Echinoderms: Hierarchically Organized Light Weight Skeletons

Evaluating the single crystallinity of sea urchin calcite

Recent advancements in electron backscatter diffraction (EBSD) data evaluation enable the determination of misorientation between crystals below 0.1°, while with conventional EBSD data evaluation, the smallest misorientation precision between crystals scatters between 0.5°-1°. Sea urchin tests and spines are lightweight biomaterials with a serrated microstructure comprising interlinked calcite crystals. We investigated the microstructure and crystallographic texture of Cidaris cidaris and Paracentrotus lividus test and spine calcite with advanced EBSD measurement and data evaluation. In particular, we re-evaluated the widely accepted single-crystallinity of sea urchin calcite. We found that the test and the spines comprise calcite crystals with different fabrics and a significant variation in crystal co-orientation strength. Even the highly co-oriented calcite of C. cidaris and P. lividus is not perfectly single-crystalline. We found test and spine portions that feature significant internal misorientations (1-3°). Test c-axis orientation in C. cidaris is tangential to the outer test surface, while in the spines, it is parallel to the morphological axis of the spine. Primary and secondary spines feature a bimodal crystal texture comprising co-oriented calcite surrounded by a cortex of misoriented crystals. Crystal misorientation in the spine cortex seems to result mainly from competitive growth determinants. Deciphering the degree of crystallinity and mode of crystal organisation of biological hard tissues is vital for understanding their exceptional control of structure, material architecture and material properties. STATEMENT OF SIGNIFICANCE: Echinoids form lightweight biomineralised skeletal elements with outstanding material properties and a complex microstructure formed of interlinked calcite crystals. The degree of crystallinity and the crystallographic organisation of the calcitic tests and spines are still under debate. We investigate and discuss the crystallinity, microstructure, and texture of Cidaris cidaris and Paracentrotus lividus test and spine crystals. Unprecedented and not yet used for biomineralised carbonate tissues, we apply electron backscatter diffraction pattern matching data evaluation, enabling detection of misorientation precision below 0.1°, relative to 0.5°-1° misorientation precision obtained from conventional EBSD data evaluation. We demonstrate that sea urchin test plates and spines are not single crystals. They feature internal small-angle misorientations and poorly co-oriented, polycrystalline regions with intricate microstructures.

Bio-Informed Porous Mineral-Based Composites

Certain biominerals, such as sea sponges and echinoderm skeletons, display a fascinating combination of mechanical properties and adaptability due to the well-defined structures spanning various length scales. These materials often possess high density normalized mechanical properties because they contain well-defined pores. The density-normalized mechanical properties of synthetic minerals are often inferior because the pores are stochastically distributed, resulting in an inhomogeneous stress distribution. The mechanical properties of synthetic materials are limited by the degree of structural and compositional control currently available fabrication methods offer. In the first part of this review, examples of structural elements nature uses to impart exceptional density normalized Young's moduli to its porous biominerals are showcased. The second part highlights recent advancements in the fabrication of bio-informed mineral-based composites possessing pores with diameters that span a wide range of length scales. The influence of the processing of mineral-based composites on their structures and mechanical properties is summarized. Thereby, it is aimed at encouraging further research directed to the sustainable, energy-efficient fabrication of synthetic lightweight yet stiff mineral-based composites.

Structural and molecular distinctions of primary and secondary spines in the sea urchin Lytechinus variegatus

