Adult Rat Dorsal Root Ganglion Neurons Extend Neurites on Predegenerated But Not on Normal Peripheral Nerves In Vitro

Contact inhibition in the failure of mammalian CNS axonal regeneration

Anamniote animals, such as fish and amphibians, are able to regenerate damaged CNS nerves following injury, but regeneration in the mammalian CNS tracts, such as the optic nerve, does not occur. However, severed adult mammalian retinal axons can regenerate into peripheral nerve segments grafted into the brain and this finding has emphasized the importance of the environment in explaining regenerative failure in the adult mammalian CNS. Following lesions, regenerating axons encounter the glial cells, oligodendrocytes and astrocytes, and their derivatives, respectively myelin and the astrocytic scar. Experiments to investigate the influence of these components on axon growth in culture have revealed cell-surface and extracellular matrix molecules that inhibit axon extension and growth cone motility. Structural and functional characterization of these ligands and their receptors is underway, and may solve the interesting neurobiological conundrum posed by the failure of mammalian CNS regeneration. Simultaneously, this might allow new possibilities for treatment of the severe clinical disabilities resulting from injury to the brain and spinal cord.

Embryonic optic nerve tissue fails to support neurite outgrowth by central and peripheral neurons in vitro

The failure of axon regeneration in the injured mammalian central nervous system has been ascribed, in part, to the inhibitory effects of myelin proteins. To investigate the influence of myelination on neurite growth and regeneration by both central nervous system and peripheral nervous system neurons, isolated rat neonatal retinal ganglion cells and adult and neonatal dorsal root ganglion neurons were cultured on cryostat sections of both immature unmyelinated and mature fully myelinated adult rat optic nerve. In agreement with earlier studies using neonatal peripheral neurons, the adult optic nerve failed to support neurite outgrowth from any of the neurons tested. A new finding was that tissue sections from unmyelinated optic nerve (aged embryonic days 18 and 20, and postnatal days 1-3), also failed to support the growth of neurites from neonatal retinal ganglion cells and both neonatal and adult dorsal root ganglion neurons. Neonatal retinal ganglion cells also failed to extend neurites on sections of pre-degenerated sciatic nerve, a tissue shown in our previous work to be a good substratum for supporting neurite growth for both neonatal and adult DRG neurons. These results suggest that cells in the immature optic nerve either express widely acting axon growth inhibitory molecules unrelated to previously described myelin proteins, or do not synthesize appropriate axon growth promoting molecules. They also reveal that, for axon regeneration, central nervous system and peripheral sensory neurons require distinct substratum interactions.

Observations on the progress of Wallerian degeneration in transected peripheral nerves of C57BL/Wld mice in the presence of recruited macrophages

The first part of this study is a description of the effect of the intraneural injection of lysophosphatidyl choline into the sciatic nerves of C57BL/Wld mice. This mouse is unusual because its peripheral nerve fibres degenerate very slowly after transection, and few myelomonocytic cells are recruited into the endoneurium after traumatic injury. However, intraneural injection of lysophosphatidyl choline produced a typical demyelinating lesion in which recruited macrophages were active in removal of myelin. In the second part of the study, nerves were transected either before, at the same time as, or some days after, the intraneural injection of lysophosphatidyl choline into the distal stump; the changes within the endoneurium were compared with those seen in distal stumps which had not been injected with lysophosphatidyl choline. Immunohistochemical and ultrastructural examination during the period 1-4 weeks after transection showed that degeneration occurred in the portion of each nerve which had been injected with LPC (and which therefore contained exogenous macrophages) but failed to occur in the portion of nerve which was not penetrated by the injected bolus of lysophosphatidyl choline. It is suggested that the unusual property of sealing off of the tips of the transected axons within the distal stumps may be a significant factor in maintaining 'normal' Schwann cell-axon relationships along transected axons, even though the axons are separated from their cell bodies. Lysophosphatidyl choline destabilises the Schwann cell-axon relationship by initiating myelin breakdown within the Schwann cell. It is suggested that while the Schwann cells remain closely associated with the axons in the distal stumps, they do not behave as denervated cells and consequently may be incapable of signalling their damaged status.

Neurotoxic damage evokes regenerative responses from adult rat sensory neurones

When adult rats were pre-treated systemically with the excitotoxin, capsaicin, a subset of their dorsal root ganglion (DRG) sensory neurones responded as if they had been axotomised. Firstly, an increased proportion of capsaicin-sensitive DRG neurones (identified by using a cobalt stain) vigorously extended processes in vitro. Additionally, a subset of small dorsal root ganglion neurones expressed elevated levels of the growth-associated protein-43 (GAP-43) in situ. Both vigorous neuritogenesis and elevated GAP-43 levels are characteristics of neurones that are actively regenerating following mechanical damage. Although capsaicin-treated adult DRG neurones show these regenerative responses, their C-fibers fail to regenerate in vivo.

Macrophages and nerve regeneration

Macrophages are not only phagocytic cells but also secrete a plethora of growth factors that are potentially important for regeneration. This review will examine the emerging evidence of a likely contribution by macrophages to axonal regeneration.

Consequences of slow Wallerian degeneration for regenerating motor and sensory axons

The time course of Wallerian degeneration in the tibial and saphenous nerves was compared in Balb/c mice and mice of the C57BL/Ola strain (Lunn et al., 1989). Axons, particularly myelinated ones, in nerves of C57BL/Ola mice are very slow to degenerate, many still being present 3 weeks after axotomy. Nuclear numbers in the distal stump peak much later and do not reach the levels found in Balb/c mice; debris removal is very slow, and Schwann cell numbers only rise slightly above normal levels in the long term. Regeneration was investigated electrophysiologically and by electron microscopy (EM). Myelinated sensory axons regenerated slowly and incompletely compared with motor ones which were only slightly slowed after nerve crush (although they were significantly hindered after nerve section). Total myelinated axon numbers were still some 20% less than normal even after 200 days in sensory nerves. Even after all axons had degenerated in C57BL/Ola mice, regeneration rates of neither myelinated nor unmyelinated sensory axons reached those achieved in Balb/c mice. It is concluded that while regeneration can eventually proceed slowly when Wallerian degeneration is much delayed, the usual rapid time course of Wallerian degeneration is necessary if axons, particularly sensory ones, are to regenerate at optimal rates and to maximum extent. While local obstruction to axon growth probably impedes the early phase of regeneration in C57BL/Ola mice, it seems possible that a lack of adequate early signals affects regeneration permanently by minimizing the cell body reaction to injury.