Pathology of the spongiform encephalopathy in the Gray tremor mutant mouse.

Gray tremor (gt) is an autosomal recessive mutation mapped to chromosome 15 in the mouse. Its phenotypic feature most relevant to human disease is a noninflammatory spongiform encephalopathy which has been transmitted to genetically normal mice in a previously reported, preliminary inoculation experiment. The present study describes the histopathology, topography, developmental sequence, and ultrastructure of the inherited spongiform encephalopathy in the gray tremor homozygote (gt/gt). Vacuolation is present in the first postnatal week in spinal and cerebellar white matter, and spreads rapidly by the second postnatal month to involve gray and white matter throughout almost the entire neuraxis. Adjacent swollen and vacuolated neuronal processes, particularly dendrites, appear to coalesce to form membrane-bound vacuoles in the neuropil. Neuronal abnormalities include focal distension of intracellular membranes and distension, fragmentation, bleb formation, rupture, and disintegration of plasma membranes. White matter vacuoles result from splitting of the myelin sheath at the intraperiod line and from vesicle formation in oligodendroglial inner loop cytoplasm. These ultrastructural abnormalities targeted on subcellular and cellular membranes in neurons and oligodendrocytes implicate a membrane disorder as a fundamental component of the pathogenetic mechanism. A comparison of the pathology of gt to that caused by unconventional agents and neurotropic retroviruses suggests that gt may be valuable in conceptually unifying the whole class of noninflammatory spongiform lesions.