Immunocytochemical demonstration of laminin in the synovial lining layer of the rat temporomandibular joint

Development of synovial membrane in the temporomandibular joint of the human fetus

The development of the synovial membrane was analyzed in serial sections of 21 temporomandibular joints of human fetuses at 9 to 13 weeks of gestation. Sections of two fetuses at 12 weeks of development were used to perform immunohistochemical expression of the markers CD68 and Hsp27 on the synovial lining. Macrophage-like type A and fibroblast-like type B cells, which express CD68 and Hsp27, respectively, were observed at the twelfth week of development. Our results suggest that the development of the synovial membrane is related to the vascularization of the joint and the formation of the articular cavities.

Expression of caveolin-3 in the fibroblast-like type B synoviocytes in the rat temporomandibular joint

The present study revealed that the fibroblast-like type B synoviocytes (covering the surface of the synovial membrane in the rat temporomandibular joint) had muscle-specific caveolin-3 protein in their caveolae. The existence of two kinds of type B synoviocytes (with and without caveolin-3-immunoreactions even in the synovial lining layer) might reflect the functional difference between them.

Localization of CD44 and hyaluronan in the synovial membrane of the rat temporomandibular joint

Previous studies have pointed out a lack of adhesion structures in the synovial lining layer of the rat temporomandibular joint (TMJ) despite showing an epithelial arrangement. CD44, a major cell adhesion molecule, plays crucial roles as an anchor between cells and extracellular matrices by binding hyaluronan (HA) for the development of organs or the metastasis of tumors. The present study examined the localization of CD44 in the synovial membrane of the rat TMJ by immunocytochemistry for OX50, ED1, and Hsp25, which are markers for the rat CD44, macrophage-like type A, and fibroblast-like type B synoviocytes, respectively. Histochemistry for HA-binding protein (HABP) was also employed for the detection of HA. OX50 immunoreactions were found along the cell surface and, in particular, accumulated along the surface of the articular cavity. Observations by a double immunostaining and immunoelectron microscopy revealed that all the OX50-immunopositive cells were categorized as fibroblastic type B cells, which had many caveolae and a few vesicles reactive to intense OX50. However, the macrophage-like type A cells did not have any OX50 immunoreaction in the synovial lining layer. A strong HABP reaction was discernable in the extracellular matrix surrounding both OX50-positive and -negative cells in the synovial lining layers, exhibiting a meshwork distribution, but weak in its sublining layer. This localization pattern of CD44 and HABP might be involved in the formation of the epithelial arrangement of the synovial lining layer. Furthermore, OX50 immunonegativity in the type A cells suggests their low phagocytotic activity in the rat TMJ under normal conditions.

Isolation and characterization of synovial cells from the human temporomandibular joint

BACKGROUND

The synovial tissues with temporomandibular disorders (TMDs) often show chronic inflammatory changes and the synovial cells participate in the pathogenic processes of TMDs. The synovial membrane is composed of a synovial lining layer and a connective sublining layer. The synovial lining layer is made up of two kinds of cells: macrophage-like type A and fibroblastic type B cells. The aim of this study was to isolate and characterize synovial cells from the human temporomandibular joint (TMJ).

METHODS

Synovial cells were isolated using an explant culture method. Then, we characterized the cultured synovial cells (SGA2 cells) using immunocytochemistry.

RESULTS

SGA2 cells expressed the fibroblastic markers vimentin and prolyl 4-hydroxylase; they also expressed laminin and heat shock protein 27, all of which are markers of type B cells. However, some cells expressed the macrophage marker CD68. These CD68-positive cells simultaneously expressed laminin.

CONCLUSIONS

We isolated and cultured synovial type B cells from the human TMJ, and identified the presence of intermediate type synovial lining cells, having the phenotypic properties of both type A and type B cells, among the synovial lining cells.

Immunocytochemical localization of MAPKAPK-2 and Hsp25 in the rat temporomandibular joint

