Among all 3D structures, saddle-shaped geometries and minimal surfaces have intrigued scientists for centuries, because they can spontaneously self-assemble driven by the minimization of thermodynamic quantities such as the interfacial energy. Echinoderms, like sea urchins and sea stars, build a calcite skeleton whose microstructure is an outstanding biogenic example of saddle-shape geometry. This 3D structure, called stereom, is a porous meshwork made of calcite, whose peculiar curvature signature is conserved across different species and body parts. Several studies have addressed the morphogenesis of the stereom showing that it forms via biomineralization through a rich dynamics of branching and bridging episodes. Yet, a global, mechanistic comprehension of what controls preferential mineral deposition is still lacking. Recently, we have found that the stereom may inherit its specific geometry from a precursor made of actin-fibers that self-organize under tension close to the growing boundary. To confirm this hypothesis, the student will characterize how the stereom evolution is affected in the presence of drugs targeting the contractility and the polymerization of actin fibers. Next, we will also develop a new protocol to observe and characterize the cytoskeleton dynamics in vivo. The internship will be the opportunity to learn experimental techniques like immunochemistry and confocal microscopy, and to develop a numerical workflow for image analysis.