Reactive synaptogenesis in the CNS. A comparison of regenerating and sprouting systems.

Lesion-induced synaptogenesis was compared in the goldfish retinotectal system, which readily regenerates after optic nerve crush, and in the cat spinal cord, in which collateral sprouting has been demonstrated after dorsal rhizotomy. Quantitative electron microscopic methods were used. Reinnervation of the tectum was complete, but was characterized by a prolonged time course. Reinnervation appeared to be achieved by retinal axons and not by sprouting from nonretinal axons. Reinnervation in the cat spinal cord was also virtually complete, but was very rapid and may be mediated by some axons that are similar to those destroyed and by other axons that are different.