THE REFORMATION OF SYNOVIAL MEMBRANE IN THE KNEES OF RABBITS AFTER SYNOVECTOMY

After the removal of the synovial membrane the joint is filled with a mixture of blood and synovial fluid. A clot of cells and fibrin adheres to the denuded surface. There is an immediate polymorphonuclear leucocyte reaction which lasts about forty-eight hours, and then the polymorphs disappear and phagocytes (clasmatocytes and monocytes) wander into the joint. The fixed connective cells of the denuded area enlarge, proliferate rapidly, and invade the adherent fibrin. Capillaries grow into the fibrin, and by the sixth day it is fairly well organized. By the fourth day a definite surface zone on the adherent fibrin is demarcated by a thin hyaline layer which stains as does collagenic tissue. This surface zone for a time is not penetrated by the advancing fibroblasts, but they form a layer just beneath it. As the clot becomes organized the fibrin and extravasated blood are absorbed and by the tenth day a few collagenic fibres are present. These are rapidly increased during the next five days. Mitotic figures are frequent and the proliferation of fibroblasts continues until about the fifteenth day. At this time the fibrin has almost entirely disappeared and the hyaline surface layer has largely disappeared. The denuded area is now covered by a thick, highly cellular layer of loose fibrous tissue which is very vascular and has a well defined surface layer of fibroblasts. This membrane of young connective tissue now slowly matures to form a synovial lining which can be distinguished only with difficulty from that of the normal joint. The fibroblasts beneath the surface decrease markedly in number and those remaining shrink in size to become normal lamellar cells of connective tissue. As the fibroblasts decrease in size and number collagenic fibres are laid down. The surface or synovial cells are decreased in size and number and become flattened. They lie not on the surface but in the surface, as is true of normal synovial membrane. Joints are not lined by a layer of mesothelium but their lining is a connective tissue surface made up of cells and collagenic tissue. The cells may be likened to field stones. Some are on the surface, some are partly buried, and others are wholly covered by the collagenic matrix. The joint space up to fifteen days encroaches upon the newly formed membrane. After that time, partly from condensation of the membrane and partly by stretching and breaking down of adhesions, it becomes approximately normal in size and contour. The resultant membrane has a more irregular fibrous ground work than has the normal synovia and, as was noted by Sumita, is deficient in the subsynovial areolar tissue. In this series, exostoses on the mesial condyle were frequent. These are believed to be due to inadvertent injury to the periosteum at the time of the operation. In the rabbit the joint is again approximately normal sixty days after hemisynovectomy. The new synovial membrane is formed in situ by metaplasia of underlying connective tissue cells and there is little or no tendency for surface growth from the edges to cover the denuded area as occurs in the repair of a defect in an epithelial surface. The synovial cells are connective tissue cells slightly specialized by their location on a free connective tissue surface.