Ultrastructure of synovial cells in vitro

A morphological study of macrophage and synovial cell interactions with hydroxyapatite crystals

Hydroxyapatite crystals have been found in synovial fluids and periarticular spaces in association with osteoarthritis. This study examined the morphology of the interaction of macrophages and synovial cells with hydroxyapatite in order to determine the fate of hydroxyapatite over an extended culture period. Mouse peritoneal macrophages and bovine synovial cells were grown as monolayers. The morphology of the interaction between hydroxyapatite crystals (HAC) and these cells was studied by light microscopy, transmission and scanning electron microscopy for up to 14 days. Phagocytosis of HAC, phagosome formation, and concentration of crystal aggregates around the cells began within four minutes of their addition to the cultures. Phagocytosis and cellular aggregation continued after this time. HAC caused minimal cellular disruption despite the uptake of large amounts of crystalline material and significant cellular enlargement. HAC remained within cells for the duration of the study without any apparent dissolution by the cells. Whether macrophages are stimulated to produce interleukin I as a result of HAC uptake is currently under investigation.

The pig synovium, II. Some properties of isolated intimal cells

I. The effect of trypsinization. Certain physiological and immunological properties of isolated intimal synoviocytes were studied in various in vitro systems. Suspensions of the synoviocytes were obtained by incubating sheets of joint capsule in a solution of trypsin and then scraping off the intimal cells with a small knife. Synovium was examined immediately after trypsinization, but before scraping, by light and electron microscopy. The intimal cells had withdrawn many of their long processes, acquired lamelliform pseudopodia, and somewhat resembled cells of the A-type (Barland et al. 1962). To test the viability of the trypsinized synovium, fragments of trypsinized joint capsule were explanted in organ culture with the intima in contact with a Millipore substrate (Fell et al. 1976). After two days the intimal synoviocytes had become branched and, as in untreated control explants, now resembled B-cells. II. Phagocytosis and opsonic adherence. In a freshly prepared suspension of synoviocytes scraped from trypsinized synovial tissue many cells were still branched, but during 1 1/4 hours' incubation in serum-containing medium the majority had withdrawn their processes and become rounded. Cells in the branched form had little capacity for phagocytosis, but most of those in the rounded form were actively phagocytic. After suspensions of intimal cells had been incubated with opsonized sheep erythrocytes, most of the rounded, but none of the few remaining branched cells, had formed rosettes. Intimal cells from scraped synovial tissue were maintained in Sykes-Moore chambers for periods of up to 48 hours. Although the young pig synovium contains only a small proportion of macrophage-like (A-type) cells, in the cultures the cell population consisted of cells indistinguishable from macrophages, with a few small colonies of typical fibroblasts. In marked contrast to the fibroblasts, the macrophage-like cells in the Sykes-Moore cultures were highly phagocytic and formed conspicuous opsonic rosettes. Excessive phagocytosis inhibited subsequent rosetting by the macrophage-like cells. III. The effect of antiserum, with and without complement, on synovial cells. In view of earlier work by Fell & Barratt (1973) the effect of rabbit antiserum to pig erythrocytes (AS) with and without serum complement (C') on intimal synoviocytes in a Sykes-Moore chamber was investigated. AS+C' either lysed the macrophage-like cells or caused them to fuse into multinucleate giant cells. In the presence of AS without C', the macrophage-like cells formed large lakes of multinucleated cytoplasm. Fibroblasts in the same cultures were much more resistant to lysis by AS+C' and did not form multinucleate giant cells when exposed to AS either with or without C'.