Sea urchins (echinoids) are common model organisms for research in developmental biology and for their unusual transition from a bilaterally organized larva into a post-metamorphic adult with pentaradial body symmetry. The adult also has a calcareous endoskeleton with a multimetameric pattern of continuously added elements, among them the namesake of this phylum, spines. Nearly all echinoids have both large primary spines, and an associated set of smaller secondary spines. We hypothesize that the secondary spines of the tropical variegated urchin species, Lytechinus variegatus, are morphologically and molecularly distinct structures from primary spines and not just small versions of the large spines. To test this premise, we examined both spine types using light microscopy, micro-CT imaging, lectin labeling, transcriptomics, and fluorescence in situ hybridization (FISH). Our findings reveal basic similarities between the two spine types in mineral and cellular anatomy, but with clear differences in growth patterns, genes expressed, and in the profile of various expressed genes. In particular, secondary spines have non-overlapping, longitudinally concentrated growth bands that lead to a blunt and straight profile, and a distinct transcriptome involving the upregulation in many genes in comparison to the primary spines. Neural, ciliary, and extracellular matrix interacting factors are implicated in the differentially expressed gene (DEG) dataset, including two genes-ONECUT2 and an uncharacterized discoidin- and thrombospondin-containing protein. We show spine type-specific localizations by FISH, which will be of interest to ongoing work in urchin spine patterning. These results demonstrate that primary and secondary spines of L. variegatus have overlapping but distinct molecular and biomineralization characteristics, suggesting unique developmental, regenerative, and representation in this spiny dermal phylum.

Echinoid skeleton: an insight on the species-specific pattern of the Paracentrotus lividus plate and its microstructural variability

The skeletal plates of echinoids consist of a peculiar lightweight structure, called stereom, which is organized in a porous three-dimensional lattice-like meshwork. The stereom is characterized by an extremely complex and diverse microarchitecture, largely varying not only from species to species but also among different test plates. It consists of different basic types combined in extremely different ways according to specific functional needs, creating species-specific structural patterns. These patterns can lead to specific mechanical behaviours, which can inspire biomimetic technology and design development. In this framework, the present study aimed to characterize the species-specific pattern of the Paracentrotus lividus interambulacral plate and the main microstructural features regarding its geometrical variability and mechanical responses. The results achieved quantitatively highlighted the differences between the analysed stereom types providing new insights regarding their topological configuration and isotropic and anisotropic behaviour. Interestingly, data also revealed that the galleried stereom present at the tubercle is significantly different from the one located at the suture. These analyses and findings are encouraging and provide a starting point for future research to unravel the wide range of mechanical strategies evolved in the echinoid skeletal structure.

Hexagonal Voronoi pattern detected in the microstructural design of the echinoid skeleton

Repeated polygonal patterns are pervasive in natural forms and structures. These patterns provide inherent structural stability while optimizing strength-per-weight and minimizing construction costs. In echinoids (sea urchins), a visible regularity can be found in the endoskeleton, consisting of a lightweight and resistant micro-trabecular meshwork (stereom). This foam-like structure follows an intrinsic geometrical pattern that has never been investigated. This study aims to analyse and describe it by focusing on the boss of tubercles-spine attachment sites subject to strong mechanical stresses-in the common sea urchin Paracentrotus lividus. The boss microstructure was identified as a Voronoi construction characterized by 82% concordance to the computed Voronoi models, a prevalence of hexagonal polygons, and a regularly organized seed distribution. This pattern is interpreted as an evolutionary solution for the construction of the echinoid skeleton using a lightweight microstructural design that optimizes the trabecular arrangement, maximizes the structural strength and minimizes the metabolic costs of secreting calcitic stereom. Hence, this identification is particularly valuable to improve the understanding of the mechanical function of the stereom as well as to effectively model and reconstruct similar structures in view of future applications in biomimetic technologies and designs.

Flexible sutures reduce bending moments in shells: from the echinoid test to tessellated shell structures

In the field of structural engineering, lightweight and resistant shell structures can be designed by efficiently integrating and optimizing form, structure and function to achieve the capability to sustain a variety of loading conditions with a reduced use of resources. Interestingly, a limitless variety of high-performance shell structures can be found in nature. Their study can lead to the acquisition of new functional solutions that can be employed to design innovative bioinspired constructions. In this framework, the present study aimed to illustrate the main results obtained in the mechanical analysis of the echinoid test in the common sea urchin Paracentrotus lividus (Lamarck, 1816) and to employ its principles to design lightweight shell structures. For this purpose, visual survey, photogrammetry, three-dimensional modelling, three-point bending tests and finite-element modelling were used to interpret the mechanical behaviour of the tessellated structure that characterize the echinoid test. The results achieved demonstrated that this structural topology, consisting of rigid plates joined by flexible sutures, allows for a significant reduction of bending moments. This strategy was generalized and applied to design both free-form and form-found shell structures for architecture exhibiting improved structural efficiency.

