Long-distance regulation of regenerating frog axons.

Following a nerve crush, damaged frog motor axons regenerate to reinnervate their denervated muscle fibre targets. The axons do this by growing down their old nerve tubes to their former synaptic sites. It is the naked basal lamina of the nerve tube that appears to direct the regenerating axons by providing a substratum over which the axons will preferentially grow. Another possible mechanism for directing the regenerating axons to the end-plates is the release of molecular signals by the cells of the nerve tube or by the denervated muscle fibres. This paper provides evidence that chemical signals do direct the regeneration process. Such signals, released several millimetres from a growing nerve tip, cause it to change direction and bend towards the source. Both the cells of the nerve tube and the denervated muscle fibres release diffusible substances and thereby establish a gradient that affects the regenerating axons.