Primary synovial chondromatosis. An ultrastructural study.

Three cases of synovial chondromatosis involving the knee were studied using the electron microscope. Cells in the hyaline cartilage nodules demonstrated ultrastructural features of typical mature cartilage. Adjacent to the cartilage nodules were areas of numerous fibroblast-like cells in a matrix containing abundant coarse cartilage. A few areas contained cells that demonstrated intermediate differentiation between chondrocytes and fibroblast--elongated cells with well developed Golgi complexes. These areas most probably represent sites of active metaplasia where previously resting fibrocytes begin to secrete a chondroid matrix.