Eggshell Matrix Proteins and Natural Defenses of the Egg

The chicken eggshell proteins have been largely investigated during the last decade. Proteins and associated genes have been identified and characterized. Various experimental evidences suggest a role for eggshell matrix proteins during mineralization. Indeed, novel specific eggshell matrix proteins are only expressed in tissues where eggshell mineralization takes place and some are highly stimulated during shell formation. These proteins also interact in vitro with the growing shell mineral (calcium carbonate) and modify crystal morphology and resulting shell microstructure which influence mechanical properties of the shell. In addition, the matrix composition changes during the various phases of eggshell calcification (initiation, growth and completion) and coincides with some alterations in eggshell mechanical properties that are observed at different physiological states of the hen (aging or molt). Antibacterial proteins (lysozyme, ovotransferrine) are also present in the eggshell. Recently, we also have cloned 2 novel eggshell matrix proteins (ovocalyxin-36 and -25) that could be related to anti-bacterial proteins. Because of their role during eggshell formation and their anti-bacterial properties, eggshell proteins are thought to be involved in natural egg defenses by reinforcing the mineral structure and by expressing antibacterial activities which preserve the hygienic quality of eggs during its formation.