The structure of the hen's egg shell can be divided into five separate layers . The Innermost layer consists of two distinct membranes known as the inner and outer shell membranes. These membranes are compos ed of networks of protein/polysaccharide fibres and are =70 mm thick. Attached to the outer fibres of the outer membrane are polycrystals of calcite (calcium carbonate) which extend outward in an inverse conical manner until the cones from several sites of crystal i nit iation fuse together. The fibre/ crystal attachment sites, known as basal caps, and the cones form the mammillary knobs layer which is :: 100- 110 llm thick. After the cones fuse with each other, continuing calcite deposition produces columnar crystals 10 -30 "'m in diameter and :::200 llm in length. These crys tals form the palisade layer and are intermingled with a protein/polysaccharide matrix that differs in composition From the shell membranes. Over the columnar crystals is a thin layer (::S -8 "'m thick), known as the vertical crystal layer, of small calcite crystals that are orientated perpendicular to the shell's surface. The cuticle is the outermost layer of the shell; it is =10 mm thick and contains predominantly protein. Passing vertically through the palisade layer of the shell from "valleys" between the marrmillary knobs to the surface of the vertical crystal layer are funnel - shaped, unbranched pores, These pores are capped by the cuticle which is cracked and thus allows the diffusion of gases between the contents of the egg and its environment. The geometrical configuration of the cones in the mammillary knobs layer is related to the thickness of shell. Specific amino acids in the membrane fibres.
Hamilton, R. M. G.
"The Microstructure of the Hen's Egg Shell - A Short Review,"
1, Article 13.
Available at: http://digitalcommons.usu.edu/foodmicrostructure/vol5/iss1/13