Shell Formation

The formation of shell can be described in terms of two major phases: (1) cellular processes of ion transport, protein synthesis, and secretion and (2) a series of physicochemical processes in which crystals of CaCO3 are nucleated, oriented, and grow in intimate association with a secreted organic matrix. In providing the mineral of shell, Ca2+ and HCO3− are first transported across epithelia at the body surface and the mantle epithelium facing the inner shell surface. Movement of these ions is incompletely understood but almost certainly involves active transport of Ca2+. The physicochemical phase takes place in fluid between the outer mantle epithelium and the inner shell surface and on this shell surface. For crystals to be deposited, the following conditions are required (1) concentrations of Ca2+ and CO32- exceeding the solubility product, (2) conditions favoring crystal nucleation, and (3) the elimination of H+ resulting from CaCO3 formation. Following crystal nucleation oriented crystal growth leads to a shell constructed of a variety of crystal patterns. In the process protein secreted by the mantle surrounds the individual crystals and becomes the cement which binds them together as a shell. Our understanding of the processes of the crystal depositional phase of shell formation is fragmentary. However, we can expect that through analyses of the fluid at the site of crystallization and study of effects of organic fractions of the fluid and shell on crystallization, the conditions governing the initiation and control of crystal growth will be further defined. Substances that initiate crystallization have been proposed but have not been tested. Preliminary studies indicate that the compounds at the crystallization site include inhibitors of CaCO3 deposition. A major aspect of shell construction of great interest yet to receive experimental study is the relation of particular segments of the mantle epithelium to the structure and orientation of strikingly different types of crystal patterns. A promising approach to these crystallographic problems appears to be the application of in vitro methods of physical biochemistry to crystal development and orientation in association with fractions of the organic phase of shell. Studies indicating involvement of the nervous system and hormonal and neurosecretory changes associated with shell formation will almost certainly be expanded and contribute to the understanding of mechanisms of mineralization and their control.