Exploring the macrostructural anatomy of dendrochirotid sea cucumber's (Echinodermata) calcareous rings under micro-computed tomography and its bearing on phylogeny

Despite descending from heavily calcified ancestors, the holothuroid skeleton is fully internal and composed of microscopic ossicles and a ring of plates bound by connective tissue, the calcareous ring. The calcareous ring exhibits a complex and poorly understood morphology; as a result, establishing unambiguous homology statements about its macrostructure has been challenging and phylogenetic studies have had to simplify this important structure. Here, we provide the first broad comparative study of Dendrochirotida calcareous rings using micro-computed tomography (μCT). A detailed description of the three-dimensional macrostructure of the calcareous ring of 10 sea cucumber species, including rare and type specimens, is presented. The structures observed were highly variable at the subfamily level, especially at the point of tissue attachment. The relationship between the calcareous ring and its associated organs, and their functional morphology are discussed. To aid future phylogenetic studies, we listed 22 characters and performed a preliminary cladistic analysis. The topology obtained supports the idea that the simple, cucumariid ring is ancestral to the mosaic-like phyllophorid ring; however, it did not support the monophyly of the cucumariids. It also did not support the family Sclerodactylidae, which was described based on the ring morphology. Differently from the dermal ossicles, which are highly homoplastic, the general homoplasy index of the calcareous ring characters was relatively low. This result highlights the importance of this structure for phylogenetic inference. Unfortunately, time since collection, rough collection methods and fixation can damage the skeleton, and the calcareous ring is often overlooked in taxonomic descriptions. The data presented here will improve our understanding of holothuroid relationships and facilitate studies on holothuroid functional morphology and biomechanics.

Optimization of skeletal protein preparation for LC-MS/MS sequencing yields additional coral skeletal proteins in Stylophora pistillata

Stony corals generate their calcium carbonate exoskeleton in a highly controlled biomineralization process mediated by a variety of macromolecules including proteins. Fully identifying and classifying these proteins is crucial to understanding their role in exoskeleton formation, yet no optimal method to purify and characterize the full suite of extracted coral skeletal proteins has been established and hence their complete composition remains obscure. Here, we tested four skeletal protein purification protocols using acetone precipitation and ultrafiltration dialysis filters to present a comprehensive scleractinian coral skeletal proteome. We identified a total of 60 proteins in the coral skeleton, 44 of which were not present in previously published stony coral skeletal proteomes. Extracted protein purification protocols carried out in this study revealed that no one method captures all proteins and each protocol revealed a unique set of method-exclusive proteins. To better understand the general mechanism of skeletal protein transportation, we further examined the proteins’ gene ontology, transmembrane domains, and signal peptides. We found that transmembrane domain proteins and signal peptide secretion pathways, by themselves, could not explain the transportation of proteins to the skeleton. We therefore propose that some proteins are transported to the skeleton via non-traditional secretion pathways.

