Biomedical Engineering Reference
In-Depth Information
Fig. 5.1 TEM image of enamel in early stage of formation at dentin-enamel junction. In enamel,
thin and ribbon-like crystals grow parallel to each other, while in dentin, tiny granules deposit in
collagenous matrix. (Reproduced with permission from ref. [ 90 ]) (Copyright 1995, Springer)
the DEJ, they produce an enamel matrix in which enamel crystals grow. The
enamel formation process is conventionally considered to occur in two stages [ 1 ]:
organic matrix secretion and enamel crystal formation. In the first stage, enamel
proteins secreted by ameloblasts form an enamel matrix [ 2 ], and the matrix partially
mineralizes (to about 30% mineralization). In the second stage, the full thickness
of the partially mineralized matrix is formed, and the degree of mineralization
increases. The thickness of the enamel is specific to the animal species and tooth
type. At a certain point in the secretory (first) stage, the matrix consists of 54%
water, 31% proteins, and 15% mineral (by volume), with a density of 1.39 g cm 3
[ 3 ]. In the maturation (second) stage, the enamel proteins are degraded by enzymes
[ 4 ]. The degraded protein fractions and water are rapidly removed from the matrix
by the ruffle-ended ameloblasts (RAs). At the same time, the RAs actively transport
Ca 2C and PO 4 3 ions into the enamel matrix [ 5 - 10 ], enabling the crystal to increase
in thickness. Thus, the water, organic, and mineral contents change during enamel
formation [ 11 , 12 ]. Mature human enamel contains about 1.8 wt% protein and has
a density of about 2.9 g cm 3 [ 13 ]. Some proteins remain on the surface of the
enamel crystals after most of the proteins have been removed from the matrix. As
a result, the mineral content of the mature enamel is more than 95%. In contrast,
degradation and removal of the structural matrix protein (collagen) does not occur
in the mineralization of dentin and cementum.
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