The Study On The Synovial Lymphatic Stomata And Lymphatic Reabsorption In Knee Effusion

Background:In the normal knee joint, there exists a small amount of synovial (joint) fluid to nourish the articular cartilage, lubricate the joints and reduce the articular surface friction. Joint fluid secreted by the synovial membrane circulates continuously. Under pathological conditions, the fluid reabsorption is impaired, leading to the joint effusion and pain of the joint. However, the exact mechanism of joint effusion has not been elucidated yet. It has been demonstrated that the serous cavity is covered with lymphatic stomata, which have important function of lymphatic absorption. The lymphatic stomata have been shown to be distributed in the mice and human diaphragm peritoneum, human meninges, frog pericardial pleura and golden hamster costal pleura, mice ovarian capsule membrane, and mice and human tunica vaginalis. The structure of the synovial membrane is similar to that of the serous cavity. However, there are no reports demonstrating the existence of lymphatic stomata in the knee synovial membrane. Objective:To illustrate the mechanism of lymphatic reabsorption in knee joint effusion. The current investigation attempts to demonstrate the ultrastructure of lymphatic stomata of the knee synovial membrane in New Zealand rabbits and human in order to reveal the absorption function of the synovial lymphatic stomata and the morphological changes of lymphatic stomata.Methods:The knee synovial membranes were examined by transmission electron microscopy and scanning electron microscopy in New Zealand rabbits and human. Ultrastructural changes of the synovial lymphatic stomata were observed by using trypan blue absorption and sodium hydroxide (NaOH) digestion methods, and the animal models of synovitis.Results:New Zealand rabbits and human synovial membranes were composed of two types of synovial cells:Type A and Type B. Type A synovial cells have the macrophage appearance, while Type B synovial cells have the morphological appearance of fibroblasts. More cytoplasmic processes and microvilli were observed in Type A synovial cells with densely distributed nuclear chromatin, less rough endoplasmic reticulum and well developed Golgi complexes, indicating the phagocytic function of Type A synovial cells. In Type B synovial cells, the cytoplasmic processes were closely overlapped with loose nuclear chromatin and rich rough endoplasmic reticulum. Abundant openings of the pinocytotic vesicles existed on the cell surface, indicating the secretory function of Type B synovial cells. No lymphatic stomata were found among Type A synovial cells, whereas lymphatic stomata with the diameters ranging0.74-3.26μm were found in Type B synovial cells, and some stomata were closed. After the NaOH digestion, a number of sieve pores, similar to lymphatic stomata in size and shape, were observed in the dense fibrous connective tissue underneath the Type B synovial cells. After injecting trypan blue into the rabbit knee joint cavity, absorption of trypan blue through the lymphatic stomata was observed, suggesting the absorption function of the synovial lymphatic stomata. In the rabbit knee joint synovitis models, the synovial lymphatic stomata diameter enlarged from1.6±0.55μm to2.09±0.6μm (P<0.001, compared with the normal control). Some macrophages migrated from the lymphatic stomata, indicating that the synovial lymphatic stomata were involved in the joint effusion absorption and inflammatory response.Conclusion:Our study is the first to report the existence of synovial lymphatic stomata in the New Zealand rabbits and human knee joints. Lymphatic stomata may have an important role in the reabsorption of joint effusion.