Calcium trafficking in the mussel Mytilus galloprovincialis suggests conserved biomineralization pathways

Vesicles play a key role in biomineralization pathways by mediating ion concentration and transport while maintaining cellular homeostasis. Whereas calcium-loaded vesicles have been extensively studied in organisms such as coccolithophores and vertebrates, comparatively little is known about those involved in mollusc shell mineralization. Using electron microscopy and electron energy loss spectroscopy (EELS), we analyzed the ultrastructure and composition of three vesicle types predominant near the shell's growth surface in cryofixed juvenile Mytilus galloprovincialis: bilayered vesicles, ribosome-associated vesicles (RAVs), and mitochondria containing electron-dense granules. Calcium associated with organic compounds was detected in all of them. Mitochondrial granules were shown to be composed of an amorphous mixed phase containing substantial amounts of calcium and phosphorus, in addition to organic compounds. Identical Ca- and P-rich granules have been widely reported in the mitochondria of bone-related cells, where their presence is associated with the regulation of intracellular calcium and the initiation of hydroxyapatite formation. Their presence in calcifying cells of both molluscs and vertebrates, together with increasing evidence of phosphate involvement in the regulation of transient calcium carbonate precursors, leads us to propose a conserved pathway for calcium transport, concentration and storage across phyla, regardless of the final mineral phase.