Casts located within the tubuli of the kidney and that contain myoglobin. Myoglobin casts are composed of round granules that may line up in chains or aggregate in clusters. Their color ranges from pink to red-brown with hematoxylin and eosin stain, light brown to black with Jones methenamine silver stain, pink to bright magenta with periodic acid-Schiff stain, and bright red with trichrome stain. Immunoperoxidase staining with antibody to myoglobin is stronglypositive in the casts.Electron microscopy shows globular casts with an electron-dense core and a somewhat less-intense periphery. Substructure is absent. This feature may be accompanied by acute tubular injury with variable flattening of tubular epithelial cells, loss of brush borders, and intratubular sloughed epithelial cells. [from
HPO]