Science Fair Project Encyclopedia
Synovial fluid is a thick, stringy fluid found in the cavities of synovial joints. With its egg-like consistency (synovial comes from Latin for "egg") synovial fluid reduces friction between the articular cartilage in joints to lubricate and cushion them during movement.
The inner membrane of synovial joints is called the synovial membrane, which secretes synovial fluid into the joint cavity. This fluid forms a thin layer (approximately 50 micrometres) at the surface of cartilage, but also seeps into the articular cartilage filling any empty space . The fluid within articular cartilage effectively serves as a synovial fluid reserve. During normal movements, the synovial fluid held within the cartilage is squeezed out mechanically (so-called weeping lubrication) to maintain a layer of fluid on the cartilage surface.
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 . Hyaluronan is synthesized by the synovial membrane and secreted into the joint cavity to increase the viscosity and elasticity of articular cartilages and lubricate the surfaces between synovium and cartilage .
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. There is also some evidence that it helps regulate synovial cell growth .
Health and disease
Synovial fluid can be classified into normal, noninflammatory, inflammatory, septic, and hemorrhagic:
|WBC/mm3||<200||200-2,000||2,000-75,000||>100,000||Same as blood|
|Polys (%)||<25||<25||>50||>75||Same as blood|
|Gram stain||Negative||Negative||Negative||Often positive||Negative|
- Noninflammatory (Group I)
- Inflammatory (Group II)
- Septic (Group III)
- Pyogenic bacterial infection
- Septic arthritis
External links and references
- How are joints lubricated?, from University of Washington Medicine
- Warman W. "Delineating biologic pathways involved in skeletal growth and homeostasis through the study of rare Mendelian diseases that affect bones and joints." Arthritis Res. Ther. 2003, 5(Suppl 3):5 
- Hyaluronan: structure and properties
- Synovial fluid analysis, from the American College of Rheumatology
- Synovial fluid white blood cell count, from the Family Practice Notebook
- Synovial fluid, from the FPN
- Normal joint structure, from University College London
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