Avian eggshell mineralization

Abstract

The eggshell of the hen is a highly ordered mineral structure deposited in an acellular milieu secreted by the distal parts of the oviduct; the isthmus and uterus. Spherulitic crystal growth is initiated by deposition of calcium carbonate on organic aggregates layed quasiperiodically on the outer surface of eggshell membranes. The uterus secretes a hypersaturated milieu relative to calcite during the phase of eggshell formation. The mechanisms and hormonal control of calcium and bicarbonate fluxes have been partially identified, The uterine fluid also contained organic molecules and a matrix which pervades the calcified layer and can be extracted as soluble and insoluble fractions. SDS-PAGE reveals a complex array of proteins in uterine fluid and eggshell:extract in hens and other species of birds, among them ovocleidin-17, (OC17), a specific uterine protein and osteopontin, a bone matrix protein. Serum albumin, ovalbumin, lysozyme and ovotransferrin are also present in hen eggshell. Similarities in protein electrophoretic profiles, as well as the presence of serum albumin, ovalbumin, lysozyme, ovotransferrin and OC17 in the uterine fluid and the modification of this organic composition with different stages of shell formation support the hypothesis that the precursors of eggshell matrix are found in this milieu. Eggshell extracts or uterine fluid contain components which are able in vitro to delay, from a metastable solution of calcium and carbonate, the rate of precipitation of calcium carbonate and to modify the size and crystal morphology of the resulting calcite. Proteoglycans, primarily keratan- and dermatan-sulfate are also found in the eggshell matrix, and they also modify crystal precipitation and morphology. These observations suggest that these macromolecules may influence the organization of crystal growth by controlling size, shape and orientation of calcite crystals. The identification of matrix components and analysis of their biochemical and functional properties is a necessary prerequisite to gain insight into the mechanisms and regulation of eggshell calcification and for the development of new approaches to control eggshell quality.