Structural subunit in prisms of immature rat enamel

Abstract

THE prisms of rat enamel, like those of other mammals, are composed of lath-like crystals with maximum widths of about 30 nm, arranged longitudinally in an organic matrix. The matrix is considered either to contain groups of fibrils or septae which orientate the crystallites, or to consist of an amorphous gel. The first view is based on the fact that pairs of electron-dense linear structures have been seen in fixed demineralised sections of immature enamel. These structures were about 5 nm wide and 15 nm apart, and parallel with the long axis of the prism. Furthermore, irregularly-arranged cross bridges have been described running between them^1,2,3. Physical techniques of structure determination, however, have failed to show evidence of periodicity in demineralised immature enamel⁴ and this has supported an alternative view that the matrix is an amorphous thixotropic gel⁵. This latter concept is consistent with the change in matrix composition when enamel matures⁶ and a simultaneous reduction in volume⁷. It does not, however, offer a satisfactory explanation of the orientation of the crystallites.