Stretching Yourself to the Limit

Abstract

During embryonic development, the growth cone drives neurite outgrowth. However, the mechanisms that contribute to axonal elongation once neuronal connections have been made--that allow, for instance, a motor neuron that runs from a baby's spinal cord to his foot to extend the full length of the adult leg--have been far less clear. Pfister et al. cultured embryonic rat dorsal root ganglion neurons on overlapping membranes that they drew apart for days to weeks as a model for axonal stretch. During the first 24 hours of stretch, axon tracts could extend at a rate of about 1 mm/day without disconnecting, to about 10 times their initial length. Subsequently, however, they could be extended at rates up to 8 mm/day. Transmission electron microscopy revealed that "stretched axons" maintained their normal ultrastructural architecture and, unexpectedly, were larger in diameter than unstretched axons. This rate of growth considerably exceeds the 1 mm/day maximum rate observed for growth-cone extension as well as the 1 mm/day rate for transport of structural proteins, which led the authors to suggest that mechanical forces may stimulate an alternative axonal growth process. B. J. Pfister, A. Iwata, D. F. Meaney, D. H. Smith, Extreme stretch growth of integrated axons. J. Neurosci . 24 , 7978-7983 (2004). [Abstract] [Full Text]