Mechanisms and hormonal regulation of shell formation: supply of ionic and organic precursors, shell mineralization
Résumé
The avian egg is a giant reproductive cell protected by an eggshell, which biomineralizes in the uterus, in an acellular fluid that contains ionic and organic precursors. The distinctive features of the eggshell, as compared to bone or teeth, are its mineral composition (CaCO3 in the form of calcite) and the absence of cell-directed assembly during its mineralization on extracellular membranes. In chickens, the hen exports daily 2 g of Ca, which is a challenge for maintenance of calcium homeostasis. Hens develop numerous physiological adaptations for providing shell calcium and carbonate. One feature is the development of labile calcium reserves in medullary bone at sexual maturity. Moreover, the level of intestinal Ca uptake increases sixfold though the action of 1,25(OH)2 D3. The uterine segment of the oviduct acquires the capacity to secrete large amounts of Ca2+ and HCO3– ions and to synthesize/secrete eggshell matrix proteins that control the process of shell mineralization and its textural properties. This chapter describes the mechanisms of ion transfer from bone, intestine, and uterus, the process of shell mineralization and their hormonal regulation.