Structural changes at cut ends of earthworm giant axons in the interval between dye barrier formation and neuritic outgrowth

We describe structural changes at the cut ends of invertebrate myelinated e arthworm giant axons beginning with the formation of a dye barrier (15 minu tes posttransection or postcalcium addition) and ending with the formation of a neuritic outgrowth (2-10 days posttransection). The morphology of the cut end, and the location and morphological configuration of the dye barrie r, were assessed by time-lapse confocal, fluorescence microscopy and by ele ctron microscopy. During the interval from 15 to 35 minutes postcalcium add ition, the dye barrier continuously migrated away from a cut axonal end; th e dye barrier then remained stable for up to 5 hours. The size, packing den sity, and arrangement of membranous structures were correlated with changes in the dye barrier from 15 to 35 minutes postcalcium addition. During this interval, uptake of an externally placed hydrophilic dye by these membrano us structures was also variable. After 35 minutes postcalcium addition, the membranous structures remained stable until they completely disappeared be tween 1 and 2 days posttransection. The disappearance of membranous structu res always preceded neuritic outgrowth, which only arose from cut axonal en ds. These results demonstrate that the dye barrier and associated membranou s structures, which form after transection of earthworm giant axons, are ve ry dynamic in the short term (35 minutes) with respect to their location an d morphological configuration and suggest that axolemmal repair must be com pleted before neuritic outgrowth can occur. J. Comp. Neurol. 416:143-157, 2 000. (C) 2000 Wiley-Liss, Inc.