Effects of ionic inflow and organic matrix on crystal growth of octacalcium phosphate; relevant to tooth enamel formation

Abstract

The mechanism of the lengthwise and oriented growth of enamel apatite crystals was studied on the basis of a hypothesis that (1) enamel crystals grow initially as octacalcium phosphate (OCP) as a precursor of apatite and (2) a one-directional Ca 2+ ion supply from the ameloblasts promotes the lengthwise and oriented growth of the crystals. To test this, OCP crystals were grown in a model system of enamel formation, where a cation selective membrane was used to control the Ca 2+ ion diffusion, at 37°C and at pH 6.3–7.4. The lengthwise and oriented growth of OCP was enhanced by the Ca influx from the membrane. H + ion enhanced while F −, Mg 2+ and CO 3 2− ions decreased the growth in the c-axis direction. F − induced an epitaxial overgrowth of apatite on the (1 0 0) of OCP, embedding an OCP lamella in the center of a crystal. When 5% polyacrylamide gel was attached to the membrane, OCP grew in long plate-like or ribbon-like crystals at pHs between 6.5 and 7.4. The length of these crystals was greater than that of those grown on the membrane without gel. The crystal size decreased with an increase in gel concentration from 5 to 20%. When a thin slice of tendon was used in place of the membrane, long OCP crystals grew along the collagen fibrils; these crystals were oriented with their axis of elongation perpendicular to the direction of ionic inflow. The size of crystals grown inside the collagenous matrix was much smaller than the size of crystals formed outside of the matrix and on the membrane. Thus, crystal growth was regulated by ionic diffusion and the matrix. When there were no constituents which regulated the growth direction, OCP crystals tended to grow along the direction of ionic inflow.