Transected axons are often assumed to seal at their cut
ends by the formation of continuous membrane barriers that
allow for the restoration of function in the axonal stumps.
We have used several electrophysiological measures (membrane
potential, input resistance, injury current density) and
several morphological measures (phase-contrast, video-enhanced
differential interference contrast, light, and electron
microscopies) of living and fixed material to assess the extent
and mechanism of sealing within hours after transecting
giant axons of squid (Loligo pealeiand Sepioteuthis lessoniana)
and earthworms (Lumbricus terrestris). Our electrophysiological
data suggest that the proximal and distal ends
of transected squid giant axons do not completely seal within
2.5 hr in physiological saline. In contrast, the same set of
measures suggest that proximal and distal ends of transected
earthworm giant axons seal within 1 hr in physiological
saline. Our morphological data show that the cut ends
of both squid and earthworm axons constrict, but that a 20-
70-am-diameter opening always remains at the cut end that
is filled with vesicles. Axonal transection induces the formation
of vesicles that are observed in the axoplasm within
minutes in standard salines and that rapidly migrate to the
cut ends. These injury-induced vesicles are loosely packed
near the cut ends of squid giant axons, which do not functionally
seal within 2.5 hr of transection. In contrast, vesicles
formed a tightly packed plug at the cut ends of earthworm
medial giant axons, which do functionally seal within 1 hr of
transection in physiological saline. Since we detect no single
continuous membrane that spans the cut end, sealing does
not appear to occur by the fusion of constricted axolemmal
membrane or the formation of a membranous partition at the
cut end. Rather, our data are consistent with the hypothesis
that a tightly packed vesicular plug is responsible for sealing
of earthworm giant axons.This work was supported in part by NIH Grant NS31256 and ONR Grant N00014-90-J-1137 to H.M.F., an NIAAA fellowship to T.L.K., and an ATP grant to G.D.B.Neuroscienc