Hydroxyapatite crystal in bone matrix (HA)

Hydroxyapatite crystal is the principal inorganic component of the human skeletal system, comprising up to 70% by weight and about 50% by volume of mature human bones. Chemically defined as Ca₁₀(PO₄)₆(OH)₂, it forms small plate-like or needle-shaped nanocrystals embedded within an organic collagen matrix. This composite structure imparts both rigidity from the minerals and flexibility from collagen fibers—essential for normal mechanical function. The primary biological roles include providing structural support for movement/protection, serving as an ion reservoir (especially calcium/phosphate), enabling hardness necessary for load-bearing functions, and participating actively in processes like remodeling/regeneration through interactions with cells such as osteoblasts/osteoclasts. While not itself a druggable target molecule like receptors or enzymes, its properties underpin many therapeutic strategies—including antiresorptive drugs that act on its surface chemistry within living tissue—and it serves widely in biomedical engineering applications such as dental/bone graft materials due to its high biocompatibility.

Other names

HydroxylapatiteBone mineralCalcium apatite (in the context of its chemical family)HA

For bisphosphonates: Bind to exposed surfaces of hydroxyapatite crystals in bone, inhibiting their dissolution by osteoclasts and thus reducing bone resorption. For synthetic/natural HA materials: Serve as scaffolds promoting new bone growth via osteoconduction/osteoinduction when implanted clinically.