The authors have obtained preparations of peripheral nerve tissue in culture which provide either Schwann cell populations, essentially free of connective tissue cells such as fibroblasts, or outgrowing neurites from sensory (or autonomic) ganglia entirely free of ensheathing Schwann cells or connective tissue elements The demonstrated mitogenic capacities of the axon may explain several recent in vivo observations. Thomas has observed that repeated crushes (which enable optimal axon regrowth) of a peripheral nerve leads to a remarkable overproduction of Schwann cells in the distal nerve stump reaching a point at which these cells are applied in multiple layers around each single axon. Also, in crush lesions of the abnormal peripheral nerve roots of dystrophic mice, where a paucity of Schwann cells and myelin segments provides much less degenerating material than in normal nerve, an increase in the number of myelin segments and, presumably, Schwann cells is induced by crush injury. The importance of the axon in provoking Schwann cell proliferation is also indicated by observations, on the proximal stump of severed or crushed sciatic nerve, that proliferation occurs independently of widespread nerve fibre or myelin degeneration and in conditions in which nerve regeneration is most actively occurring.
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