Hydroxyapatite crystal, bone matrix (None established; sometimes abbreviated as "HA" or "HAP" in literature, but for structured representation, use the full name.)

Hydroxyapatite crystal is the primary inorganic component of bone and dental matrices, composed predominantly of calcium and phosphate in the stoichiometric form Ca₁₀(PO₄)₆(OH)₂. In bone, these crystals are interwoven with collagen and non-collagenous proteins, forming nanocrystalline platelets with numerous defects and substitutions (such as carbonate or magnesium), making them distinct from pure synthetic hydroxyapatite[1][3][4][5]. Hydroxyapatite provides bones with structural rigidity and acts as a reservoir for mineral ions[3][5]. It is formed within the collagen fibrils of bone by a process of nucleation and mineralization[4][1]. Hydroxyapatite crystals are subject to resorption by osteoclasts in bone remodeling and are targeted by several drug classes in osteoporosis and imaging[3]. Abnormal deposition of hydroxyapatite is involved in a variety of pathological calcifications in soft tissues and in systemic diseases[2][5][4]. However, hydroxyapatite is not a druggable target in the conventional sense but serves as the locus for drug actions modulating bone metabolism and turnover.

Other names

Hydroxyapatite (HA, HAP)Bone mineralCarbonated apatite (for bone-specific form)Calcium phosphate nanocrystal

Bisphosphonates: bind with high affinity to hydroxyapatite, are taken up by osteoclasts during bone resorption, interfere with osteoclast function and survival Tetracyclines: bind by chelating calcium ions in HA crystal lattice (for imaging, not therapy) Chelators: dissolve or modify the HA crystal by binding calcium