Ultrastructure of brown tumor of hyperparathyroidism

The ultrastructure of two cases of brown tumor of hyperparathyroidism is described. The most frequent cells found are multinucleated giant cells and fibroblasts. The giant cells have ultrastructural features similar to osteoclasts such as numerous mitochondria, dilated rough endoplasmic reticulum, and short filopodia. However, the ruffled borders typical of osteoclasts are not seen. In this manner, the giant cells of brown tumor are inactive osteoclasts. The fibroblasts show myofibroblastic differentiation. We think that brown tumor represents a reparative cellular process similar to giant cell reparative granuloma of bone.