Subgingival calculus forms below the gumline and is typically darkened in color by the presence of black-pigmented bacteria, whose cells are coated in a layer of iron obtained from heme during gingival bleeding. These areas experience high salivary flow because of their proximity to the parotid and sublingual salivary glands. Supragingival calculus formation is most abundant on the buccal (cheek) surfaces of the maxillary (upper jaw) molars and on the lingual (tongue) surfaces of the mandibular (lower jaw) incisors. The processes of calculus formation from dental plaque are not well understood. Trace amounts of host, dietary, and environmental microdebris are also found within calculus, including salivary proteins, plant DNA, milk proteins, starch granules, textile fibers, and smoke particles. The organic extracellular matrix in calculus consists primarily of proteins and lipids (fatty acids, triglycerides, glycolipids, and phospholipids), as well as extracellular DNA. The cells within calculus are primarily bacterial, but also include at least one species of archaea ( Methanobrevibacter oralis) and several species of yeast (e.g., Candida albicans). Cell density within dental plaque and calculus is very high, consisting of an estimated 200,000,000 cells per milligram. The organic component of calculus is approximately 85% cellular and 15% extracellular matrix.
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