Eggshell

Some arthropod eggs do not actually have shells, rather, their outer covering is actually the outermost embryonic membrane, the choroid, which protects inner layers.

Eggs which must survive in dry conditions usually have hard eggshells, made mostly of dehydrated or mineralized proteins with pore systems to allow respiration.

[3][4][2] Without the protein, the crystal structure would be too brittle to keep its form and the organic matrix is thought to have a role in deposition of calcium during the mineralization process.

[5][6][7] The structure and composition of the avian eggshell protects the egg from damage and microbial contamination, prevents desiccation, regulates embryonic gas and water exchange and provides calcium for embryogenesis.

Eggshell formation requires gram amounts of calcium being deposited within hours, which must be supplied via the hen's diet.

[2] The avian eggshell is constructed from an organized assembly of mineral nanoparticles whose size confers different hardness properties to different regions of the shell.

[2] In the distal (red) isthmus mammillae or mammillary knobs are deposited on the surface of the outer membrane in a regular array pattern.

In the shell gland (similar to a mammalian uterus), mineralization starts at the mammillae and around the outermost membrane fibers.

This attachment between dissimilar materials is facilitated by a structural interdigitation of fibers into each mammillae at the microscale, and reciprocally, at the nanoscale, mineral spiking into fibers directly at the interface [11] While the bulk of eggshell is made of calcium carbonate, it is now thought that the protein matrix has an important role to play in eggshell strength.

[citation needed] As they contain mainly calcium carbonate, bird eggshells dissolve in various acids, including the vinegar used in cooking.

[15] The disposal methods for waste eggshells are 26.6% as fertilizer, 21.1% as animal feed ingredients, 26.3% discarded in municipal dumps, and 15.8% used in other ways.

[citation needed] Hatching birds, amphibian and egg-laying reptiles have an egg-tooth used to start an exit hole in the hard eggshell.

[22] Chicken eggshells have been additionally incorporated as a calcium precursor into the synthesis of calcium-based metal-organic frameworks (MOFs).

[24] Recently, researchers have utilized chicken eggshells as a biofiller with a conducting polymer to enhance its sensing properties.