Avian eggshell mineralization: biochemical and functional characterization of matrix proteins

The eggshell of the hen is a highly ordered and mineralised structure, which is sequentially deposited within an acellular milieu - the uterine fluid secreted by the distal oviduct. Spherulitic crystal growth of calcite is initiated on organic aggregates on surface of the eggshell membranes, followed by competition between radial crystallites for space to form a compact columnar biomineral. The exceptional mechanical properties associated with the well-defined eggshell ultrastructure and texture arise from the control of crystal morphology and growth by the organic matrix, and, amongst them, proteins specific to the uterus and eggshell (ovocleidins and ovocalyxins). The changes in uterine fluid constituents with stages of egg calcification, their effects on morphology of calcite grown in vitro, and the relationship between eggshell texture and mechanical properties point to this control of eggshell fabric.