Ultrastructural Studies of the Dying-back Process

Abstract

Organotypic tissue cultures, composed of structurally and functionally coupled explants of mouse spinal cord, dorsal root ganglia, and striated muscle, have been used to create a model of the distal (dying-back) axonopathy found in animals and humans with aliphatic hexacarbon neuropathy. Mature explants were treated with 50-650 micrograms/ml of the following hexacarbons dissolved in nutrient fluid: n-hexane, 2-hexanol, 2,5-hexanediol, methyl n-butyl ketone, 5-hydroxy-2-hexanone, 2,5-hexanedione (all neurotoxic), or 2,4-hexanedione (a non-neurotoxic diketone). High concentrations (400-650 micrograms/ml) induced pancytotoxic damage and necrosis of tissue within days, while the lower doses (50-100 micrograms/ml) induced no pathological changes over a period of several weeks. Continuous exposure of explants to 245-325 micrograms/ml (2.8 mM) of the neurotoxic hexacarbons caused specific pathological changes to develop in distal nerve fibers after three to six weeks. Initial changes seen in distal, nonterminal regions of myelinated fibers included: nodal elongation, axonal swellings on proximal-side paranodes, and paranodal myelin retraction. Prolonged treatment was associated with Wallerian-like degeneration of distal nerve fibers. Denuded paranodal swellings in more proximal regions of affected myelinated fibers adopted a more-normal size and underwent remyelination; this occurred during and after the course of treatment. Remyelination by lateral extension from adjacent Schwann cells was documented in living and fixed tissue. The observations confirm the spatial-temporal evolution of hexacarbon distal axonopathy previously suggested from comparable studies in vivo.