Morphogenesis of the diamond-type stereom microlattice and the origin of saddle-shaped minimal surfaces in the echinoderm skeleton

Echinoderm endoskeleton has a unique trabecular microstructure, called stereom, which can exhibit highly ordered geometries. A striking example of these geometries is the recently discovered "diamond-type" stereom, characterized by a diamond triply periodic minimal surface (D-TPMS) and distinguished by exceptional mechanical and structural properties. Despite its promise for engineering applications, the morphogenesis of this microarchitecture remains poorly understood. Here, we applied a multimodal imaging and labeling approach to investigate the developmental processes underlying the formation of the diamond-type stereom in adults of a starfish Protoreaster nodosus. We showed that this stereom type develops through two principal marginal growth patterns: trabecular trifurcation, which typically occurs horizontally on flat external plate surfaces, oriented approximately along the crystallographic {111} planes of the d-TPMS; and trabecular bifurcation, which generally occurs along plate edges and between trifurcating zones, aligned with the crystallographic {100} planes of the d-TPMS. Although these growth patterns may proceed at different rates, they are tightly coordinated, producing the coherent D-TPMS microarchitecture. Furthermore, we demonstrated that F-actin cytoskeletal deposition is consistently associated with active biomineralization fronts in both diamond-type and less ordered, galleried stereom. Notably, the formation of lateral bridges between adjacent stereom trabeculae is often preceded by catenoid-like F-actin structures, suggesting a guiding and templating role for the cytoskeleton in building trabecular connectivity and shaping its curvature. Given the recurrence of saddle-like features across stereom types, we hypothesize that minimal-surface geometries may emerge from tension-driven cytoskeletal dynamics acting as physical templates during biomineralization. Our observations underscore the critical involvement of the cytoskeleton in adult echinoderm biomineralization. STATEMENT OF SIGNIFICANCE: Triple periodic minimal surfaces (TPMS) are geometrically regular, three-dimensionally repeating surfaces that minimize area by maintaining zero mean curvature. Among them, the diamond-type (D-TPMS) is notable for exceptional mechanical stability, uniform porosity, and low surface area-to-volume ratio. Although TPMS-like microarchitectures occur in nature, D-TPMS structures, with large lattice parameters (>10 µm) are exceedingly rare; one natural example is the stereom of certain echinoderms. Yet the mechanism governing the formation of such ordered microstructures remains unresolved. We applied a multi-modal approach to investigate principles of diamond-type stereom morphogenesis. Our results provide key insight into the growth dynamics of this microarchitecture and highlight the essential role of the cytoskeleton, particularly F-actin, in guiding trabecular connectivity and the emergence of minimal-surface geometry, in both periodic and more irregular stereom types.