Normal salivary gland myoepithelia are contractile cells with hybrid epithelial/myogenic ultrastructural features. It is known that these cells co-express the intermediate filaments
cytokeratin,
vimentin, and occasionally GFAP. This complex cytoskeletal immunophenotype is also reflected in multiple morphologic cell types of
pleomorphic adenoma. At present, the myofilament
complement of normal and neoplastic myoepithelium is not well defined. We have evaluated the expression of
desmin and smooth and sarcomeric muscle
actins in 11 normal salivary glands (six snap-frozen and five
methacarn fixed) and 26
pleomorphic adenomas (11 snap-frozen and 15
methacarn fixed) by ABC-immunoperoxidase method. Two of 11 frozen
pleomorphic adenomas contained the muscle-specific intermediate filament
desmin, which is not found in the normal glands. This novel finding was confirmed by gel electrophoresis and immunoblot. Using specific
antibodies, normal gland myoepithelial cells consistently contained muscle actin
isoforms of the smooth muscle type but not sarcomeric muscle
actins. Muscle actin expression by the neoplastic cells of
pleomorphic adenoma was found in 13 of 26
tumors (six of 11 frozen
tumors (
desmin negative) and seven of 15
methacarn fixed
tumors). In comparison to the normal myoepithelial cell, the transformed myoepithelial-like cells of
pleomorphic adenoma are not always characterized by a muscle actin cytoskeleton. Expression of
desmin intermediate filaments in
pleomorphic adenomas appears to be a rare event that is independent of a muscle actin cytoskeleton.