Mollusc Shell - Formation

Formation

A mollusc shell is formed, repaired and maintained by a part of the anatomy called the mantle. Any injuries to or abnormal conditions of the mantle are usually reflected in the shape and form and even color of the shell. When the animal encounters harsh conditions that limit its food supply, or otherwise cause it to become dormant for a while, the mantle often ceases to produce the shell substance. When conditions improve again and the mantle resumes its task, a "growth line" is produced.

The mantle edge secretes a shell which has two components. The organic constituent is mainly made up of polysaccharides and glycoproteins; its composition may vary widely: some molluscs employ a wide range of chitin-control genes to create their matrix, whereas others just express one, suggesting that the role of chitin in the shell framework is highly variable; it may even be absent in monoplacophora. This organic framework controls the formation of calcium carbonate crystals, (never phosphate with the questionable exception of Cobcrephora), and dictates when and where crystals start and stop growing, and how fast they expand; it even controls the polymorph of the crystal deposited, controlling positioning and elongation of crystals and preventing their growth where appropriate.

The shell formation requires certain biological machinery. The shell is deposited within a small compartment, the extrapallial space, which is sealed from the environment by the periostracum, a leathery outer layer around the rim of the shell, where growth occurs. This caps off the extrapallial space, which is bounded on its other surfaces by the existing shell and the mantle. The periostracum acts as a framework from which the outer layer of carbonate can be suspended, but also, in sealing the compartment, allows the accumulation of ions in concentrations sufficient for crystallization to occur. The accumulation of ions is driven by ion pumps packed within the calcifying epithelium. Calcium ions are obtained from the organism's environment through the gills, gut and epithelium, transported by the haemolymph ("blood") to the calcifying epithelium, and stored as granules within or in-between cells ready to be dissolved and pumped into the extrapallial space when they are required. The organic matrix forms the scaffold that directs crystallization, and the deposition and rate of crystals is also controlled by hormones produced by the mollusc. Because the extrapallial space is supersaturated, the matrix could be thought of as impeding, rather than encouraging, carbonate deposition; although it does act as a nucleating point for the crystals and controls their shape, orientation and polymorph, it also terminates their growth once they reach the necessary size. Nucleation is endoepithelial in Neopilina and Nautilus, but exoepithelial in the bivalves and gastropods.

The formation of the shell involves a number of genes and transcription factors. On the whole, the transcription factors and signalling genes are deeply conserved, but the proteins in the secretome are highly derived and rapidly evolving. engrailed serves to demark the edge of the shell field; dpp controls the shape of the shell, and Hox1 and Hox4 have been implicated in the onset of mineralization. In gastropod embryos, Hox1 is expressed where the shell is being accreted; however no association has been observed between Hox genes and cephalopod shell formation. Perlucin increases the rate at which calcium carbonate precipitates to form a shell when in saturated seawater; this protein is from the same group of proteins (C-type lectins) as those responsible for the formation of eggshell and pancreatic stone crystals, but the role of C-type lectins in mineralization is unclear. Perlucin operates in association with Perlustrin, a smaller relative of lustrin A, a protein responsible for the elasticity of organic layers that makes nacre so resistant to cracking. Lustrin A bears remarkable structural similarity to the proteins involved in mineralization in diatoms – even though diatoms use silica, not calcite, to form their tests!