Ultrastructural and immunohistochemical investigations of the giant cells in nodular fasciitis.

The giant cells of histologically typical cases of nodular fasciitis were studied by electron microscopy and immunohistochemistry. Ultrastructurally, these cells were found to be similar to the adjacent spindle and large fibroblast-like cells in many ways. These giant cells exhibited numerous intracytoplasmic, well-formed longitudinally oriented bundles of myofilaments and hemi-desmosome-like structures with associated basement membrane-like material, abundant endoplasmic reticulum, and numerous dense bodies. The observed myofilaments did not show beading or Z-line formation. Immunohistochemical staining by means of the indirect peroxidase-antiperoxidase technique failed to localize the proven antigenic markers of histiocytes within the giant cells. The results support the concept that the giant cells in nodular fasciitis are of myofibroblastic origin. It is thus proposed that these giant cells are neither histiocytes nor rhabdomyoblasts as previously thought, but are rather modified fibroblasts ("myofibroblasts").