Quantitative 3D structural analysis of the cellular microstructure of sea urchin spines (II): Large-volume structural analysis

Biological cellular materials have been a valuable source of inspiration for the design of lightweight engineering structures. In this process, a quantitative understanding of the biological cellular materials from the individual branch and node level to the global network level in 3D is required. Here we adopt a multiscale cellular network analysis workflow demonstrated in the first paper of this work series to analyze the biomineralized porous structure of sea urchin spines from the species Heterocentrotus mamillatus over a large volume (ca. 0.32mm³). A comprehensive set of structural descriptors is utilized to quantitatively delineate the long-range microstructural variation from the spine center to the edge region. Our analysis shows that the branches gradually elongate (~50% increase) and thicken (~100% increase) from the spine center to edge, which dictates the spatial variation of relative density (from ~12% to ~40%). The branch morphology and network organization patterns also vary gradually with their positions and orientations. Additionally, the analysis of the cellular network of individual septa provides the interconnection characteristics between adjacent septa, which are the primary structural motifs used for the construction of the cellular structure in the edge region. Lastly, combining the extracted long-range cellular network and finite element simulations allows us to efficiently examine the spatial and orientational dependence of local effective Young's modulus across the spine's radius. The structural-mechanical analysis here sheds light on the structural designs of H. mamillatus' porous spines, which could provide important insights for the design and modeling of lightweight yet strong and damage-tolerant cellular materials. STATEMENT OF SIGNIFICANCE: Previous investigations on the cellular structures of sea urchin spines have been mainly based on 2D measurements or 3D quantification of small volumes with limited structural parameters. This limits our understanding of the interplay between the 3D microstructural variations and the mechanical properties in sea urchin spines, which hence constrains the derivation of the underlying principles for bio-inspired designs. This work utilizes our multiscale 3D network analysis, for the first time, to quantify the 3D cellular network and its variation across large volumes in sea urchin spines from individual branch and node level to the cellular network level. The network analysis demonstrated here is expected to be of great interest to the fields of biomineralization, functional biological materials, and bio-inspired material design.

Structural stress response of segmented natural shells: a numerical case study on the clypeasteroid echinoid Echinocyamus pusillus

The skeleton of Echinocyamus pusillus is considered as an exceptional model organism for structural strength and skeletal integrity within the echinoids as demonstrated by the absence of supportive collagenous fibres between single plates and the high preservation potential of their skeletons. The structural principles behind this remarkably stable, multi-plated, light-weight construction remain hardly explored. In this study, high-resolution X-ray micro-computed tomography, finite-element analysis and physical crushing tests are used to examine the structural mechanisms of this echinoid's skeleton. The virtual model of E. pusillus shows that the material is heterogeneously distributed with high material accumulations in the internal buttress system and at the plate boundaries. Finite-element analysis indicates that the heterogeneous material distribution has no effect on the skeleton's strength. This numerical approach also demonstrates that the internal buttress system is of high significance for the overall skeletal stability of this flattened echinoid. Results of the finite-element analyses with respect to the buttress importance were evaluated by physical crushing tests. These uniaxial compression experiments support the results of the simulation analysis. Additionally, the crushing tests demonstrate that organic tissues do not significantly contribute to the skeletal stability. The strength of the echinoid shell, hence, predominantly relies on the structural design.

Structural design of the minute clypeasteroid echinoid Echinocyamus pusillus

The clypeasteroid echinoid skeleton is a multi-plated, light-weight shell construction produced by biomineralization processes. In shell constructions, joints between individual elements are considered as weak points, yet these echinoid skeletons show an extensive preservation potential in both Recent and fossil environments. The remarkable strength of the test is achieved by skeletal reinforcement structures and their constructional layouts. Micro-computed tomography and scanning electron microscopy are used for microstructural and volumetric analyses of the echinoid's skeleton. It is shown that strengthening mechanisms act on different hierarchical levels from the overall shape of the skeleton to skeletal interlocking. The tight-fitting and interlocking plate joints lead to a shell considered to behave as a monolithic structure. The plate's architecture features distinct regions interpreted as a significant load-transferring system. The internal support system follows the segmentation of the remaining skeleton, where sutural layout and stereom distribution are designed for effective load transfer. The structural analysis of the multi-plated, yet monolithic skeleton of Echinocyamus pusillus reveals new aspects of the micro-morphology and its structural relevance for the load-bearing behaviour. The analysed structural principles allow E. pusillus to be considered as a role model for the development of multi-element, light-weight shell constructions.