General structure and histological pathophysiology of the tunica synovialis: Standardization of histological synovialis diagnostics

The tunica synovialis is composed of three compartments: the synovial covering cell layer, the synovial stroma and lymph follicles. These compartments may be present in varying quantities depending on the underlying disease. For standardization a uniformly applicable synovitis score has been established that enables a differentiation into low-grade and high-grade synovitis. This scoring system is internationally regarded as the current gold standard for the morphological description of pathological alterations of the tunica synovialis. This article focuses particularly on the importance of synovial fibroblasts and problems concerning the nomenclature. Furthermore, the pathophysiological principles are discussed. Modern histopathological diagnostics include histochemical (PAS, Grocott, Goldner, Prussian blue staining, Congo red staining of Puchtler), immunohistochemical (numerous antibodies in routine application) and molecular methods (in situ hybridization, polymerase chain reaction and next generation sequencing), which were supplemented by proteomic methods in recent years. These so-called omics methods will lead to the inclusion of many new biomarkers in the diagnostics.