Organic macromolecules transport a significant proportion of the calcium precursor for nacre formation

The mechanism of nacre formation in gastropods involves a vesicular system that transports organic and mineral precursors from the mantle epithelium to the mineralization chamber. Between them lies the surface membrane, a thick organic structure that covers the mineralization chamber and the forming nacre. The surface membrane is a dynamic structure that grows by the addition of vesicles on the outer side and recedes by the formation of interlamellar membranes on the inner side. By using a combination of electron microscopy imaging and spectroscopy, we have monitored the journey of the vesicles from the mantle epithelium to the mineralization chamber, focusing on the elemental composition of the organic structures at each stage. Our data reveal that transport occurs in lipid bilayer vesicles through exocytosis from the outer mantle epithelium. After release into the surface membrane, chitin undergoes a process of self-assembly and interaction with proteins, resulting in progressive changes of the internal structure of the surface membrane until the final structure of the interlamellar membranes is acquired. Finally, these detach from the inner side of the surface membrane. Elemental analysis revealed the transport of a considerable amount of calcium bound to proteins, likely forming calcium-protein complexes. STATEMENT OF SIGNIFICANCE: The formation of nacre tablets occurs through the incorporation of organic and mineral precursors extruded from the mantle epithelium. Although much attention has been paid to the presence of amorphous phases in recent decades, the calcium transport system has not yet been elucidated. We have monitored the packaging of organic and mineral precursors in the form of vesicles from the mantle epithelium to the mineralization chamber. We have shown that the surface membrane represents the zone where chitin and protein polymerization takes place, acquiring the final structure for the formation of interlamellar membranes. Interestingly, these organic structures transport a considerable amount of organic calcium into the mineralization chamber, which support a transport system based on acidic calcium-binding proteins.