Avian eggshell mineralization

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 isthmu s and uterus. Spherulitic crystal growth is initiated by deposition of calc ium carbonate on organic aggregates layed quasiperiodically on the outer su rface of eggshell membranes. The uterus secretes a hypersaturated milieu re lative to calcite during the phase of eggshell formation. The mechanisms an d hormonal control of calcium and bicarbonate fluxes have been partially id entified, The uterine fluid also contained organic molecules and a matrix w hich pervades the calcified layer and can be extracted as soluble and insol uble fractions. SDS-PAGE reveals a complex array of proteins in uterine flu id and eggshell:extract in hens and other species of birds, among them ovoc leidin-17, (OC17), a specific uterine protein and osteopontin, a bone matri x protein. Serum albumin, ovalbumin, lysozyme and ovotransferrin are also p resent in hen eggshell. Similarities in protein electrophoretic profiles, a s well as the presence of serum albumin, ovalbumin, lysozyme, ovotransferri n and OC17 in the uterine fluid and the modification of this organic compos ition with different stages of shell formation support the hypothesis that the precursors of eggshell matrix are found in this milieu. Eggshell extrac ts or uterine fluid contain components which are able in vitro to delay, fr om a metastable solution of calcium and carbonate, the rate of precipitatio n 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 precipi tation and morphology. These observations suggest that these macromolecules may influence the organization of crystal growth by controlling size, shap e and orientation of calcite crystals. The identification of matrix compone nts and analysis of their biochemical and functional properties is a necess ary prerequisite to gain insight into the mechanisms and regulation of eggs hell calcification and for the development of new approaches to control egg shell quality.