Microtubule stability in severed axons

We examined severed axons of cat sympathetic nerves and severed neurites of cultured chick sensory neurons for evidence of extensive microtubule depolymerization. Cat sympathetic fibres fixed at various times after severing were cross-sectioned for electron microscopy from both cut ends. The number density of microtubules was determined at various times after severing for matched proximal and distal regions equidistant from cut ends. These data show that the number density of microtubules was nearly identical in proximal and distal fragments at 10, 20 and 60 min and at distances between 10 and 50 micron from the cut ends. In chick sensory neurites the microtubule array was examined in longitudinal sections. In order to define objectively a normal microtubule array, the distance between 100 microtubule pairs was measured in seven control neurites, giving a mean distance (+/- S.D.) of 33 nm +/- 19 nm. A normal array of microtubules was defined as having microtubules within 52 nm of their nearest lateral neighbour. Among 11 neurites at times from 1 to 15 min after transection, the mean distance from the cut tip to the first microtubule was 1.3 micron in proximal fragments and 0.5 micron in distal fragments. The mean distance to the normal microtubule array was 2.8 micron in proximal fragments and 2.1 micron in distal fragments. There was no trend or pattern with respect to the times after severing that the neurite was fixed and the amount of microtubule depolymerization. Our results show no evidence for stabilization of axonal/neurite microtubules by capping structures at their ends. We conclude that microtubule instability is unlikely to play a role in the response of axons to axotomy.