New insight in eggshell formation.

The matrix proteins that participate in crystalization fulfill important functions during the formation of the calcified tissues and contribute to the biomechanical properties of the mature product. We suggest that osteopontin (OPN) is part of an array of macromolecules synthesized and secreted by the cells adjacent to the mineralization front that self-assemble outside the cell and direct crystal formation. The OPN meets the theoretical requirements for involvement in the mineralization process. The phosphorylated residues of acidic phosphoprotein have been shown to exist in the protein as reactive monoesters that are available for interaction with other ions, among them crystal constituents such as calcium ions. In addition, sulfation of OPN was also found to be associated with mineralization of other tissues. In contrast to the calbindin gene, whose expression is dependent on the calcium flux, the regulation of OPN synthesis is at least in part dependent on the mechanical strain imposed by the resident egg. These results demonstrate the complexity of the regulation of the matrix genes governing eggshell formation.