Endochondral bone formation, matrix vesicles
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Matrix vesicle in Phase 1 of calcification. Matrix vesicles serve as the site for deposition of the earliest recognizable crystals of hydroxyapatite (Ca)10(PO4)6(OH2) and therefore are believed to initiate calcification. The mechanism of initiation appears to involve calcium-binding plus provision of phosphate by matrix vesicle membrane phosphatase including alkaline phosphatase.