A myofibroblastic tumor. Infantile digital fibroma (recurrent digital fibrous tumor of childhood).

Infantile digital fibromas are distinctive tumors both clinically and morphologically. A unique light microscopic features in the presence of intracytoplasmic, spherical, eosinophilic inclusion bodies. Previous electron microscopic studies have shown these bodies to consist of fibrils; bundles of fibrils have also been described in the cytoplasm. We have examined one such tumor, confirmed earlier findings, and established that the component cells are typical myofibroblasts. The latter are a variety of fibroblasts first described in granulation tissue; they are endowed with contractile properties and are characterized ultrastructurally by bundles of fibrils containing "dense bodies" such as are found in smooth muscle cells. Since our case is identical to those previously described, we propose that this tumor be called infantile digital myofibroblastoma.