Figure 1.

Mast cells within the rheumatoid synovium. Shown is fixed, paraffin-embedded synovial tissue obtained during arthroplasty from a patient with chronic rheumatoid arthritis. This tissue was stained with safranin-O, which labels mast cell granule proteoglycans red, and counterstained with hematoxylin. Note the frequent safranin-O-positive mast cells present within the synovial sublining (several indicated with arrows). A fold of thickened synovial lining is seen at the bottom left of the image (outlined with a dotted line) and a blood vessel (BV) is visible in the middle of the field, with erythrocytes staining blue. (Section 5 μm thick; original magnification ×400.)