Expression of Muscle-specific Desmin in the Rat Temporomandibular Joint Synoviocytes

Objectives: Desmin, a myogenic marker, is a major muscle-specific intermediate filament protein. Our recent studies have shown an intense immuno-expression of muscle-specific caveolin-3 (Cav3) protein in the fibroblast-like type B synoviocytes in the temporomandibular joint (TMJ) of the adult and developing rats. The present study was undertaken to investigate desmin-expression in the synovial membrane of the rat TMJ by immunocytochemical techniques.

Methods: Male Wistar rats (4-, 8-week-old, n=3 each) were used. Serial frozen sections including TMJ were processed for the ABC method using antibodies to desmin and Cav3. Some immunostained sections were embedded in epoxy resin, and examined under a transmission electron microscope. Double labeling immunocytochemistry for desmin and either Cav3, heat shock protein 25 (Hsp25)--a marker of type B synoviocytes, smooth muscle actin (SMA), or F-actin were also employed.

Results: Numerous desmin-immunopositive cells were localized in the synovial lining layer of the 4- and 8-week-old rats. On the other hand, Cav3-immunopositive cells decreased in number at 8 rather than at 4 weeks of age. Ultrastructurally, desmin-positive cells possessed long cytoplasmic projections, numerous caveolae, well-developed rER, and secretory granules, suggesting they were the fibroblast-like type B synoviocytes. Some of the type B cells showed desmin/Hsp25- or desmin/Cav3-colocalization, while the others exhibited either one of them. These desmin-immunoreactive cells were less intensely labeled with SMA and F-actin.

Conclusions: The present findings on the strong expression of muscle-specific desmin but not actin in the type B synoviocytes indicate their mechanical strength without contractile ability. In addition, the various immunoreaction patterns for desmin and Cav3 among the type B cells imply that these proteins reflect differences in their maturation stages or functions. This work was supported by Grants-in-Aid for Young Scientists (B) from the Ministry of Education, Science, Sports, and Culture of Japan (No. 18791346 to K. N-I.).