Ultrastructure of fibrous dysplasia of bone: a study of its fibrous, osseous, and cartilaginous components.

Eight cases of fibrous dysplasia were evaluated by electron microscopy and the findings were correlated with the light microscopy. Fibroblasts and myofibroblasts were the cells seen in the fibrous component. The osseous component consisted of immature woven bone trabeculae lined by abnormal osteoblasts with a fibroblastlike appearance. The cartilaginous component resembled hyaline cartilage. The process of mineralization of both osseous and cartilaginous tissues appeared similar to normal bone and cartilage. In fibrous dysplasia there is a slow turnover of bone, and this correlates well with the flattened shape of the bone-lining cells. When compared with normal ossification, the process of bone formation appears to be arrested after an early stage resembling membranous ossification. It is possible that the abnormal osteoblastic maturation of the bone-forming mesenchyme is one of the most important alterations of this condition.