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.