Macromolecular structure of the organic framework of nacre in Haliotis rufescens: implications for growth and mechanical behavior

We have performed a macromolecular structural analysis of the interlamellar and intertabular parts of the organic framework of the nacreous part of the shell of Haliotis rufescens, including the identification of structural chitin. Using histochemical optical microscopy we have mapped the locations of carboxylates and sulfates of proteins and chitin on the surfaces and within the core of the interlamellar layers and the intertabular matrix that together form the external organic matrix of composite nacre. This extends the earlier work of Nudelmann et al. [Nudelman, F., Gotliv, B.A., Addadi, L. and Weiner, S. 2006. Mollusk shell formation: mapping the distribution of organic matrix components underlying a single aragonite tablet in nacre. J. Struct. Biol. 153, 176–187] and Crenshaw and Ristedt [Crenshaw, M.A., Ristedt, H. 1976. The histochemical localization of reactive groups in septal nacre from Nautilus pompilius. In: Omori, M., Watabe, N. (Eds.) The Mechanisms of Biomineralization in Animals and Plants. Tokai University Press, Toyko] on Nautilus pompilius. Our mapping identifies distinct regions, defined by the macromolecular groups, including what is proposed to be the sites of CaCO₃ nucleation and that play a key role in nacre growth. Using AFM scanning probe microscopy we have identified a fibrous core within the framework that we associate with chitin. The structural picture that is evolved is then used to develop a simple structural model for the organic framework which is shown to be consistent with mechanical property measurements. The role of the intracrystalline matrix within the nacre tablets in mediating nacre’s mechanical response is noted within the framework of our model.