Ultrastructural Studies of Rabbit Synovitis Induced by Autologous IgG Fragments: I. Proliferation of the Lining Cells

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Abstract
The synovial lining cells of rabbits with experimental synovitis induced by intra-articular injection of cathepsin D-digested autologous IgG fragments (Fab2) have been subjected to electronmicroscopic study. From 3 to 50 such injections resulted in hyperplasia of the lining layer with an increase in the numbers of phagocytic and synthetic cells. Morphologically the phagocytes were classified into monocyte-like cells, mature and immature phagocytes, and epithelioid-like cells, indicating that synovial M cells may originate from blood monocytes that differentiate in situ like the monocytes in other tissues. The finding of “undifferentiated” (mesenchymal), transitional and mature synthetic cells in the lining layer suggests that synovial F cells are derived from the undifferentiated mesenchymal cells persisting in the synovial membrane in postnatal life. In the animals with synovitis, the synthetic cells were found to undergo mitosis but not the phagocytic cells. It is concluded that the hyperplasia of the lining layer is due to two distinct processes, namely invasion by precursors of M cells (monocytes) and local proliferation of F cells. As far as immune reactions involved in the synovitis are concerned, the possible roles played by lysosomal substances in these two processes are also discussed.