Calcium Pyrophosphate: An inorganic pyrophosphate which affects calcium metabolism in mammals. Abnormalities in its metabolism occur in some human diseases, notably HYPOPHOSPHATASIA and pseudogout (CHONDROCALCINOSIS).Chondrocalcinosis: Presence of calcium salts, especially calcium pyrophosphate, in the cartilaginous structures of one or more joints. When accompanied by attacks of goutlike symptoms, it is called pseudogout. (Dorland, 27th ed)Diphosphates: Inorganic salts of phosphoric acid that contain two phosphate groups.Microscopy, Polarization: Microscopy using polarized light in which phenomena due to the preferential orientation of optical properties with respect to the vibration plane of the polarized light are made visible and correlated parameters are made measurable.Crystallization: The formation of crystalline substances from solutions or melts. (McGraw-Hill Dictionary of Scientific and Technical Terms, 4th ed)Scleral Diseases: General disorders of the sclera or white of the eye. They may include anatomic, embryologic, degenerative, or pigmentation defects.Calcium Phosphates: Calcium salts of phosphoric acid. These compounds are frequently used as calcium supplements.Synovial Fluid: The clear, viscous fluid secreted by the SYNOVIAL MEMBRANE. It contains mucin, albumin, fat, and mineral salts and serves to lubricate joints.Phosphate Transport Proteins: Membrane proteins that are involved in the active transport of phosphate.Gout: Hereditary metabolic disorder characterized by recurrent acute arthritis, hyperuricemia and deposition of sodium urate in and around the joints, sometimes with formation of uric acid calculi.Pyrophosphatases: A group of enzymes within the class EC 3.6.1.- that catalyze the hydrolysis of diphosphate bonds, chiefly in nucleoside di- and triphosphates. They may liberate either a mono- or diphosphate. EC 3.6.1.-.Uric Acid: An oxidation product, via XANTHINE OXIDASE, of oxypurines such as XANTHINE and HYPOXANTHINE. It is the final oxidation product of purine catabolism in humans and primates, whereas in most other mammals URATE OXIDASE further oxidizes it to ALLANTOIN.TetramisoleCalcinosis: Pathologic deposition of calcium salts in tissues.Joint DiseasesOsteoarthritis: A progressive, degenerative joint disease, the most common form of arthritis, especially in older persons. The disease is thought to result not from the aging process but from biochemical changes and biomechanical stresses affecting articular cartilage. In the foreign literature it is often called osteoarthrosis deformans.Durapatite: The mineral component of bones and teeth; it has been used therapeutically as a prosthetic aid and in the prevention and treatment of osteoporosis.Knee Joint: A synovial hinge connection formed between the bones of the FEMUR; TIBIA; and PATELLA.Thiamine Pyrophosphate: The coenzyme form of Vitamin B1 present in many animal tissues. It is a required intermediate in the PYRUVATE DEHYDROGENASE COMPLEX and the KETOGLUTARATE DEHYDROGENASE COMPLEX.Cartilage, Articular: A protective layer of firm, flexible cartilage over the articulating ends of bones. It provides a smooth surface for joint movement, protecting the ends of long bones from wear at points of contact.ArthritisHydroxyapatites: A group of compounds with the general formula M10(PO4)6(OH)2, where M is barium, strontium, or calcium. The compounds are the principal mineral in phosphorite deposits, biological tissue, human bones, and teeth. They are also used as an anticaking agent and polymer catalysts. (Grant & Hackh's Chemical Dictionary, 5th ed)Technetium Tc 99m Pyrophosphate: A radionuclide imaging agent used primarily in scintigraphy or tomography of the heart to evaluate the extent of the necrotic myocardial process. It has also been used in noninvasive tests for the distribution of organ involvement in different types of amyloidosis and for the evaluation of muscle necrosis in the extremities.Calcium Signaling: Signal transduction mechanisms whereby calcium mobilization (from outside the cell or from intracellular storage pools) to the cytoplasm is triggered by external stimuli. Calcium signals are often seen to propagate as waves, oscillations, spikes, sparks, or puffs. The calcium acts as an intracellular messenger by activating calcium-responsive proteins.Polyisoprenyl Phosphates: Phosphoric or pyrophosphoric acid esters of polyisoprenoids.Phosphoribosyl Pyrophosphate: The key substance in the biosynthesis of histidine, tryptophan, and purine and pyrimidine nucleotides.