Isolated satellite cells of a peripheral nerve direct the growth of regenerating frog axons

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Abstract
Frog motor axons regenerate and grow back to reinnervate their targets, the original motor end plates, after a lesion. When the cutaneous pectoris muscle is cut away and a segment of peripheral nerve is placed in the vicinity of regenerating axons they turn and grow toward it. This is in marked contrast to the random pattern of axonal outgrowth seen in the absence of a target. The influence on the direction of axonal growth of motor neurons can be produced by a 1-mm segment of nerve satellite cells over a distance of more than 8 mm. The nerve satellite cells have no influence on the direction of growth of the regenerating axons after all the cells in the nerve segment have been killed, leaving only the Schwann cell basal lamina tubes intact. These results show that the cells in the segment of the nerve trunk contain cues that actively direct the growth of motor neurons. Two possible explanations for this effect might be (1) that the cells act indirectly by influencing the organization of the substructure over which axons regenerate or (2) that the nerve satellite cells release a diffusible substance that acts directly on the regenerating axons.