Anomalies associated with dye exclusion as a measure of axolemmal repair in invertebrate axons

After axonal injury, dye exclusion is often used as a measure of the re-est ablishment of a structural barrier. We now report that this use of dye excl usion is equivocal in two situations. (1) When a negatively-charged hydroph ilic fluorescent dye (HFD) was placed in the physiological saline (PS) surr ounding a crayfish medial giant axon (CMGA) before transection, this dye di d not readily diffuse into the cut ends after transection whereas uncharged or neutralized dyes did do so. (2) When axoplasm flowed out of the cut end s of a transected squid giant axon (SGA), this outflow markedly slowed hydr ophilic fluorescent dyes from diffusing into the cut ends. These anomalies suggest that dye exclusion by an injured axon does not always indicate that a structural barrier has formed. Therefore, dye assessments of axonal repa ir require control experiments that rule out anomalous exclusion due to dye interactions (biochemical and fluid dynamics) with components (axoplasm, a xolemma, glial sheath, etc.) of the particular axon under study. (C) 1998 E lsevier Science Ireland Ltd. All rights reserved.