A novel model for the study of synovial microcirculation in the mouse knee joint in vivo

A novel model for the investigation of the microcirculation in synovial tissue of the mouse knee joint is presented. The mouse knee joint was exposed on a specially designed plexiglass stage with a slight flexion. After partial resection of the skin, the patella tendon was cut transversally, which allowed for visualization of the "Hoffa's fatty body", an intraarticular fatty tissue containing synovial cells on the interior surface of the joint. An intravital fluorescence microscope was adjusted to observe the microcirculation of this intraarticular synovial tissue without opening of the joint capsula. For staining of the plasma, fluorescein isothiocyanate (FITC)-dextran was used, and for the staining of leukocytes rhodamine 6G was used. The tissue investigated presents with a high-density honeycomb-like capillary network, containing some postcapillary venules and a few arterioles. The following parameters were assessed off-line using a computer-assisted microcirculation analysis system: flow and diameter of arterioles and postcapillary venules, as well as functional capillary density. Moreover, leukocyte-endothelial cell interaction was quantified by counting the number of rolling cells and cells adhering to the endothelium in postcapillary venules. As an indication of endothelial leakage, macromolecular extravasation was also assessed. To validate the model, we investigated these parameters at three time points during an observation period of 60 min. There was no change in functional capillary density, nor in vessel diameter after 60 min of observation. Moreover, there was neither a change in the number of rolling cells, nor in the number of cells adhering to the endothelium nor in extravasation of FITC-dextran, thus indicating the stability of the preparation. The new model allows the quantitative analysis of the intraarticular microcirculation of the synovial fatty tissue in vivo. It provides insight into the dynamics of synovial microcirculation and leukocyte-endothelial cell interaction in acute or chronic joint inflammation.