Axon sprouting in the spinal cord: growth promoting and growth inhibitory mechanisms

After lesions in the central nervous system (CNS), the affected nerve fibers usually cannot regenerate and reconnect to their original target cells. One important reason for this failure to regenerate is the presence of neurite growth inhibitory molecules in the myelin sheath of central nerve fibers. Despite the absence of regeneration fiber growth can occur after CNS lesions from intact nerve fibers unaffected by the lesion. These fibers can form new collaterals and sprout into the region denervated by the lesion, thereby increasing their terminal arbors in a process called collateral sprouting. A certain functional compensation for the nerve fibers lost by the lesion can be achieved by this mechanism. In the spinal cord, collateral sprouting is extensive after lesions in young postnatal animals and decreases with increasing age. In the spinal cord of adult animals, axon sprouting can be observed but is strongly restricted. The factors that determine the amount of sprouting found after lesions at different ages are still largely unknown. Recent evidence suggests that the myelin-associated neurite growth inhibitors that suppress regeneration also restrict collateral sprouting in the spinal cord. In addition, the expression of growth-associated molecules, in particular the growth-associated protein GAP-43, by the sprouting nerve fibers appears to be an important determinant of the sprouting response. The robustness of the sprouting response is thus likely to be controlled by intrinsic growth determinants of the sprouting neuron as well as by the growth promoting and growth inhibitory properties of the microenvironment of the sprouting fibers.