One series of our research has shown an intense expression of immunoreaction for heat shock protein 25 (Hsp25) in various cellular elements in the rat temporomandibular joint (TMJ). This protein is the major substrate of mitogen-activated protein kinase-activated protein kinase-2 (MAPKAPK-2), which mediates an intracellular stress-activated signaling pathway to stimulate cytosolic actin reorganization under various stresses. The present study was undertaken to examine the localization of MAPKAPK-2 in the rat TMJ by immunocytochemical techniques. Furthermore, confocal microscopy with double staining was employed to demonstrate the colocalization of MAPKAPK-2 and Hsp25. Immunocytochemistry for MAPKAPK-2 showed an intense immunoreaction in the cytoplasm of the synovial lining cells, the endothelial cells, and the fibroblasts in the synovial membrane of the rat TMJ. Double immunostaining under a confocal microscope succeeded in demonstrating the colocalization of MAPKAPK-2 and Hsp25 immunoreactions in the cytoplasm of fibroblastic type B synoviocytes in the TMJ. On the other hand, the macrophage-like type A-cells expressed MAPKAPK-2 immunoreactions but lacked Hsp25 immunoreactivity. The cells in the articular disk and the chondrocytes in the maturative and hypertrophic layer of the mandibular cartilage also showed intense immunoreactions for MAPKAPK-2 and Hsp25. In addition to cytoplasmic localization, MAPKAPK-2 immunoreactions were found in the nucleus of some synovial lining cells, cells in the articular disk, and chondrocytes. Current observations imply the presence of the phosphorylation of Hsp25 via activated MAPKAPK-2 in the cytoplasm. MAPKAPK-2 and Hsp25 possibly participate in the induction of cytoskeletal changes to the various cellular elements in rat TMJ under normal conditions.

Development of the synovial membrane in the rat temporomandibular joint as demonstrated by immunocytochemistry for heat shock protein 25

The synovial lining layer of the temporomandibular joint (TMJ) consists of macrophage-like type A cells and fibroblast-like type B cells. Until now, little information has been available on the development of the synovial membrane in TMJ. In the present study we examined the development of the synovial lining layer in the rat TMJ by light- and electron-microscopic immunocytochemistry for heat shock protein (Hsp) 25, which is a useful marker for type B cells. At embryonic day 19 (E19), a few Hsp25-positive cells first appeared in the upper portion of the developing condyle. During the formation of the upper articular cavity (E21 to postnatal day 1 (P1)), a few positive cells were arranged on its surface. Immunoelectron microscopy demonstrated that these cells had ultrastructural features of fibroblast-like type B cells. In addition, some Hsp25-positive cells moved to the deep portion by extending their cytoplasmic processes toward the articular cavity at P3. At that time, the presence of typical macrophage-like type A cells in the lining layer was confirmed by immunoelectron microscopy. The slender processes of Hsp25-positive cells showed a continuous covering with the synovial surface at P7, followed by a drastic increase in the Hsp25-positive cells at P15 and later, when active jaw movement occurred. These findings suggested that the arrangement and morphological maturation of type B cells are closely related to the formation of the articular cavity in the embryonic period and the commencement of active jaw movement after birth, respectively.

Synovial membrane in the temporomandibular joint--its morphology, function and development

This paper reviews recent findings of the synovial membrane, in particular the morphology, function and development of synovial lining cells, in the temporomandibular joint (TMJ). Electron microscopic studies have confirmed the synovial membrane in TMJ consists of macrophage-like type A cells and fibroblast-like type B cells identical to those in other systematic joints. The macrophage-like type A cells react with anti-macrophage and macrophage-derived substances including the major histocompatibility class II molecule, and show a drastic increase in their number in the inflamed synovial membrane. In addition, they have the ability to produce substances involved in the progression of TMJ inflammation such as nitric oxide and inducible nitric oxide synthase. Observation of osteopetrotic mice revealed that macrophage-like type A cells in TMJ are derived from monocyte lineage. Immunocytochemistry for 25kDa heat shock protein was able to depict the entire shape of fibroblast-like type B cells including their unique processes. The expression of an estrogen receptor alpha-immunoreaction in the fibroblast-like type B cells may explain the etiology of temporomandibular disorders at a higher frequency in females than in males, suggesting that TMJ is a target tissue for estrogen. Furthermore, fibroblast-like type B cells are equipped with a basement membrane to serve as an adhesion molecule for the fibroblast-like type B cells to keep their epithelial arrangement. A clear understanding of the morphology of the intact synovial membrane will serve to clarify the etiology and development of temporomandibular disorders.

