Quantification of sheet nacre morphogenesis using X-ray nanotomography and deep learning.

High-resolution three-dimensional imaging is key to our understanding of biological tissue formation and function. Recent developments in synchrotron-based X-Ray tomography techniques provide unprecedented morphological information on relatively large sample volumes with a spatial resolution better than 50 nm. However, the analysis of the generated data, in particular image segmentation - separation into structure and background - still presents a significant challenge, especially when considering complex biomineralized structures that exhibit hierarchical arrangement of their constituents across many length scales - from millimeters down to nanometers. In the present work, synchrotron-based holographic nano-tomography data are combined with state-of-the-art machine learning methods to image and analyze the nacreous architecture in the bivalve Unio pictorum in 3D. Using kinetic and thermodynamic considerations known from physics of materials, the obtained spatial information is then used to provide a quantitative description of the structural and topological evolution of nacre during shell formation. Ultimately, this study establishes a workflow for high-resolution three-dimensional analysis of fine highly-mineralized biological tissues while providing a detailed analytical view on nacre morphogenesis.

Morphological and textural evolution of the prismatic ultrastructure in mollusc shells: A comparative study of Pinnidae species.

Molluscan shells, exhibiting a variety of complex three-dimensional architectures, are an exemplar model system to study biogenic mineral formation by living organisms. Recent studies have demonstrated that the deposition process of some shell ultrastructures can be described using classical analytical models borrowed from materials physics, which were developed to predict the structural evolution of man-made and geological polycrystalline composite assemblies. In the current study, we use this newly developed capacity to quantitatively describe the morphogenesis of the prismatic ultrastructure in three shells from the bivalve family Pinnidae towards establishing a correlation between structure, texture, growth kinetics, topology and phylogeny of the species. Using data collected by electron microscopy, synchrotron-based microtomography, electron backscatter diffraction analysis (EBSD) and X-ray diffraction we demonstrate that the prismatic ultrastructures in Pinnidae are formed following either ideal or triple-junction-controlled kinetics, which are shown to be closely linked to the morphological and topological characteristics, as well as crystallographic texture of these biocomposites. The experimental and analytical framework presented in this comparative study can serve as an additional tool for classifying molluscan shell ultrastructures on the levels of structural and textural morphogenesis. STATEMENT OF SIGNIFICANCE: The ability to quantitatively describe the structural evolution of the prismatic architecture in mollusc shells is used for the first time to derive and compare between analytical parameters that define the growth kinetics and morphological and topological evolution during the growth of three shells from the family Pinnidae from two different genera. Furthermore, these parameters are linked to the evolution of crystallographic texture in the studied architectures. The developed experimental and analytical framework not only enables us to quantitatively describe species-specific growth mechanisms but also suggests a direct correlation between the evolution of morphology and texture.