Cytochemical localization of hyaluronic acid in human synovium with special reference to its possible process of degradation.

Fibroblast-like type B cells are known to produce hyaluronic acid (HA), but the process of its degradation remains unknown. In order to examine the possible route for the degradation of HA in normal human synovium, histochemical and immunohistochemical techniques were applied to the synovial tissue, using biotinylated HA binding region (HABR) and antibodies against CD44 and cathepsin B. Reaction products for HA and CD44 were detected on the cell surface of all synovial lining cells, while half of these lining cells contained intracellular stainings of HA and CD44. Electron microscopically, the lining cells containing intracellularly stained HA and CD44 extended cytoplasmic processes (type A cells), while the other lining cells possessed a smooth cell surface (type B cells). By light microscopic double staining, the intracellular stainings of HA and CD44 appeared co-localized in the cells immunopositive for cystatin beta, an endogenous cysteine proteinase inhibitor which has been shown to be localized in alveolar macrophages and osteoclasts. Moreover, these intracellular stainings of HA and CD44 were co-localized with immunodeposits for cathepsin B, a representative cysteine proteinase in lysosomes. In the extracellular staining of HA, dot-like reaction products appeared on fibrous structures with a periodicity of 41.7 nm. These results suggest that Type A cells in the normal human synovium participate in the degradation of HA by its CD44 mediated intake. Furthermore, HA may be closely associated with fibrous structures, probably type III collagen molecules.