Calcification and silicification: a comparative survey of the early stages of biomineralization.

Most of the studies on biomineralization have focused on calcification and silicification, the two systems that predominate in nature in the construction of skeletal or integumental hard tissues. They have, however, been studied separately, as if they were completely distinct processes, in spite of their several points of contact, especially as far as the organic-inorganic relationships during the early mineralization stages are concerned. A very tight association of the inorganic substance with organic macromolecules, in fact, initially characterizes both systems. Although the mechanism of biomineralization remains elusive, a number of old and new findings, which have been taken into account in this review, support the view that, both in calcification and in silicification, genetically controlled organic macromolecules induce the formation of composite, organic-inorganic nanoparticles, behave as templates for the subsequent assemblage of the nanoparticles into micro- to macroarchitectures of complex pattern, and, eventually, are mostly reabsorbed. There are still many gaps left in our knowledge of this process. Comparative studies of the two biomineralization systems may help to fill them.