Figure 4. Proposed mechanism of vesicle‐mediated mineral formation .

Top, cell‐derived vesicles are preloaded with a nucleational core (NC), composed of annexins A2, A5 and A6, as well as a complex of PS, Ca²⁺ and P_i. Calcium and phosphate ions are accumulated within the vesicle lumen via multiple routes: intravesicular P_i cleavage from PChol and PEA via PHOSPHO1; P_i cleavage from ATP in the extravesicular space via TNAP and NPP1/3, and import via Pit1/2; and calcium import via annexins A2, A5 and A6. These ions are patterned as calcium phosphate mineral at the NC. Bottom, over time, mineral crystals propagate through the vesicle membrane, where their continued growth is regulated by the ratio of PP_i, an inhibitor of crystal growth, to its derivative P_i. Note that the hypothesis that annexins form transmembrane ion channels, as depicted in this figure, is controversial (given that annexins are believed to primarily function as peripheral membrane proteins), thus meriting further investigation.