Synovial fluid
Synovial fluid, also called synovia, is a viscous, non-Newtonian fluid found in the cavities of synovial joints. With its egg white–like consistency, the principal role of synovial fluid is to reduce friction between the articular cartilage of synovial joints during movement. Synovial fluid is a small component of the transcellular fluid component of extracellular fluid. Structure. The inner membrane of synovial joints is called the synovial membrane and secretes synovial fluid into the joint cavity. Synovial fluid is an ultrafiltrate from plasma, and contains proteins derived from the blood plasma and proteins that are produced by  cells within the joint tissues. The fluid contains hyaluronan secreted by fibroblast-like cells in the synovial membrane, lubricin (proteoglycan 4; PRG4) secreted by the surface chondrocytes of the cartilage and interstitial fluid filtered from the blood plasma. This fluid forms a thin layer (roughly 50 μm) at the surface of cartilage and also seeps into microcavities and irregularities in the articular cartilage surface, filling all empty space. The fluid in articular cartilage effectively serves as a synovial fluid reserve. During movement, the synovial fluid held in the cartilage is squeezed out mechanically to maintain a layer of fluid on  the cartilage surface (so-called "weeping lubrication"). The functions of the synovial fluid include: Composition. Synovial tissue is sterile (meaning microorganisms are not typically found there) and composed of vascularized connective tissue that lacks a basement membrane. Two cell types (type A and type B) are present: Type A is derived from blood monocytes, and it removes the wear-and-tear debris from the synovial fluid. Type B produces hyaluronan. Synovial fluid is made of hyaluronic acid and lubricin, proteinases, and collagenases. Synovial fluid exhibits non-Newtonian flow characteristics; the viscosity coefficient is not a constant and the fluid is  not linearly viscous. Synovial fluid has thixotropic characteristics; viscosity decreases and the fluid liquifies when under stress. 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 to lubricate the surfaces between synovium and cartilage. Synovial fluid contains lubricin (also known as PRG4) as a second lubricating component, secreted by synovial fibroblasts. Chiefly, it is responsible  for so-called boundary-layer lubrication, which reduces friction between opposing surfaces of cartilage. There also is some evidence that it helps regulate synovial cell growth. It also contains phagocytic cells that remove microbes and the debris that results from normal wear and tear in the joint. Clinical significance. Collection. Synovial fluid may be collected by syringe in a procedure termed arthrocentesis, also known as joint aspiration. Classification. Synovial fluid may be classified into normal, noninflammatory, inflammatory, septic, and hemorrhagic: Glucose (mg/dl) concentration in synovial fluid is nearly equal to serum. "Normal": "Normal or decreased": "Decreased": Pathology. Many synovial  fluid types are associated with specific diagnoses: Analysis. Glucose (mg/dl) concentration in synovial fluid is nearly equal to serum. The cytological and biochemical analysis of human synovial fluid began around 1940 using cadaver-derived fluid and comparing characteristics to those of, for instance, bovine synovial fluid. Chemistry. The mucin clot test is a very old approach to determining if an inflammatory infiltrate is present. In this test, acetic acid is added to the synovial fluid specimen. In a normal specimen, this should lead to a congealing of the hyaluronic acid, forming a 'mucin clot.' If inflammation is present, a  mucin clot is not formed (the hyaluronic acid is degraded). Lactate is elevated in septic arthritis, usually above 250 mg/dL. Complement factors are decreased in rheumatoid arthritis and lupus arthritis. Microscopy. Microscopic analysis of synovial fluid is performed to evaluate for cell count and crystals. Crystals include monosodium urate, calcium pyrophosphate, hydroxyapatite and corticosteroid crystals. Monosodium urate crystals are seen in gout or gouty arthritis and appear as needle-shaped negatively birefringent crystals varying in length from 2 to 20 μm. With negative birefringence, the crystals appear yellow in parallel light and blue with perpendicular light. Calcium pyrophosphate crystals  are seen in pseudogout (also known as calcium pyrophosphate deposition disease or, CPPD). These crystals are rod-shaped or rhomboids varying in length from 2 to 20 μm and with positive birefringence (blue with parallel light, yellow with perpendicular light). Hydroxyapatite crystals are small and negatively birefringent. They are usually only detectable with an Alizarin Red S stain. Corticosteroid crystals may be seen following therapeutic corticosteroid injection into the joint space. They appear blunt, jagged, and show variable birefringence. Cracking joints. When the two articulating surfaces of a synovial joint are separated from one other, the volume within the joint  capsule is increased and a negative pressure results. The volume of synovial fluid within the joint is insufficient to fill the expanding volume of the joint and gases dissolved in the synovial fluid (mostly carbon dioxide) are liberated and quickly fill the empty space, leading to the rapid formation of a bubble. This process is known as cavitation. Cavitation in synovial joints results in a high frequency 'cracking' sound. Etymology and pronunciation. The term "synovia" () came to English around 1640 (the anglicized form "synovial" is first recorded in the mid 18th century) from New Latin, where it was coined perhaps  by Paracelsus from Greek "συν-" "with" and Latin "ovum" "egg" and "-ia" because it resembles egg white in consistency and external appearance. The term "synovium" is a much more recent pseudo-Latin coinage for what is less confusingly called the synovial membrane. It is not recorded in general dictionaries, and medical dictionaries only explain its meaning, not its etymology, but it is apparently derived from the term "synovia", i.e. the obfuscated etymology of mixed Greek and Latin elements of the singular term "synovia" was misunderstood and the word was erroneously reinterpreted as the plural of the previously non-existent term  synovium (perhaps in analogy to other plural terms for liquids such as "waters" for amniotic fluid). If one insists on using this pseudo-Latin term "synovium" for the synovial membrane, the non-Latinate plural "synoviums" is better and less confusing than "synovia".