BACKGROUND AND PURPOSE: Spontaneous animal mutants affected by abnormal 
formation of myelin in the central nervous system (CNS) are useful in studies on 
myelinogenesis and remyelination leading to better understanding of cellular and 
molecular interactions involved in myelin repair. A novel rat mutant, Bouncer 
Long Evans (LE-bo) is severely dysmyelinated, but with exceptional longevity, 
and its clinical and pathologic phenotype are described.
METHODS: Clinical observations, genetic studies, and determination of longevity 
were performed in a colony of rats, including carriers of LE-bo phenotype 
producing the mutant animals. Comprehensive histologic studies were performed on 
all perfusion-fixed tissues, and ultrastructural examination of the optic nerve 
and thoracic part of the spinal cord also was done in rats 1 to 14 weeks old.
RESULTS: The LE-bo phenotype is characterized by whole body tremor, 
progressively severe ataxia, and severe seizure activity. The LE-bo phenotype is 
transferred as an autosomal recessive trait and is stable. The LE-bo rat can 
survive in good health beyond 45 weeks. Neuropathologic changes include severe 
global dysmyelination, with thin uncompacted myelin sheaths in young rats 
forming no major dense line, whereas the myelin sheaths of the peripheral 
nervous system appear normal. Oligodendrocytes degenerate with apparently 
progressing accumulation of membranous material in the perikaryon. Large numbers 
of immature glial cells were detected in the CNS of LE-bo rats at 4 to 14 weeks.
CONCLUSION: The LE-bo rat is severely dysmyelinated due to inability of its 
oligodendrocytes to form myelin sheaths. Similarities of the LE-bo rat and Long 
Evans Shaker (les) rat neuropathologic features, such as severe dysmyelination, 
lack of major dense line in uncompacted myelin sheaths, apparent proliferation 
of oligodendroglial cells, and considerable longevity, are striking and suggest 
that a LE-bo mutation may functionally affect the myelin basic protein gene.