The effect of fluoride on the size and morphology of apatite crystals grown from physiologic solutions

In adult human bone, fluoride uptake is accompanied by an increase in apatite crystal size. This increase, however, is not isotropic but is restricted primarily to growth in width and/or thickness, with no measurable change in length. In the present study, seeded growth experiments were conducted in vitro to determine whether this anisotropic effect is physicochemical in origin, i.e., a direct result of F- selectively enhancing lateral crystal growth, or is an indirect consequence of F--induced alterations in cellular function and matrix development. The growth reactions were maintained at 37 degrees C under physiologic-like solution conditions (1.33 mmol/liter Ca2+, 1.0 mmol/liter total phosphate, 0 or 26 mmol/liter carbonate, 270 mmol/kg osmolality, pH 7.4) using constant-composition methods. When new accretions accumulated to three times the initial seed mass, the solids were collected and net crystal growth was assessed by X-ray diffraction line broadening analysis. The X-ray results revealed that the carbonate constituent in our physiologic-like solutions promoted the proliferation of new crystals at the expense of further growth of the seed apatite. Solution F- concentrations of approximately 2 micromol/liter partially offset the repressive effect that carbonate had on primary crystal growth. Moreover, F- stimulated seed crystal growth in the same anisotropic manner as had been observed for adult human bone apatite, a finding that suggests that the latter growth in vivo was the consequence, in part, of direct F--mineral interactions.