The localization of cathepsins B and D in the synovial lining cells and distribution of the lining cells containing these cathepsins in the normal rat temporomandibular joint (TMJ) were examined im-munocytochemically in thick (8-10 μm) and semithin (1 μm) cryosections with the avidin-biotin-perox-idase complex method and in ultrathin sections with the gold-labeled IgG method. In the thick sections, strong immunoreactivity for cathepsins B and D probably in the type A cells (not clear at this level), was observed simultaneously in the superficial layer of the synovial membrane. This reactivity was strong at the following portions of the synovial membrane located adjacent to the blood vessels: 1) at the anterior portion and 2) medial-posterior portion facing the upper joint cavity, and 3) at the anterior portion and 4) lateral portion facing the lower joint cavity. In semithin cryosections, strong immunoreactivity for cathepsins B and D was simultaneously found in the type A cells which have numerous pseudopodia and vacuoles, while weak granular immunoreaction products for both cathepsins were also observed in the type B cells. In control sections, no type A or B cells showed immunoreactivity for these cathepsins. In the ultrathin sections, numerous gold particles indicating cathepsins B and D were detected in the lysosomes and phagolysosomes of the type A cells facing the lateral intercellular spaces and joint cavity. In the phagolysosomes, these cathepsins were characteristically co-localized. On the other hand, in the type B cells, a few gold particles were localized only in the lysosomes. Judging from these findings, it is suggested that type A cells predominantly contain cathepsins B and D, and that these cathepsins may be restricted within the cells (lysosomes) without extracellular release in the normal rat TMJ. It is also suggested that materials such as cell debris in the lateral intercellular spaces between the type A cells or in the joint cavity are endocytosed by phagosomes nearby, and are digested by proteolytic enzymes (cathepsins B and D) in the lysosomes. In addition, it is considered that this function in the type A cells is most conspicuous at the anterior portion of the synovial membrane of the normal rat TMJ.
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