The fine structure of the synovial membrane of the knee joint in rats with special reference to regional differences.

The synovial membranes (SM) in the infrapatellar fat pad (fat pad) and the suprapatellar recessus (recessus) of the rat knee joint were studied by transmission electron microscopy. The architecture of microvasculature in these two regions was also examined by scanning electron microscopy, using the resin casting method. The SM of the rat did not exhibit any species-specific variance in basic structure, however, there were apparent differences in ultrastructure between the two regions; i.e., synovial cells and capillary networks in the fat pad were more tightly packed than those in the recessus. In addition, the occurrence of fenestrated capillaries was higher in frequency in the fat pad than in the recessus. In the experiments using horseradish peroxidase (MW, 40,000) as a tracer material, it was strongly suggested that the SM in the fat pad mediated a more rapid and massive exchange of synovial fluid between the joint cavity and the bloodstream than the SM in the recessus, in which the fenestrated capillaries played a most important role.