The eggshell: structure, composition and mineralization.

The calcareous egg is produced by all birds and most reptiles. Current understanding of eggshell formation and mineralization is mainly based on intensive studies of one species - the domesticated chicken Gallus gallus. The majority of constituents of the chicken eggshell have been identified. In this article we review eggshell microstructure and ultrastructure, and the results of recent genomic, transcriptomic and proteomic analyses of the chicken eggshell matrix to draw attention to areas of current uncertainty such as the potential role of amorphous calcium carbonate and the specific nature of the molecules that initiate (nucleate) mammillary cone formation and terminate palisade layer calcification. Comparative avian genomics and proteomics have only recently become possible with the publication of the Taeniopygia guttata (zebra finch) genome. Further rapid progress is highly anticipated with the soon-to-be-released genomes of turkey (Meleagris gallopavo) and duck (Anas platyrhynchos). These resources will allow rapid advances in comparative studies of the organic constituents of avian eggshell and their functional implications.