Ressource pédagogique : Prof. Alejandro Rodriguez Navarro - Mechanisms of eggshell biomineralization

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The avian eggshell is a thin mineral layer (350 µm thick in chicken) that protects the egg content against mechanical impacts, dehydration and microorganism contamination. Eggshell formation is a highly controlled and rapid mineralization process occurring while the egg is residing in the uterus during the night. The composition of the uterine fluid changes at each stage of eggshell formation (initiation, linear growth, termination) with the expression of specific proteins that actively regulate calcium carbonate precipitation, selecting the mineral phase to form (calcite), inducing the nucleation of crystals and controlling their growth, morphology and size. Eggshell formation is terminated with the deposition of the cuticle about 2 hours before oviposition. Egg laying is highly demanding and female birds have developed specific physiological adaptations for it. They need a large and continuous supply of calcium. A stimulated production of vitamin D leads to an increase in calcium absorption by intestinal and uterine tissues. Also, hens develop a new type of bone within the marrow cavities of their long bones (medullary bone) that serves as a calcium reservoir for eggshell calcification during the night when hens are not eating and the intestinal calcium supply is exhausted. The formation and resorption of medullary bone is synchronized with the egg daily cycle. Laying hens also provide enough respiratory CO2 to form carbonate ions needed for eggshell calcium

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