DELAMINATING MYELIN MEMBRANES HELP SEAL THE CUT ENDS OF SEVERED EARTHWORM GIANT-AXONS

Transected axons are often assumed to seal by collapse and fusion of t he axolemmal leaflets at their cut ends, Using photomicroscopy and ele ctronmicroscopy of fixed tissues and differential interference contras t and confocal fluorescence imaging of living tissues, we examined the proximal and distal cut ends of the pseudomyelinated medial giant axo n of the earthworm, Lumbricus terrestris, at 5-60 min post-transection in physiological salines and Ca2+-free salines, In physiological sali nes, the axolemmal leaflets at the cut ends do not completely collapse , much less fuse, for at least 60 min post-transection. In fact, the a xolemma is disrupted for 20-100 mu m from the cut end at 5-60 min post -transection. However, a barrier to dye diffusion is observed when hyd rophilic or styryl dyes are placed in the bath at 15-30 min post-trans ection. At 30-60 min post-transection, this barrier to dye diffusion n ear the cut end is formed amid an accumulation of some single-layered and many multilayered vesicles and other membranous material, much of which resembles delaminated pseudomyelin of the glial sheath. In Ca2+- free salines, this single and multilayered membranous material does no t accumulate, and a dye diffusion barrier is not observed. These and o ther data are consistent with the hypothesis that plasmalemmal damage in eukaryotic cells is repaired by Ca2+-induced vesicles arising from invaginations or evaginations of membranes of various origin which for m junctional contacts or fuse with each other and/or the plasmalemma. (C) 1997 John Wiley & Sons, Inc.