Structure and calcification in avian egg shell

Abstract

Optical and radiographic studies show that the hen's egg shell is a highly ordered structure throughout its entire thickness. The usual division of the shell into transverse “mammillary” and “spongy” layers is not supported by the present observations. In polarized light, distinct mineral columns are seen to extend radially from the mammillary projections to the surface of the shell. Within the calcified tip of each shell mammilla is a fibrous ring which is attached directly to the outer shell membrane and is responsible for the firm attachment of the membrane to the shell. It is suggested that these rings also serve as initial nucleation centers during egg shell formation. A differentiation is made between the optically opaque “mineral cores” in the center of the mammillae and the “organic matrix cores” at the tips of the mammillae. Whereas it could be confirmed by X-ray microdiffraction that the mineral component of the shell is calcite, the presence of keratin in shell membrane could not be ascertained.