Muscle-specific actin expression in chondroblastomas

Chondroblastomas are unusual cartilage benign lesions of bone that have well-characterized histological features. We reviewed and immunohistochemically examined the paraffin block material of 20 cases, and in seven tumors of this collection we found distinct cytoplasmic muscle-specific actin positivity of some tumor chondroblasts and chondrocytes. Muscle-specific actin-positive cells had the histological and ultrastructural features typical of chondroblasts. Moreover, in their cytoplasm they contained bundles of microfilaments with focal densities, as is typical of myofilaments. We did not observe any basal lamina around these cells, which were surrounded by intercellular matrix of the cartilage tissue type. Therefore, we suppose that muscle-specific actin-positive cells occurring in some chondroblastomas do not represent an admixture of myofibroblasts but chondroblasts with actin expression. The unusual immunophenotype of some chondroblasts might be the result of aberrant actin expression or of the plasticity of their phenotype modulated by microenvironmental stimuli. It is a question of whether, analogously to the terminology of myofibroblasts, such cells could be designated as myochondroblasts.