Mollusc Shell Patterning
|Pixel Dimension||0.5 um X 5 um|
|Pattern Size||10 cm X 10 cm|
The mollusc shell is made up of both organic and inorganic components. The organic components are chitin, proteins with many aspartic acid residues, and silk fibroin protein(1). The inorganic component is calcium carbonate CaCO3. The calcium carbonate can crystalize into aragonite or calcite.
The mollusc mantle is a sheet of epithelial cells that secrete various proteins necessary for shell formation. The mantle epithelial cells thus control the shell structure (Figure 2A)(2). The secreted proteins organize and crystalize the shell in between the periostracum (secreted by the mantle) and the mantle in a space called the extrapallial space.
Organic matrices that are made of chitin, decorated with acidic proteins, and filled with silk fibroin gel, are first layed down. Nucleating proteins localized in the center of the matrix start CaCO3 crystallization. It is not known how the start of crystallization is positioned at the center of the organic matrix. Crystalization continues upwards (towards the mantle) through the chitin layers by mineral bridges (3). Thus, several grains grow against each other towards the mantle (Figure 2B,C).
One paper demonstrated that mollusc shell formation can be modeled entirely by thermodynamically driven grain growth(4). These results are consistent with the current hypothesis that mineral bridges continue crystallization of CaCO3 upwards through the chitin layers.
- T. Furuhashi, C. Schwarzinger, I. Miksik, M. Smrz, A. Beran, Molluscan shell evolution with review of shell calcification hypothesis. Comparative Biochemistry and Physiology, Part B. 154, 351–371 (2009).
- F. Heinemann, M. Launspach, K. Gries, M. Fritz, Gastropod nacre: Structure, properties and growth — Biological, chemical and physical basics. Biophysical Chemistry. 153, 126–153 (2011).
- F. Nudelman, Nacre biomineralisation: A review on the mechanisms of crystal nucleation. Seminars in Cell and Developmental Biology, 1–9 (2015).
- B. Bayerlein et al., Self-similar mesostructure evolution of the growing mollusc shell reminiscent of thermodynamically driven grain growth. Nat Mater. 13, 1102–1107 (2014).