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R Soc Open Sci
2025 Jun 25;126:241439. doi: 10.1098/rsos.241439.
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The microarchitectural variability in the echinoid skeleton: a 3D geometrical and stiffness characterization of Paracentrotus lividus.
Perricone V
,
Cesarano P
,
Deb M
,
Lublin D
,
Mutalipassi M
,
Pappalardo L
,
Kisailus D
,
Marmo F
.
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The sea urchin skeleton is a lightweight yet load-bearing hierarchical structure composed of calcitic plates with a species-specific three-dimensional (3D) trabecular meshwork known as stereom. Interestingly, the stereom architecture is extremely complex and variable in different basic types, each one characterized by a unique geometry and structural behaviours. The present study provides an in-depth analysis of the microarchitectural variability in the sea urchin Paracentrotus lividus. Accordingly, micro-CT scans, image analysis, 3D modelling, mean intercept length and linear elastic finite-element analysis were conducted to provide the first comprehensive insights on the structural variability of the different stereom types, their anisotropy and their mechanical behaviour calculated for tensile and shear loading. The findings demonstrate distinct structural adaptations, with anisotropic stereoms specializing in directional stress transfer and more isotropic stereoms facilitating a uniform stress distribution. These results provide critical insights into the mechanical functions of stereom variability and support bioinspired designs for lightweight and strong materials.
Figure 1. . The hierarchical design of the skeleton of Paracentrotus lividus. At the macroscale, the skeleton exhibits a pentameric symmetry with a tessellated test, composed of alternating ambulacral (az) and interambulacral (iz) zones arranged in double columns. At the mesoscale, individual skeletal plates feature rounded tubercles, which serve as attachment sites for spines via soft tissues, including muscles and the catch apparatus. The spine–tubercle system consists of the mamelon, platform, boss and areola. At the microscale, the structure is composed of stereom, a highly porous biomineralized network. At the nanoscale, the skeletal material is primarily Mg-calcite. The bottom panel presents a longitudinal section from a micro-CT scan, illustrating stereom variability: galleried stereom at the tubercle boss and suture areas, labyrinthic stereom in the central region of the plate and perforated stereom at the basal zone.
Figure 2. . Stereom selection and trabecular analysis. (A) Micro-CT scan of the skeletal plate, showing the selected stereom sub-volume along with its corresponding x, y, z orientation. (B) 3D reconstructions of the selected stereom regions. (C) Trabecular analysis, highlighting the identification of pore spaces and the trabecular network, characterized by nodes and segments.
Figure 3. . Boundary conditions for tensile and shear loading in FEM.
Figure 4. . Trabecular analysis. Box plots of node–segment configuration, descriptors of the trabecular system, tortuosity, connectivity and porosity comparing the investigated stereom types.
Figure 5. . Averaged eigenvalues of fabric tensor eigenvalues for each stereom type.
Figure 6. . Principal longitudinal stiffness components (top left), principal longitudinal stiffness coupling components (top right) and principal shear stiffness components (bottom) for each stereom type.
Figure 7. . Averaged values of engineering material parameters for each stereom type: Young's moduli (left) and Poisson's ratios (right).
Figure 8. . Micro-CT scans of the analysed stereom types (RVE1) (left column) and related 3D directional representation of fabric tensor (centre column) and longitudinal components of the stiffness tensor (right column).
Figure 9. . Micro-CT scan of P. lividus plate from different plane perspectives: (A) Coronal plane, highlighting the perforated stereom 6 with detailed views of its two sub-layers (p1 and p2), as well as the suture area featuring skeletal protrusions (finger joints). (B) transversal plane and (C) sagittal plane, showing the galleries departing from the centre of the primary tubercle with growth bands (indicated by blue arrows). (D) Angled plane relative to the coronal plane, revealing the galleried structure of stereom 5 and a representative RVE, with a layer cut at 63°, exposing the angled galleries.
Figure 10. . Stereom organization in the species-specific pattern of P. lividus and its comparison with previous literature [14]. The main stereom types analysed are illustrated, emphasizing key differences respect to Smith's model, including the distinction between the galleried stereoms 2 and 3, as well as the identification of stereom 5 as an angled galleried stereom, previously identified as labyrinthic.