Three-dimensional ultrastructure of synoviocytes in the knee joint of rabbits and morphological changes in osteoarthritis model

The synovial intima is composed of two types of synoviocytes: absorptive macrophages and secretory, fibroblast-like F cells. Many studies have tried to observe synoviocytes by scanning electron microscopy (SEM) but failed to reveal the entire shape of synoviocytes because they are deeply embedded in the interstitial matrix. The present study, primarily employing SEM observation of NaOH macerated samples, reveals the distribution and three-dimensional ultrastructure of the synoviocytes in the normal knee joint of rabbits, and the morphological changes of synoviocytes in an osteoarthritis model of this animal. F cells were broadly distributed throughout the synovial intima, while macrophages showed a restricted distribution on fatty tissues around the patella. F cells were classified into a flat type, which covered the surface of synovial membrane like an epithelium, and a dendritic type, which extended long processes to form a characteristic meshwork on the surface. The flat type predominated in regions adhering to the femur, while the dendritic type predominated in ambilateral parts of both the patella and tendon of the musculus quadriceps femoris, and on the peripatellar fatty tissue. Intermediate forms of flat and dendritic types appeared in middle regions between the patella and periphery of the joint capsule. In the synovial membrane of the osteoarthritis model, both types of synoviocytes increased in number and changed their morphology, indicating their elevated activities in absorption and secretion. It is suggested that the ultrastructural changes in synoviocytes reflect pathological conditions of the synovial membrane, and synoviocytes play important roles in the pathogenesis of osteoarthritis.

Morphology and functional roles of synoviocytes in the joint

The joint capsule exhibits a unique cellular lining in the luminal surface of the synovial membrane. The synovial intimal cells, termed synoviocytes, are believed to be responsible for the production of synovial fluid components, for absorption from the joint cavity, and for blood/synovial fluid exchanges, but their detailed structure and function as well as pathological changes remain unclear. Two types of synoviocytes, macrophagic cells (type A cells) and fibroblast-like cells (type B cells) have been identified. Type A synoviocytes are non-fixed cells that can phagocytose actively cell debris and wastes in the joint cavity, and possess an antigen-presenting ability. These type A cells, derived from blood-borne mononuclear cells, can be considered resident macrophages (tissue macrophages) like hepatic Kupffer cells. Type B synoviocytes are characterized by the rich existence of rough endoplasmic reticulum, and dendritic processes which form a regular network in the luminal surface of the synovial membrane. Their complex three-dimensional architecture was first revealed by our recent scanning electron microscopy of macerated samples. The type B cells, which are proper synoviocytes, are involved in production of specialized matrix constituents including hyaluronan, collagens and fibronectin for the intimal interstitium and synovial fluid. The proliferative potentials of type B cells in loco are much higher than type A cells, although the transformation of subintimal fibroblasts into type B cells can not be excluded. In some mammals, type B cells show features suggesting endocrine and sensory functions, but these are not recognized in other species. The synoviocytes, which form a discontinuous cell layer, develop both fragmented basement membranes around the cells and junctional apparatus such as desmosomes and gap junctions. For an exact understanding of the mechanism of arthritis, we need to establish the morphological background of synoviocytes as well as their functions under normal conditions.

Fibroblast biology. Signals targeting the synovial fibroblast in arthritis

Fibroblast-like cells in the synovial lining (type B lining cells), stroma and pannus tissue are targeted by many signals, such as the following: ligands binding to cell surface receptors; lipid soluble, small molecular weight mediators (eg nitric oxide [NO], prostaglandins, carbon monoxide); extracellular matrix (ECM)-cell interactions; and direct cell-cell contacts, including gap junctional intercellular communication. Joints are subjected to cyclic mechanical loading and shear forces. Adherence and mechanical forces affect fibroblasts via the ECM (including the hyaluronan fluid phase matrix) and the pericellular matrix (eg extracellular matrix metalloproteinase inducer [EMMPRIN]) matrices, thus modulating fibroblast migration, adherence, proliferation, programmed cell death (including anoikis), synthesis or degradation of ECM, and production of various cytokines and other mediators [1]. Aggressive, transformed or transfected mesenchymal cells containing proto-oncogenes can act in the absence of lymphocytes, but whether these cells represent regressed fibroblasts, chondrocytes or bone marrow stem cells is unclear.

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 rat 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 comprising the synovial lining layer. Confocal microscopic observation revealed two characteristic profiles of the Hsp25-positive cells with cytoplasmic processes: one extended thick and long processes towards the articular cavity, and the other prejected horizontally slender processes which covered the synovial membrane. Under the electron microscope, the immunoreactive synovial lining cells were characterized by a well-developed rough endoplasmic reticulum and secretory granules, suggesting that they can be categorized as fibroblastic type B cells. The covering by the cytoplasmic extensions was confirmed by immuno-electron microscopic observations. This cytoplasmic covering presumably performs a barrier function and expedites the effective secretion/resorption of synovial fluids. Since it has been proposed that Hsp 25 is 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 Hsp25 might be involved in the inhibition of apoptosis of those cells.