Preferential growth of neonatal rat dorsal root ganglion cells on homotypic peripheral nerve substrates in vitro

Electrical stimulation restores the specificity of sensory axon regeneration

Electrical stimulation at the time of nerve repair promotes motoneurons to reinnervate appropriate pathways leading to muscle and stimulates sensory neurons to regenerate. The present experiments examine the effects of electrical stimulation on the specificity of sensory axon regeneration. The unoperated rat femoral cutaneous branch is served by 2-3 times more DRG neurons than is the muscle branch. After transection and repair of the femoral trunk, equal numbers of DRG neurons project to both branches. However, 1 h of electrical stimulation restores the normal proportion of DRG neurons reinnervating skin and muscle. To ask if the redistribution of stimulated neurons results from enhanced specificity of target reinnervation, we developed a new technique of sequential double labeling. DRG neurons projecting to the femoral muscle branch were prelabeled with Fluoro Gold 2 weeks before the nerve was transected proximally and repaired with or without 1 h of 20-Hz electrical stimulation. Three weeks after repair, the muscle nerve was labeled a second time with Fluororuby. The percentage of regenerating neurons that both originally served muscle and returned to muscle after nerve repair increased from 40% without stimulation to 75% with stimulation. Electrical stimulation thus dramatically alters the distribution of regenerating sensory axons, replacing normally random behavior with selective reinnervation of tissue-specific targets. If the enhanced regeneration specificity resulting from electrical stimulation is found to improve function in a large animal model, this convenient and safe technique may be a useful adjunct to clinical nerve repair.

Schwann cells and endoneurial extracellular matrix molecules as potential cues for sorting of regenerated axons: a review

Besides very well elaborated microsurgical management of severed peripheral nerves, the clinical results of functional recovery following surgical repair of mixed nerves are disappointing. An improvement of functional recovery after peripheral nerve lesion requires the accurate regeneration of axons to their original target tissues and structures. Therefore, better clinical results could be obtained by a greater understanding of the cellular and molecular biology of selective nerve regeneration. The studies concerning Schwann cells and their endoneurial extracellular matrix as potent cues for selective promotion and influence of regenerating motor and sensory axons are reviewed. Knowledge of the sorting mechanisms of regenerated motor and sensory axons is needed not only for improvement of functional recovery, but also for the development of biocompatible nerve prostheses.

Computer-assisted quantitative analysis of immunofluorescence staining of the extracellular matrix in rat dorsal and ventral spinal roots

The endoneurial extracellular matrix (ECM) is produced by Schwann cells and fibroblasts under the control of axons. Dorsal and ventral spinal roots contain different types of axons, but information is not available on differences in the composition of their ECM. A comparison was made of the intensity of immunofluorescence staining of chondroitin sulfate proteoglycan, fibronectin, tenascin and thrombospondin in the endoneurial ECM of rat dorsal and ventral spinal roots. Sections of dorsal and ventral roots were incubated simultaneously for indirect immunofluorescence detection of the epitopes studied. Brightness of immunofluorescence staining was assessed by computer-assisted image analysis using interactive segmentation of digitized images to select areas to be analyzed. Our results revealed quantitative differences in the composition of endoneurial ECM of spinal dorsal and ventral roots, probably due to the presence of different types of axons. The ECM composition of the endoneurium in dorsal and ventral roots may be related with the creation of extrinsic conditions that support differential regeneration of afferent and motor axons after injury.

Disruption of spinal cord white matter and sciatic nerve geometry inhibits axonal growth in vitro in the absence of glial scarring

BACKGROUND

Axons within the mature mammalian central nervous system fail to regenerate following injury, usually resulting in long-lasting motor and sensory deficits. Studies involving transplantation of adult neurons into white matter implicate glial scar-associated factors in regeneration failure. However, these studies cannot distinguish between the effects of these factors and disruption of the spatial organization of cells and molecular factors (disrupted geometry). Since white matter can support or inhibit neurite growth depending on the geometry of the fiber tract, the present study sought to determine whether disrupted geometry is sufficient to inhibit neurite growth.

RESULTS

Embryonic chick sympathetic neurons were cultured on unfixed longitudinal cryostat sections of mature rat spinal cord or sciatic nerve that had been crushed with forceps ex vivo then immediately frozen to prevent glial scarring. Neurite growth on uncrushed portions of spinal cord white matter or sciatic nerve was extensive and highly parallel with the longitudinal axis of the fiber tract but did not extend onto crushed portions. Moreover, neurite growth from neurons attached directly to crushed white matter or nerve tissue was shorter and less parallel compared with neurite growth on uncrushed tissue. In contrast, neurite growth appeared to be unaffected by crushed spinal cord gray matter.

CONCLUSIONS

These observations suggest that glial scar-associated factors are not necessary to block axonal growth at sites of injury. Disruption of fiber tract geometry, perhaps involving myelin-associated neurite-growth inhibitors, may be sufficient to pose a barrier to regenerating axons in spinal cord white matter and peripheral nerves.

White matter of the CNS supports or inhibits neurite outgrowth in vitro depending on geometry

Axonal regeneration is normally limited within myelinated fiber tracts in the CNS of higher vertebrates. Numerous studies suggest that CNS myelin contains inhibitors that may contribute to abortive axonal growth. In contrast to the evidence of myelin-associated neurite inhibitors, embryonic neurons transplanted into the CNS can regenerate extensively within myelinated tracts in vivo. It has been speculated that embryonic neurons do not yet express the appropriate receptors for myelin-associated inhibitors. Recently, however, extensive regeneration from transplanted adult neurons has also been reported within myelinated tracts of the CNS, casting doubt on the role myelin-associated inhibitors play in abortive regeneration. The present study reexamined the potential of white matter to support neurite growth in vitro. By the use of Neurobasal medium, neurons were cultured onto unfixed cryostat sections of mature rat CNS tissue. As documented previously, robust neuronal attachment and neurite outgrowth occurred on gray matter but these neurites were sharply inhibited by white matter. In addition, however, increased rates of neuronal attachment directly to white matter occurred with neurite outgrowth comparable in length with that on gray matter but limited to directions parallel to the fiber tract. Frequently, the same section of white matter was found to inhibit neurite outgrowth from neurons on gray matter while supporting parallel neurite outgrowth from neurons on white matter. These results suggest that whether white matter supports or inhibits axonal growth depends on the geometric relationship between the axon and the fiber tract; more specifically, white matter supports parallel growth but inhibits nonparallel growth.

Regulation of fibronectin alternative splicing during peripheral nerve repair

Wallerian degeneration following peripheral nerve injury is associated with increased production of fibronectin and other extracellular matrix molecules that are thought to enhance repair. We have shown previously that alternative splicing of the mRNA for fibronectin also changes following sciatic nerve lesions so as to reexpress forms of mRNA seen during embryogenesis. In the present study, we have examined the role of the regenerating axons in the regulation of this splicing. We have compared the patterns of fibronectin mRNA splicing seen in sciatic nerve development with that seen in cut nerves (that do not regenerate), crushed nerves (that regenerate successfully), and Schwann cells cultured in forskolin so as to mimic axonal signals. By using a reverse transcriptase polymerase chain reaction assay to examine all three regions of fibronectin mRNA splicing in a quantitative manner, we found that embryonic patterns of fibronectin mRNA splicing appear rapidly following injury and are not then altered by reestablishment of axons in the nerve. In addition, we found that forskolin has no effect on fibronectin mRNA splicing in cultured cells. We conclude that axonal signals do not regulate the pattern of fibronectin alternative splicing in peripheral nerve repair.