Evolution of the synovial joints in Osteichthyes : Cells displaying functional similarities to synoviocytes in zebrafish Danio rerio

Abstract: Synovial joints enable frictionless movement between the adjacent bones. Lubricating fluid enclosed within the joint reduce the pressure to allow free movement. Previously synovial joints have been associated with a terrestrial life and have mainly been studied in tetrapods, such as mammals and chicken, under the assumption that the joint did not evolve before animals adapted to a life on land. However, a recent study described the production and function of lubricin in the jaw and pectoral fin joints in ray-finned fishes, such as zebrafish, suggesting previous assumption to be questionable. The current study aim to further investigate the jaw joints in the ray-finned zebrafish, by examininge the morphology and presence of specific cell types, called synoviocytes, associated with the synovial joint. The synoviocytes are divided into two groups: macrophage-like cells and fibroblast-like cells. Tissue sections were histologically investigated with nuclear staining and immunohistochemically with antibodies to chondroitin sulphate (CS-56) and uridine diphosphoglucose dehydrogenase (UDPGD, UGDH) to investigate the presence of fibroblast-like cells. Small and circular cells were observed in the joint line and surrounding cartilage nascent to the joint. These cells could not be identified as chondrocytes or part of the perichondrium, and their UDPGD activity witness on similarities to the fibroblast-like cells. UDPGD is associated to the synthesis of hyaluronan (HA) which plays a key role in joint cavitation, hence, suggesting functional similarities in the development of zebrafish jaw joint to synovial joints. Furthermore a double transgenic zebrafish, fli1:EGFP/mpeg1:mCherry, was used to examine the presence of macrophages within the joint area. Macrophages were observed on the surface of the jaw joint in close association with blood vessels but did not show signs of incorporation within the joint. These results present further similarities of joints in ray-finned fish to the synovial joint but also the need to further examine similarities in order to understand the development and evolution of the joints. The possibility to implement the zebrafish model would facilitate future studies on the healthy synovial joint as well the diseases related to it.