Biominerals: nano-scale characterization of calcium phosphate crystals forming the “calcification” of the human heart valves

Calcium phosphates, mainly ‘bioapatite’, make up the inorganic part of bone and teeth but also occur as pathological deposits. Among pathological biomineralizations, better known in the medical field with the generic term calcification, the deposition of calcium phosphate nanocrystals in the valve tissues of the human heart is a worldwide important topic associated with major morbidity, mortality and health economic costs. Nowadays the mechanisms leading to these pathological products are an open question despite all effort devoted to their comprehension. The nanometer scale structure of such pathological crystals was investigated using a dual beam Zeiss Auriga 405 HR-FESEM with resolution of 1 nm, and low accelerating voltage (< 15 kV) to obtain information about biomineral/organic structure interface. TEM analyses were performed on powdered samples by a Jeol JEM 2010 operating at 200KV with LaB6 source, nominal point resolution of 1.9 Å, and spherical aberration of 0.5 mm. Ultrastructural investigations brought to light the lowest units constituting the pathological deposits of the human valve tissues. These are represented by needle- and rod-like nanocrystals having the typical structure of hexagonal hydroxylapatite, but different chemical composition as [CO3]2- is present both for [PO4]3- and (OH)-. Characteristic aggregation properties of ‘bioapatite’ result in the formation of micrometer sized spherical particles. These latter appear to be also associated to mixed aggregates formed by nanocrystals and organic matrix, and to individual organic structures but without any relation to nanobacteria. All nanocrystals show the typical features of hydroxylapatite crystals precipitated in aqueous solutions, and a wide range of crystallite size, from a few to several hundreds of nanometers, that appears to be associated to local growth conditions and to different mineralization sites. The presence of localized compartments within the organic tissue, similar to “vugs” in rocks, in which a locally and progressive increase of ions concentration can take place, seems to be the pivotal requirement for the bioapatite precipitation. This confirms the important role of purely physicochemical processes in the biomineralization process of the valve tissues of the human heart and allows to ascribe to the organic matrix only the function of spatial template. However the presence of nanocrystals directly formed onto the organic substrate and oriented respect to this latter, also suggests a surface-induced mineralization process and a possible involvement of matrix components inducing ‘bioapatite’ nucleation.