Synovial Lining Cells in the Temporomandibular Joint

The synovial membrane in the temporomandibular joint (TMJ) consists of a lining and connective tissue sublining layers. Its synovial lining layer contains two types of synovial lining cells—macrophage-like type A and fibroblast-like type B cells. Ultrastructurally, the type A cell is characterized by numerous vacuoles, lysosomes, nuclei with rich heterochromatin, and cell surface filopodia while the type B cell possesses a well-developed rough endoplasmic reticulum, a nucleus with less chromatin, cytoplasmic projections, and cell surface caveolae. However, they are hardly distinguishable under the light microscope without specific cell markers. Immunocytochemistry and in situ hybridization histochemistry have revealed that fibroblast-like type B cells in the murine TMJ show an intense reaction for 25 kDa-heat shock protein (Hsp25), indicating that this protein is a useful cell marker for the type B cells. On the other hand, the macrophage-like type A cells react with ED1, which recognizes the macrophage/monocyte lineage because the type A cells are derived from monocyte. Double immunostaining for Hsp25 and ED1 demonstrated regional differences in the population of the lining cells among the synovial membrane of rat TMJ—the type A cell is predominant in the anterior portion while the type B cell is abundant in the posterior synovial folds. Hsp25 protein is the major substrate of mitogen-activated protein kinase-activated protein kinase-2 (MAPKAPK-2), which mediates a stress-activated signaling pathway. The activation of Hsp25 by MAPKAPK-2 has been considered to regulate cytoskeletal actin organization, resulting in cellular morphological changes. MAPKAPK-2 and Hsp25 are co-localized in the cytoplasm of the fibroblast-like type B cells in the rat TMJ, implying that these proteins possibly participate in the induction of morphological changes of the type B cells.