Histopathology of the late-onset motor neuron degeneration (Mnd) mutant in the mouse.

The motor neuron degeneration (Mnd) is characterized by a progressive deterioration of motor function (stiff-legged gait, abnormal limb placements and grasping, and finally paralysis; moving from rear to forelimbs). There is a dramatic degeneration of spinal cord motor neurons, more severe in the lumbosacral than in the other regions, as well as variable pathology in the lower cranial nerves. Upper motor neurons of the red nucleus, reticular formation of the pons and medulla, and restricted areas of the cerebral cortex are also affected. Degenerating motor neurons share many characteristics seen in the human disease amyotropic lateral sclerosis, including loss of Nissl substance, increases in lipofuscin and abnormal cytoplasmic inclusions. Additionally, Mnd, like ALS, is a disease of later life.