Cardiac rod body: hypertrophic Z-line in an aged pony

Numerous rod bodies were found in a heart sample from a 33-year-old pony by a conventional electron-microscopic technique. The rod bodies were concentrated in localized areas of both atria and ventricles, without a specific pattern of distribution. The rods appear to have a typical crystalline-like structure which presumably contains actin backbone filaments and alpha-actinin as major protein constituents. Diminution, fragmentation, and disorganization of the myofibrils, random expansion of electron-dense materials, especially at the fasciae adherens of the intercalated disc, an increase of intercellular space, frequently associated with connective tissue elements, and accumulation of glycogen granules, lipofuscin, mitochondria, and nonviable organelles in the rod body-containing cells suggested that rod-containing myocytes are in a degenerating stage and that rod body formation occurs in atrophic cardiomyocytes rather than normal or hypertrophic cardiomyocytes in the aged pony heart.