Regeneration of Axons in the Vertebrate Central Nervous System

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Abstract
The author cites instances of axon regeneration in the spinal cord and optic nerve of bony fishes; in the larval and adult spinal cord, brain, optic nerve, and optic tracts of amphibians; and in the spinal cord of reptiles. In mammals three mechanisms were shown: (1) in the very young mammal, outgrowth can occur from neuroblasts undifferentiated at the time of making the lesion; (2) in adults, regeneration often takes place from several axons of cells of the central nervous system or of ganglia outside the neur-axis; and (3) sprouting of new processes from intact central axons after damage and degeneration of adjacent neurons has been demonstrated. Past failure to recognize fully the possibility of central regeneration may have been due, partly, to casual experimentation with inadequate or even inept techniques, unskilled surgical procedures resulting in massive tissue damage, improper selection of histological methods to reveal regeneration and to wrong timing of experiments. However, it was mainly lack of knowledge regarding the means of creating or maintaining a milieu favorable to growth of sprouting axons in brain and cord lesions that prevented earlier recognition of the possibilities. Now, it may be only a matter of time before means will be found to make one or more of the processes of regeneration practicable.