Some ultrastructural aspects of biological apatite dissolution and possible role of dislocations

High resolution electron microscope techniques now make it possible to study the mineralization of calcified tissues at the crystal structure level. Dislocations play an important part in the course of crystal maturation and modifications. At least two origins of dislocations are known. The first is due to phenomena related to the incorporation of fluoride ions into the lattice. The second is due to mechanical stresses occurring between crystals during their maturation. Dislocations are the starting-points of acid dissolution which proceeds along dislocation-lines, sometimes inducing a splitting of the crystals. In the present study, dislocations have been visualized, either isolated in the crystal core and perhaps the two-dimensional surface representation of a screw-shaped dislocation, or forming nets of dislocations producing a spiral staircase on the sides faces of the crystals. There is evidence of a maturation cycle of the crystals, which may grow, split, coalesce or dissolve; liberated elements and small crystal fragments may allow development of near-by crystals. This remodelling explains the existence of a standard size as well as the perfect fitting of the crystals. These morphological data involve physico-chemical properties of biological apatites.