Ferritin in synovial cells in patients with rheumatoid arthritis.

Although the lesions of rheumatoid arthritis occur throughout the body tissues, it is the synovial membrane which shows the most constant and usually the earliest changes. The characteristic features revealed by the light microscope are proliferation of synovial lining cells, infiltration with inflammatory cells including collections of plasma cells and lymphocytes, and increased vascularity. Areas of fibrinoid degeneration may occur and the synovial surface may be covered with plaques of fibrin itself. Unlike the pathological features of the rheumatoid nodule, however, none of these changes can be considered specific for this disease. For this reason the ultrastructural details of the rheumatoid synovial membrane have particular interest. In recent years the fine structure of the normal synovium in man and animals has been described. Barland, Novikoff, and Hamerman (1962) first noted that the synovial lining cells could be classified into two morphological types (which may represent two states of functional activity of a single cell type) designated as types A and B. The type A cell was characterized by numerous filopodia, large vacuoles, many vesicles, a prominent Golgi zone, scanty endoplasmic reticulum, and numerous mitochondria. The less common type B cell contained abundant endoplasmic reticulum but few vacuoles and vesicles. It must be added that from personal experience with human, rabbit, and guinea-pig material, it is clear that cells which are less well defined or are intermediate in form are commonly found. Unfortunately, the Japanese workers Hirohata, Mizuhara, Fujiwara, Sato, Imura, and Kobayashi (1963) have called the cell types M and F respectively, with FM as an intermediate type. The designation "M" stands for macrophage and support for this view of the cells' function has been provided by the experiments of Chapman, Muirden, Ball, and Hyde (1962), Muirden (1963), and Cochrane, Davies, and Palfrey (1965). In these studies electron dense material injected into joints was found to localize predominantly in the