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The eggshell is an important structure for two reasons. Firstly it forms an embryonic chamber for the developing chick, providing mechanical protection and a controlled gas exchange medium. Secondly it is a container for the market egg, providing protection of the contents and a unique package for a valuable food. The superficial structure of the shell has been known for over 100 years. The shell consists of 97% calcium carbonate, and this is provided to the hen in the diet. However, the chemical must be broken down in the digestive system and then re-synthesized in the shell gland to form the shell. This results in a turnover of blood Ca of ~ 100 times each 24 hours. Provision of calcium to layers has been researched at length, but the key work, conducted at Cornell University in the 1960's, showed the necessity for sources with large particle size, for example, oyster shells. Subsequent research has defined guidelines that, if correctly followed, will yield reliable and optimum eggshell quality. Breakage or cracking of eggshells in market channels is a serious concern. Cracks result from a combination of shell strength and integrity, and the extent of the "insult" received by the egg during handling. Measurement of shell strength, and assessment of "insults" is important to the poultry industry. Most recent research has identified the ultra-structure of the shell, with resulting opportunities for industrial remedies, and possible new selection criteria, to ensure maximum shell quality throughout the egg industry.
Peter Hunton (Wed,) studied this question.