Biomineralization and Eggshells: Cell-Mediated Acellular Compartments of Mineralized Extracellular Matrix

Publisher Summary This chapter summarizes cell biology, morphological organization, crystallography, chemical composition, process of mineralization, and biological function of avian eggshells. It also discusses emerging concepts of biomineralization and speculates about the mechanism of eggshell assembly and its implications for fabrication of polymer-ceramic composites. The eggshell is a microenvironmental compartment for housing developing embryos of a number of species. This unique microenvironment provides physical protection to the embryo and regulates gas, water, and ionic exchange. The avian eggshell can be characterized as a multilayered, polymer–ceramic composite. The three main layers include an outer mucous layer, an intermediate calcified zone, and an inner fibrous membrane layer. The shell membranes are the most internal layer of the eggshell and are formed by two nonmineralized fibrillar sublayers, the outer membrana testae externa, and the inner membrana testae interna or putaminis. The most external layer of the eggshell is referred to as the cuticle. This proteinaceous layer covers the entire calcified portion of the shell to a depth of about 10 μ m. The complementary use of biological, chemical, and crystallographic approaches demonstrates that the avian eggshell is a very promising model for the study of biomineralization. Avian eggshell is one of the most rapidly mineralizing biological systems known.