Scientists have followed the growth process of a giant Mediterranean fan mussel by creating a 3D reconstruction of part of its shell, using X-rays at the European Synchrotron, the ESRF. The study of the mollusk has revealed fundamental detail about how the shell forms, and confirms classical theories about how crystals within such a complex system are expected to grow. While verification of so-called ‘grain growth’ theories has been demonstrated in non-biological materials, this is the first time it has been proven within a living organism. The results, published in Nature Materials this week (19 October 2014), are a fundamental step towards understanding how the complex structures within biological organisms form.
Mollusk shells are regularly used to investigate the mineralisation process of calcium carbonate by a living organism. The shells have two mineralised layers beyond the outer surface, both with completely different properties, and therefore provide two different examples of calcium carbonate structures within the same organism. Using high-resolution synchrotron-based microtomography where cross sections of a sample are created using X-rays to form a 3D model, the scientists examined in this case, a shell from the fan mussel ‘Pinna nobilis’.
The team, led by Igor Zlotnikov from the Department of Biomaterials at the Max Planck Institute of Colloids and Interfaces, focused on the layer of the shell just beneath surface level, known as the prismatic layer which consists of elongated calcite prisms that are bound together to form a honeycomb-like microstructure. By taking slices of the layer but from different depths within it, they were able to track the growth process throughout the shell to gain a greater understanding of the forces at play as biomineralisation occurs.
“Think of it like the entire process of growth being frozen in the structure”, explains Igor Zlotnikov. “The mineralisation is occurring from the outside of the shell to the inside, leaving tracks along the way, such that the distance along the growth direction perpendicular to the shell’s surface is just representing time. By taking cross sections of the layer and reconstructing them into a 3D image, we can gain a full picture of the growth history”.
The team found that the processes at play in the Pinna nobilis exactly follow text book theories of grain growth in an engineering material when heated, for example. That is to say that using these theories, the properties of the microstructure of the prismatic layer in a living organism can be fully predicted.
As well as giving a greater understanding of the process of biomineralisation in mollusk shells, the result also contributes to a broader understanding of the development of biomineralised tissues. The study shows that the shape evolution of the prisms can be explained by thermodynamics, suggesting that it does not need to be controlled by the biological organism except for creating the appropriate boundary conditions.
Peter Fratzl, also named on the paper and director at the Max Planck Institute, said: “For me, this study has been very exciting as it shows the extent to which living organisms are able to take advantage of thermodynamics and kinetics of materials to help them growing biologically useful structures, such as a mollusk shell”.
The work was carried out using ESRF’s ID19 beamline which offers a combined absorption- and phase-contrast enhanced imaging mode. Igor Zlotnikov said: “The combination of the high quality resolution available on ID19 and the expertise of Alexander Rack at the ESRF made the facility the perfect choice for this work”.
The team now plan to apply the same techniques to other living organisms with different properties.
Self-similar mesostructure evolution of the growing mollusc shell reminiscent of thermodynamically driven grain growth.
B. Bayerlein, P. Zaslansky, Y. Dauphin, A. Rack, P. Fratzl, I. Zlotnikov
Nature Materials, 19 October 2014.