LENGTHWISE AND ORIENTED GROWTH OF OCTACALCIUM PHOSPHATE ON CATION SELECTIVE-MEMBRANE IN A MODEL SYSTEM OF ENAMEL FORMATION

The mechanism of the lengthwise and oriented growth of enamel apatite was studied using a model system on the basis of the hypothesis that (1) ribbon-like enamel crystals grow initially as octacalcium phosphate (OCP) and (2) the ionic diffusion through the layer of ameloblasts promotes the lengthwise and oriented growth of OCP crystals. A synthetic cation selective membrane was used to control the ionic diffusion in this model system. Reactions were carried out in 3.6mM-14.4mM PO4 solutions at 37-degrees-C and at various pH levels ranging from 5.8 to 7.4 with and without CO3(2-), Mg2+, and F- ions being present. Under these conditions, oriented growth of OCP was observed. The growth of OCP along the c-axis increased with a decrease in pH and/or PO4 concentration, resulting in a ribbon-like morphology at pH ranging from 6.0 to 6.8 CO3(2-), Mg2+, and F- ions disturbed the growth of OCP along the c-axis, particularly when they coexisted. The inhibitory activity of these ions decreased with a decrease in pH. Under the coexistence of these ions, the lengthwise growth of OCP in the c-axis direction took place at pH 6.0. These results indicate that the one-directional supply of Ca2+ ions through the membrane is important in enhancing the lengthwise and oriented growth, and increase the possibility that OCP grows in a milieu containing inorganic growth inhibitors.