The structure and development of mammalian enamel
Enamel development and structure have been studied in a number of placental and marsupial mammals, by light microscopy; electron-microscopy; and scanning electron microscopy. The relationship between the formative cells of the enamel and its structural organisation into "prisms" and interprismatic regions has been studied in particular. The crystallites in developing enamel tend to be oriented perpendicular to its mineralising front; but their orientation may be modified by either the translatory movement which may occur between certain surfaces of the TOMES' processes of the ameloblasts and the mineralising front, or the self directed growth of groups of groups of crystallites. The presence of a repetitive (prism) pattern of crystallite orientation in formed enamel is determined by changes of orientation of and within the mineralising front: these changes are 1) the result of the peculiar mode of secretion of the enamel precursor substances from and about projections from the ameloblasts; and 2) absent during the formation of the first and last layers of enamel (formed at the enamel-dentine junction and the true enamel surface respectively) by a given group of ameloblasts: hence there are no prisms in these regions. Abrupt changes in orientation of the mineralising front determine abrupt changes in crystallite orientation in the enamel (equivalent to the "prism-sheaths" of adult enamel). The secretory territories of individual ameloblasts are only equivalent to prisms in one particular pattern: one ameloblast may be related to more than one prism. Decussation of prisms is associated with the depressions in the mineralising front filling in from alternate sides in "zones". Zone formation begins as a spiral over cusp centres. Light scattering from enamel depends on 1) the size; and 2) the orientation of its ultrastructural elements and 3) the wavelength of the incident radiation; blue light being scattered preferentially; hence the visibility of: - 1) the incremental striae; and 2) the decussating zones of prisms; and 3) the brown colour of the incremental striae when viewed by transmitted light. The calcium content in developing enamel measured by the x-ray emission microanalytical method was found to increase steadily, from the surface of the developing enamel inwards.