1. Synovial cultures are differentiated in tissue cultures from other tissues of mesenchymal origin by their type of growth and cell function.

2. In these respects they are more closely allied to chondroblasts and osteoblasts than to fibroblasts.

3. Synovial cells in tissue cultures develop marked globular cytoplasmic granulations that stain easily with neutral red and sometimes with toluidine blue; they show marked polymorphism with all transitions from round to spindle, polygonal and star shapes and eventually form an epithelial-like membrane, composed of cells with numerous syncytial bridges.

4. In cultures of typically growing synovial cells a mucin-like substance is elaborated. Typical growth and maximal mucin production is best maintained in media containing a minimum of growth-stimulating substances. Transformation of synovial cell growths into fibroblastic growth is accompanied by a loss of mucin production. Dying cells apparently do not produce mucin.

5. Amitotic cell division and the formation of macrophage-like cells were observed.

6. Marked tendency to liquefaction of the plasma about the growths was observed and attributed to the elaboration of a proteolytic ferment.

7. The specific designation "synovioblasts" is proposed for these cells.

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