On the Possible Relation between Morphology and Precursors of the Crystallities in Calcified Tissues

Abstract

Crystal structure data on hydroxyapatite, octocalcium phosphate and brushite have been used in order to predict their crystal morphology on the basis of the Hartman-Perdok theory. The predicted forms are pencil-like for hydroxyapatite, board-like for octocalcium phosphate and flattened needle-like for brushite. Although the biominerals in bone, dentine and tooth enamel have an apatite structure, their form is nbt pencil-like. This may partially be due to the fact that precursor phases are nucleated first in these tissues during mineralization and that these precursor phases are transformed later by topotactical reactions into compounds with apatite structure or that they serve as nuclei for ongrowth of apatite. The form of the mineral particles in mature bone and dentin is board-like which indicates that octocalcium phosphate might be their precursor phase. However, the form of the crystals in mature enamel is flattened needle-like which indicates that brushite is their precursor phase. It is argued that a possible difference in the nature of the precursor phase may be due primarily to differences in the cellular activities of the odontoblasts and osteoblasts as compared to those of the ameloblasts, and secondarily to matrix effects. In both cases, however, the main effect of the matrix seems to be that it acts as a mechanical barrier leading to a limited form and size and, of course, a certain orientation of the crystals, rather than as an agent for heterogeneous nucleation. The validity of the present considerations depends as yet on the assumption that the ions and molecules occurring in body fluids do not dominate the habit of calcium phosphate crystals.