Synovial nature of pathologic periarticular structures, including subcutaneous nodules. descent from embryonic...

In the embryo the periarticular fibroblasts were the producers of the greater part of the joint they surround in later life, as well as of the tendon sheaths and bursae. It is postulated that adult fibroblasts have retained atavistic arthrogenic properties, and may react to traumatizing, inflammatory and oncogenic stimuli by forming periarticular joint-like structures: "arthromas" such as ganglia, meniscal cysts, synovial cysts, synovial sarcoma and subcutaneous nodules. The arthroid nature of these growths manifests itself by the presence of a central cavity, which can be identified as a synovial cavity by histologic, histochemical and electron-microscopic methods. In case of affection of the joint all of these adnexa may be involved. A resemblance of these structures to embryonic joint tissues has been noted for years. The nature of the subcutaneous nodule is discussed at some length. It may contain one or more synovial clefts; synovial elements may be found in its centre by histochemistry and electron-microscopy. Recent and personal findings shed a new light on palisading cells, which may be fibroblasts, having taken up again their embryonic task as synovioblasts. From periarticular fibroblasts thus three kinds of tumors may arise; benign (ganglia, cysts, subcutaneous nodules), malignant (synovial sarcoma) and "semi-malignant" (pannus in rheumatoid arthritis).