synovial-fluid Synovial fluid is a thick, stringy fluid found in the cavities of synovial joints. It reduces friction between the articular cartilage and other tissues in joints by lubricating and cushioning them during movement.
During movement, the synovial fluid held in the cartilage is squeezed out mechanically to maintain a layer of fluid on the cartilage surface. Normal synovium contains synovial lining cells that are 1-3 cells deep. Synovium lines all intracapsular structures except the contact areas of articular cartilage.
The synovial lining cells reside in a matrix rich in type I collagen and proteoglycans. There are two main types of synovial lining cells, but these can be differentiated only by electron microscopy.
Type A cells are macrophage-like and have primarily a phagocytic function.
Type B cells are fibroblast-like and produce hyaluronate, which accounts for the increased viscosity of synovial fluid.
Other cells found in the synovium include antigen-presenting cells called dendritic cells and mast cells.
Synovial tissue also contains fat and lymphatic vessels, fenestrated microvessels, and nerve fibers derived from the capsule and periarticular tissues.
Hyaluronic acid, synthesized by synovial lining cells (type B), is secreted into the synovial fluid, making the fluid viscous.
Synovial fluid is made of hyaluronic acid and lubricin, proteinases, and collagenases.
Normal synovial fluid contains 3-4 mg/ml hyaluronan (hyaluronic acid), a polymer of disaccharides composed of D-glucuronic acid and D-N-acetylglucosamine joined by alternating beta-1,4 and beta-1,3 glycosidic bonds.
Synovial fluid also contains lubricin secreted by synovial cells. It is chiefly responsible for so-called boundary-layer lubrication, which reduces friction between opposing surfaces of cartilage. It may also have a role in synovial cell growth.
Physical characteristics of normal synovial fluid
* Colorless and transparent
* Amount – thin film covering surfaces of synovium and cartilage within joint space
* Cell count – <200/mm3 . Less than 25 percent are neutrophils
* Protein – 1.3-1.7 g/dl (20 % of normal plasma protein)
* Glucose – within 20 mg/dl of the serum glucose level
* Temperature – 32 degree Celsius (peripheral joints are cooler than core body temperature)
* String sign (measure of viscosity) – 1-2 inches ; When a drop is taken between the thumb and finger as in pinching and then they are separated, the drop forms a string till 1-2 inches
Albumin 3 mg/ml
Protease inhibitor cocktail