IMMUNOCYTOCHEMICAL LOCALIZATION OF CATHEPSIN-L IN THE SYNOVIAL LININGCELLS OF THE RAT TEMPOROMANDIBULAR-JOINT

Localization of cathepsin L in the synovial lining cells of the normal rat temporomandibular joint was investigated by the avidin-biotin-per oxidase complex method for semithin (1 mu m) cryosections and the coll oidal gold-labelled IgG method for ultrathin sections of LR gold resin . At the light-microscopic level, type A (macrophage-like) and B (fibr oblast-like) cells formed the synovial lining layer. Extensive immunor eactivity for cathepsin L was observed in many granules and vacuoles o f type A cells, while in the type B cells, immunoreactivity was found in very few granules. In the sublining layer, macrophages and a few fi broblasts were positive for cathepsin L. By electron microscopy, at th e peripheral cytoplasm of the type A cells close to the lateral interc ellular spaces and joint cavity, numerous coated vesicles and vacuoles (probably early endosomes) indicating endocytotic function were found . Gold particles indicating cathepsin L were localized in the vesicles (primary lysosomes) in the perinuclear cytoplasm and in the larger am orphous vacuoles (1 mu m dia) as phagolysosomes. In type B cells, gold particles were limited to the vesicles only (primary lysosomes). The cathepsin L-positive primary lysosomes were numerous in a few fibrobla sts in the sublining layer. These results indicate that type A cells c ontain a large amount of cathepsin L, and suggest that these cells end ocytose surplus substances such as collagen and proteoglycan fragments in normal rat TMJ, effecting their digestion and degradation by the a ction of this proteolytic cathepsin.