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. 2012 Jun 4;7(6):e38393. doi: 10.1371/journal.pone.0038393

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This is an open-access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication.

This is an open-access article distributed under the terms of the Creative Commons Public Domain declaration, which stipulates that, once placed in the public domain, this work may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose.

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Cementum apposition depends on precipitation of calcium (Ca²⁺) and phosphate (P_i) ions on the root surface, and pyrophosphate (PP_i) acts as a potent inhibitor of hydroxyapatite crystal precipitation. Local pericellular PP_i concentration is controlled primarily by three cellular factors: Tissue nonspecific alkaline phosphatase (TNAP; hydrolyzes PP_i to P_i), progressive ankylosis protein (ANK; regulates transport of PP_i from the intracellular to extracellular space), and ectonucleotide pyrophosphatase phosphodiesterase 1 (NPP1; produces PP_i from hydrolysis of nucleotide triphosphates). Cementoblasts express TNAP, ANK, and NPP1 in order to regulate local PP_i homeostasis and control the amount of cementum apposition. Cementoblasts are responsive to apposition by some type of outside-in feedback mechanism, and are capable of modulating expression of PP_i regulators as well as production of secreted matrix proteins such as OPN and DMP1, which may influence mineralization or properties of the matrix.