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.