Immunohistochemical analysis of canine and feline muscle disorders using formalin-fixed, paraffin-embedded tissues

Histochemical techniques used in examination of muscle biopsies typically require frozen sections. Given that most of the specimens submitted to a veterinary laboratory for diagnosis are formalin-fixed, the choice of staining methods is limited. We aimed to further advance the diagnostic capabilities of pathologists presented with formalin-fixed muscle samples and to describe the differences in immunohistopathologic findings between neurogenic and myogenic muscle disorders. Based on hematoxylin and eosin staining, we defined in dogs the histologic lesions in 4 neurogenic disorders (degenerative myelopathy and polyneuropathy) and 2 myogenic disorders (dystrophin-deficient muscular dystrophy). In cats, we defined the lesions in 2 neurogenic disorders (lymphoma of nerve roots and spinal cords) and 1 myogenic disorder (laminin α2-deficient muscular dystrophy). Immunohistochemistry for slow and fast myosins revealed angular and group atrophy of type 1 and type 2 fibers in dogs and cats, and fiber type grouping in dogs. These immunohistopathologic findings were specific to neurogenic muscle disorders. Immunohistochemistry for nestin and myogenin revealed nestin-positive fibers and myogenin-positive nuclei in dogs and cats. They were not specific, but these fibers in myogenic disorders can be interpreted as regenerating fibers. The immunohistochemical method described herein appears to be useful for discriminating neurogenic and myogenic disorders in formalin-fixed, paraffin-embedded muscle tissue of dogs and cats.