Spontaneous regeneration of cut axons in adult rat brain

Lesions with precisely defined boundaries were made, in adult rat brains, with a new type of cutting device that remained implanted during various survival times. The subsequent events were followed histologically in silver-stained (including Fink-Heimer) horizontal sections, with myelin and nuclear counterstains. The devices, inserted vertically through the dorsal surface of the brain until they met the floor of the skull, consisted of a horizontal cutting wire (0.9--2.6 mm long) between two vertical support wires, all 90 micrometers diameter. Following fixation of the brain, the device was removed from its ventral surface; thus, the channels left by the support wires appeared as two holes in horizontal sections, clearly marking the limits of the cut. The extents of the tissue cut by, and that passively deflected around, such devices were characterized from mearsurements of brains examined immediately after an incision was made and from degeneration studies after 3-day implantations. After implantations of 18--230 days, the severed axons no longer abutted the line of such lesions, as seen initially; now axons were often observed to extend alongside the lesion, then to bend and course around the support wire. Such "detours' ' curved back into the injured tracts, seeming to reconnect them appropriately. Detours had begun by 3--8 days, and end-swellings were frequently seen on their reoriented axons. Neither collateral sprouting of spared axons nor passive deformation of brain tissue was consistent with the findings. It is concluded that a massive and reconstructive regeneration of cut axons occurred around the ends of lesions made by the cutting devices.