Mechanical recovery of cultured axons after dynamic stretch: correlation to cytoskeletal changes

In this study, we used a novel device to stretch cultured CNS axons up to 77% of their initial length under mechanical conditions mimicking traumatic injury. The morphological response immediately following stretch was recorded using video microscopy techniques, and the cytoskeletal response to stretch was examined using immunocytochemistry techniques. Below the threshold for primary axotomy, axons maintained their overall position on the substrate but displayed periodic distortions at points along their length. These strain induced changes in axonal morphology were observed at all applied strains used in the current study. Moreover, a large fraction of the cultured axons showed a relatively uniform frequency of distensions along their length; only a small minority of the axons in the field showed a few or single larger distension along their length. Interestingly, all axons showing distensions gradually recovered to a straightened pre-stretch shape over a period of 45 min, with most of the relaxation occurring in the first 20 minutes. Phenomenological models developed for the observed mechanical recovery phenomena suggest a molecular-based recovery process (τ_ave approx. 24 minutes) that is consistent with previous towed growth experiments on neurites. Unlike previous towed growth experiments, though, permanent changes in the neurofilamentous cytoskeleton were observed at 2 hr. post-stretch. These data suggest there is an endogenous mechanism for axons to regain morphology after stretch despite permanent changes to parts of the cytoskeleton that occur after even a single mechanical stimulus.