NANOINDENTATION OF TOOTH TISSUES - Nanoindentation with Biological Applications - page 236

2. Formation and Structure of Teeth
For better understanding of its compositional and structural
characteristics and the relationship to mechanical behaviour, it is
essential to briefly review the formation of teeth, with an emphasis
mainly on enamel and dentin.
2.1. Formation and structure of enamel
The formation of enamel includes secretion and maturation stages.
During the initial secretion stage, the ameloblasts develop at their apical
(secretory) surface, a single fairly large cell process, the Tomes' process,
which is a unique feature of the secretory stage of the ameloblast. The
presence and the shape of Tomes' processes are significant for the
structure of the developing enamel.
When an ameloblast has completed its principal secretory activities
and the developing enamel has reached its full thickness locally, the
secretion stage comes to an end. The ameloblast next undergoes a series
of cytological changes in preparation for enamel maturation. During
enamel maturation, there is a sharp increase in matrix degradation and
replacement by tissue fluid. Finally, most of the matrix (specifically the
amelogenins) and the fluid have been replaced gradually and the mineral
volume fraction component increases steeply from 10-20% to 80-90%.
One of the most important steps in the enamel maturation stage is the
organization of HAP crystals into bulk enamel. This organization is
achieved by extracellular protein matrices, which are secreted from the
Tomes' processes of ameloblasts. Amelogenin undergoes self-assembly
to form nano-spheres in vitro and in vivo . 7 These nano-spheres are
located adjacent to the a - and b -surfaces of the HAP crystallites in vitro ,
possibly favoring crystallite elongation along the c -axis resulting in long
and thin crystals arranged in parallel. 7 As enamel mineralization
proceeds, the organic matrix is progressively removed allowing the
crystallites to grow in thickness. The resulting mineral phase forms as a
consequence of a unique organic matrix that is competent to direct its
own replacement by mineral. The interaction between matrix protein and
HAP crystals during maturation indicates an intimate relationship of
these two phases in mature enamel.