The structure and development of mammalian enamel.
Abstract
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.
Authors
Boyde, AlanCollections
- Theses [3831]