Ultrastructure of osteoid osteoma

The ultrastructure in five cases of osteoid osteoma is described. The osteoblasts generally had a morphology similar to that of normal osteoblasts with a few differences. They contained irregular indented nuclei, glycogen particles, abundant fine intracytoplasmic fibrils, and rare iron containing lysosomes. In several osteoblasts in two cases there were atypical mitochondria with a lobulated or "honeycomb" appearance. These atypical mitochondria were also observed in two osteoclasts; otherwise the cells resembled normal osteoclasts. Other cells present in osteoid osteoma besides osteocytes included osteoprogenitor cells resembling Scott type A and B cells and cells in transitional stages of differentiation. The osteoblasts most likely originated from Scott type A cells of preosteoblasts. The areas of mineralized matrix conformed to the morphology of coarse woven bone. Varying amounts of osteoid were noted. In two cases the osteoid contained, in addition to collagen, fine granular material, which probably represents polysaccharides. No nerve fibers were identified in the material studied. It is thought that osteoid osteoma is a benign neoplastic lesion. A case of osteoblastoma was studied for comparison; the osteoblastoma cells were found to have basically the same morphology as the cells in osteoid osteoma, including the atypical mitochondria. Our ultrastructural observations support the idea that osteoid osteoma and osteoblastoma are closely related lesions.