fibers in both layers lack a clear association with CD31+ blood vessels and appear to be organized in a dense net-like meshwork to detect mechanical distortion of periosteum and bone. This organization would explain why stabilization/fixation causes a marked attenuation of movement-evoked fracture pain. Understanding the organization, plasticity and molecular characteristics of sensory and sympathetic nerve fibers innervating the skeleton may permit the development of novel mechanism-based therapies for treating non-malignant skeletal pain.

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

“…Thus, in the young, healthy skeleton, there are no sensory nerve fibers in the articular cartilage, a very low density in mineralized bone, low to moderate density in bone marrow, and a very high density in the periosteum [14]. This sensory innervation of the skeleton appears to be tightly regulated in order to minimize sensory innervation in skeletal compartments that undergo significant mechanical stress and loading (ie, articular cartilage and mineralized bone) and maximize sensory innervation in compartments that do not undergo mechanical loading, but instead detect injurious mechanical distortion (ie, periosteum) [14,62,68]. …”

“…The initial pain probably arises primarily from mechanical distortion and activation of the normally silent mechanosensitive sensory nerve fibers in the periosteum expressing ion channels Piezo 2, transient receptor potential A1 channel (TRPA1), TRPV1, and TRPV4 [22,56,68,82]. Following the initial sharp, arresting pain, a hematoma is formed around the fracture site and inflammatory cells release immune mediators such as prostaglandins, bradykinins, histamine, and nerve growth factor [6,29,32,34,48,52,55,57,60] that sensitize nerve fibers present in and around the fracture site [35,66,81].…”

Bone pain mechanism in osteoporosis: a narrative review

“…The periosteum receives the majority of sensory innervation of any other compartment of the skeleton. In the periosteum, the A-delta and C-sensory nerve fibers are arranged in a fishnet-like pattern, which appears to be designed to act as a 'neural net' to detect mechanical injury or distortion of the underlying cortical bone (29). In the cortical bone, both A-delta and C-fibers typically colocalise with blood vessels that run through the Haversian and Volkmann canals; however it is clear that the majority of blood vessels in these canals is not innervated by sensory nerve fibers.…”

Behavioral signs of pain and functional impairment in a mouse model of osteogenesis imperfecta

“…Multiple mechanisms can contribute to the pain phenotype observed in this model. Pain in the Col1a1 Jrt /+ model may be related to multiple fractures [16]. Bone abnormalities and fracture disrupt the periosteum, rich in sensory innervation, and allow stimulation of the peripheral nerve endings supplying bone tissues [17].…”