Chemoviscoelasticity of the interfibrillar matrix of the dermis of the black sea cucumber Holuthuria atria

Mutable connective tissues of the sea cucumbers' dermis can assume three different mechanical states (soft, standard and stiff) according to the chemical changes in the water. There is broad consensus that variable cross-linking of the extracellular matrix is responsible for such changes. This paper uses Small-angle X-ray Scattering (SAXS) measurements, a micromechanical viscoelastic model, and a molecular extended reptation theory to look for other causes beyond cross-linking. We conclude that in potassium-ions enriched seawater, the interfibrillar matrix stiffens due to increased cross-linking, but this must also imply macromolecular chain scission change in molecular weight and increased friction between the chains. In softening water solution (calcium-ions deprived seawater), the interfibrillar matrix softens because of decreased cross-linking, and simultaneously macromolecules chain recombine and friction between the chains decreases. These findings allow us to conclude that the zero-shear viscosity increases more than five times during stiffening and reduces to 3% of its standard value during softening. Also, we find that the fibril strains measured through SAXS seem to suggest that, in reference conditions, the interfibrillar matrix (artificial sea water) behaves similarly to a covalently cross-linked gel; instead, during softening and stiffening, it appears that the matrix shows stress relaxation akin to an ionic cross-linked gel.

Automated summary

The connective tissues of sea cucumbers can change their mechanical states based on the chemical changes in the water. This study explores the causes beyond cross-linking by using Small-angle X-ray Scattering measurements and molecular extended reptation theory. It is found that increased cross-linking leads to stiffening in potassium-ions enriched seawater, while decreased cross-linking results in softening in calcium-ions deprived seawater.