Ultrastructural Studies of the Dying-Back Process

Abstract

The spatio-temporal evolution of peripheral giant axonal degeneration has been studied in rats during the development of concurrent peripheral (PNS) and central (CNS) nervous system dying-back disease after chronic intoxication with the neurotoxic hexacarbons n-hexane (CH3CH2CH2CH2CH2CH3), methyl n-butyl ketone (MBK) (CH3COCH2CH2CH2CH3), or 2,5-hexanedione (CH3COCH2CH2CHOCH3), a neurotoxic metabolite of MBK. Each compound caused animals insidiously to develop identical, symmetrical peripheral neuropathies characterized by eversion and drop of hindfeet, inability to extend hindlimbs and upper extremity weakness. Teased fiber studies demonstrated that giant axonal swellings first developed on the proximal sides of multiple paranodes sited in distal, non-terminal regions of large myelinated fibers. Later, swellings developed at internodal sites. Smaller myelinated and unmyelinated fibers also underwent multifocal, giant axonal swelling. In affected myelinated fibers, swollen nodal and paranodal axons were frequently associated with retracted paranodal myelin sheaths. Adjacent distal internodes were attenuated and corrugated. Demyelinated paranodes apparently underwent local shrinkage and remyelination before complete distal fiber breakdown commenced. The proximal limits of chains of homogeneous myelin ovoids were interfaced with proximal, preserved regions at sites of giant axonal swellings. Regeneration of myelinated axons also occurred during intoxication. Regenerating fibers wre composed of multiple, short, branched internodes which sometimes appeared multifocally swollen. Interfaces between regenerating and preserved portions of fibers were unswollen. Thick section studies showed that pronounced endoneurial edema accompanied fiber degeneration in peripheral nerve trunks. Ultrastructural studies revealed multifocal, giant axonal swellings containing masses of 10 nm neurofilaments and sometimes, clustered mitochondria, neurotubules and smooth endoplasmic reticulum. Enlarged granular mitochondria, interdigitated Schwann cell/axon networks and corrugated myelin sheaths were common findings. Dense granules, vesicles and hexagonal particles were also noted in the axoplasm. These findings provide new insights into the nature of the dying-back process: although there was a retrograde, temporal spread of axonal swelling up affected nerve trunks, axonal degeneration neither began in the nerve terminal nor spread seriatim centripetally along individual nerve fibers. The dying-back process was further examined in a companion study in this issue (32) which analyzed some of the factors determining the differential vulnerability of PNS and CNS fibers in animals intoxicated either with these neurotoxic hexacarbons or with acrylamide.