Contribution of synovial lining cells to synovial vascularization of the rat temporomandibular joint

The lining layer of the synovial membrane in the temporomandibular joint (TMJ) contains two types of lining cells: macrophage‐like type A and fibroblast‐like type B cells. The type B cells are particularly heterogeneous in their morphology and immunoreactivity, so that details of their functions remain unclear. Some of the type B cells exhibit certain resemblances in their ultrastructure to those of an activated capillary pericyte at the initial stage of the angiogenesis. The articular surface, composed of cartilage and the disc in the TMJ, has few vasculatures, whereas the synovial lining layer is richly equipped with blood capillaries to produce the constituent of synovial fluid. The present study investigated at both the light and electron microscopic levels the immunocytochemical characteristics of the synovial lining cells in the adult rat TMJ, focusing on their contribution to the synovial vascularization. It also employed an intravascular perfusion with Lycopersicon esculentum (tomato) lectin to identify functional vessels in vivo. Results showed that several type B cells expressed desmin, a muscle‐specific intermediate filament which is known as the earliest protein to appear during myogenesis as well as being a marker for the immature capillary pericyte. These desmin‐positive type B cells showed immunoreactions for vimentin and pericyte markers (neuron‐glial 2; NG2 and PDGFRβ) but not for the other markers of myogenic cells (MyoD and myogenin) or a contractile apparatus (αSMA and caldesmon). Immunoreactivity for RECA‐1, an endothelial marker, was observed in the macrophage‐like type A cells. The arterioles and venules inside the synovial folds extended numerous capillaries with RECA‐1‐positive endothelial cells and desmin‐positive pericytes to distribute densely in the lining layer. The distal portion of these capillaries showing RECA‐1‐immunoreactivity lacked lectin‐staining, indicating a loss of blood‐circulation due to sprouting or termination in the lining layer. The desmin‐positive type B and RECA‐1‐positive type A cells attached to this portion of the capillaries. Some capillaries in the lining layer also expressed ninein, a marker for sprouting endothelial cells, called tip cells. Since an activated pericyte, macrophage and tip cell are known to act together at the forefront of the vessel sprout during angiogenesis, the desmin‐positive type B cell and RECA‐1‐positive type A cell might serve as these angiogenic cells in the synovial lining layer. Tomato lectin perfusion following decalcification would be a highly useful tool for research on the vasculature of the mineralized tissue. Use of this technique combined with immunohistochemistry should permit future extensive investigations on the presence of the physiological angiogenesis and on the function of the lining cells in the synovial membrane.