Nacreous structure layer

Nacreous structure layer a layer consisting of stacked platy aragonite crystals in brick-like layers and an interstitial protein layer. 

Figure 19.7 Molecular correspondence of the inorganic-organic interface in the nacreous shell layer of Nautilus repertus. (a) Structural relationships between protein sheets, aragonite crystals and chitin fibres, (b) Possible complementarity of Ca binding. (From Mann et al., 1989. Reproduced with permission from John Wiley Sons., Inc.)...

It is well known that well-ordered (3-chitin (a polysaccharide) associated with a less ordered protein in the (3-sheet conformation is the main component of nacreous organic matrix in shell. The amino acid sequence of such proteins is very similar to those of silk fibroins. Indeed, the amino acid sequence of a major protein from the nacreous shell layer of the pearl oyster resembles that of spidroin (Sudo et al., 1997 Weiner and Traub, 1980). The question of whether silk-like proteins play an important role in shell formation is raised. When Falini et al. (1996) did the experiment with the proteins from the shell, they assembled a substrate in vitro that contained (3-chitin and natural silk fibroin and concluded that the silk fibroin may influence ion diffusion or the accessibility to the chi tin surface or both. Furthermore, cryo-TEM study of the structure of the Atrina shell nacreous organic matrix without dehydration... 

Figure 14.2. Structures forming the shells ofbivalves [6]. Parts (a)-(d) indicate how the nacreous layer of pearl is formed. A =platy aragonite crystals 0 = organic sheet ...

Mutvei, H. Ultra structure of the mineral and organic components of molluscan nacreous layers, Biomineralisation 2, 48 (1970)... 

A particularly well-studied structure is that of the nacreous layer (= Mother-of-Pearl) of molluscs, which will be used below to illustrate our concepts17 34,46,233-241). 

Also, increase in water temperature favors -conformation. Inasmuch as the conformation of CP probably determines the tertiary structure of MM, slight changes in CP conformation introduced by cellular or environmental effects may alter MM conformation. This in turn is reflected in the mineral form and structural pattern of the inorganic phases. Perhaps, nacreous layers in molluscs represent an almost ideal situation where MM and CP are aligned in a symmetrical way. In fish otoliths, the fibrous organic matrix is a mixture of helices and 0-pleated sheets. It is tentatively concluded that the morphology of shell structures is a macroscopic expression of the molecular interactions between MM and CP which are controlled in part by cellular activities and in part by the environment. 

Transmission electron microscope studies have shown that the organic interlamellar membranes have a lamellar structure (Figure 16(a), (b)). This structure varies according to the taxa. Some fenestrations or holes have been described, and they have been considered to favor the passage of fluid substances needed for the minerahzing process. The decalcificafion process of the nacreous layer, and later all the mollusc layers and invertebrate tests, has shown... 

Shell beads are made om the same material as the pearls themselves, so are naturally nacreous. However, the nacre does not cover the entire bead, but appears in layers due to the structure of the shell (Fig. 10.13). When dyed black, shell beads make convincing imitations of black peails, thou they tend to be too regular in shape. 

Any shell that is suCBciendy solid and attractive can be used for decorative purposes. Those with nacreous interiors, or with a structure made up of layers of different colours, are the most commonly used. 

Viewed with a lOx lens, non-nacreous shell displays a structure of faint striations. The orientation of the striations in each layer of colour is approximately perpendicular to the next layer. The exception to this rule is conch shell, which has striations in the same orientation (Figs 10.21 and 10.22). 

Fig. 4.10. Schematic representation of the structural relationships between protein sheets, aragonite crystals, and chitin fibres in the nacreous layer of N. repertus ...

Consider how this mechanism of interface failure could apply to a natural laminated material like mother of pearl. This nacreous substance is formed by the deposition of thin calcium carbonate crystals by the mollusc. Such plate-hke crystals are typically 10 pm long and 0.3 pm thick, separated by a tenuous layer of polymeric protein material which forms only a few percent of the composite volume. Yet, despite this low polymer content, nacre displays significant plastic deformation when stretched, with strains up to 0.018, accompanied by white deformation bands in the composite structure. " ... 

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