Immunocytochemical demonstration of heat shock protein 25 in the rat temporomandibular joint

The expression of heat shock protein 25 (Hsp 25) was investigated in the ra t temporomandibular joint by immunocytochemistry combined with confocal and electron microscopy. Immunostaining with an antibody to Hsp25 was able to demonstrate various cellular elements in the synovial membrane of the joint . Intense immunoreaction for Hsp25 was recognized in certain cells comprisi ng the synovial lining layer. Confocal microscopic observation revealed two characteristic profiles of the Hsp25-positive cells with cytoplasmic proce sses: one extended thick and long processes towards the articular cavity, a nd the other prejected horizontally slender processes which covered the syn ovial membrane. Under the electron microscope, the immunoreactive synovial lining cells were characterized by a well-developed rough endoplasmic retic ulum and secretory granules, suggesting that they can be categorized as fib roblastic type B cells. The covering by the cytoplasmic extensions was conf irmed by immune-electron microscopic observations. This cytoplasmic coverin g presumably performs a barrier function and expedites the effective secret ion/resorption of synovial fluids. Since it has been proposed that Hsp 25 i s associated with an estrogen receptor, the immunopositive synovial lining cells were considered estrogen-target cells. Immunoreactivity for Hsp25 was also observed in the chondrocytes of the maturative and hypertrophic cell layers as well as in the cells of the articular disk. A suggestion was made that Hsp 25 might be involved in the inhibition of apoptosis of those cell s.