The mineralization process of insoluble elastin fibrillar structures: Ionic environment vs degradation

Despite its long half-life and physiological role, elastin undergoes irreversible changes (i.e elastolysis and/or calcification) impairing resilience of soft connective tissues. At present, it is still undefined: 1) to which extent elastin fibers have to be fragmented in order to increase their susceptibility to calcify; 2) which is the contribution of ionic environment on elastin mineralization; 3) why, in the same tissue area, mineralized coexist with non-mineralized fibers. The in vitro mineralization process was investigated on insoluble elastin, hydrolyzed or not-hydrolyzed, and incubated in different cell-free ionic environments. Mineral deposition is favored on hydrolyzed fibrillar structures due to exposure of multiple charged sites increasing the adsorption of Ca²⁺ that can attract phosphate and increase the local ion concentration over the point of supersaturation, representing the minimum requirement for hydroxyapatite nucleation sites. At physiological pH, the degree of elastin mineralization is influenced by hydrolysis and complexity of medium composition, since ionic species, as sodium, potassium, magnesium, in addition to calcium and phosphorus, interfere with the calcification process. These findings broaden the knowledge on the factors controlling hydroxyapatite deposition on insoluble elastin and can also explain why, in vivo, calcified and non-calcified fibers can be observed within the same tissue.