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Synovial Lining Cells, continued

A cluster of synovial lining cells is indicated by the arrow in the image on this page. This smear is from a patient with rheumatoid arthritis.

Observe the size of these synovial lining cells relative to the lymphocytes and neutrophil present in the picture. Synovial lining cells are moderately large with abundant cytoplasm and a round-to-oval nucleus, although bi-nucleated forms may be seen. The cytoplasm typically has a grainy or foamy appearance and stains unevenly. The synovial membrane is lined with both type A and type B synovial lining cells. The type A cells can be phagocytic and, at times, difficult to differentiate from monomacrophages. The type B cells, which tend to be a bit larger, secrete hyaluronic acid.

While these cells are found in a cluster, it is still possible to make out indistinct cytoplasmic boundaries. Just like mesothelial cells, these may occasionally be bi-nucleate.