Mineral-Matrix Interactions in Bone and Cartilage

Mineral-matrix interactions regulate the process of hydroxyapatite formation in bones and teeth. In mineralizing tissues, many anionic macromolecules bind to mineral. By means of this binding, such molecules are able to regulate the size and shape of the mineral crystals, determine the site of initial crystal deposition, and determine the type of mineral crystals deposited. Collagen, which provides a template for hydroxyapatite deposition; extracellular matrix vesicles, which provide a protected environment for crystal deposition; and noncollagenous matrix proteins that have high affinities for hydroxyapatite have all been shown to affect mineralization in vitro. Some of the noncollagenous proteins have been shown to be capable of promoting and inhibiting mineral formation and growth, depending on their concentration and whether they are immobilized or free in solution. This review surveys the current understanding of mineral-matrix relationships involved in endochondral, intramembranous, and appositional bone formation, outlining the way in which mineral deposition is controlled in mammalian calcified tissues. The structural basis for the interaction of the matrix molecules with hydroxyapatite is presented, as is the in vitro and in situ data implicating the matrix molecules that interact with hydroxyapatite to control mineralization.