Biomedical Engineering Reference
In-Depth Information
enamel HAP at a pH higher than 7.1. Thus, the compositions of the enamel fluid
and enamel crystal change during enamel formation. Direct staining of developing
bovine enamel with pH indicators demonstrated that the enamel matrix has alternate
acidic (pH5.8-6.0) and neutral (pH7.0-7.2) areas [ 71 ] at certain locations and times
during enamel HAP formation.
During enamel HAP formation, Ca 2C ions are actively transported from the
ameloblasts into the enamel matrix, and the amount and timing of the Ca 2C
inflow are strictly controlled by the ameloblasts (2.2). The inflow of the Ca 2C
increases rapidly during the maturation stage [ 10 ]. This could increase the total
Ca 2C concentration in the matrix. However, the a Ca is decreased due to the chemical
equilibrium of the ionic components in the enamel fluid. These components include
lactate, H 2 PO 4 ,HPO 4 2 ,PO 4 3 ,HCO 3 ,andCO 3 2 .
The a Ca is also controlled by the surrounding amelogenins. The 25 kDa amel-
ogenins are cleaved by enzymes step-by-step during enamel formation, forming
20, 13, 11, and 6 kDa fragments. The cleaved fragments show different binding
abilities with Ca 2C and the developing crystal. The concentration of 13 and 11 kDa
fragments is 2.8 w/v% in the secretory stage of porcine enamel matrix [ 23 ]. This
corresponds to a 2 mM protein concentration when calculated using a molecular
weight of 13 kDa. This is several times higher than the concentration of Ca 2C in the
enamel fluid (0.3-0.5 mM). The binding coefficient of the proteins and Ca 2C ions is
of the order of 10 4
M[ 72 ], strongly suggesting that the fragments work as a Ca 2C
reserve.
5.3
Mineral Component of Tooth Enamel
5.3.1
Composition and Solubility of Enamel HAP
Enamel HAP contains several impurity ions, vacancies, and structural water [ 73 ].
The chemical composition solubility of porcine enamel in the early and late
secretory stage and maturation stage has been determined [ 74 , 75 ]. The chemical
formula for the thin ribbon-like crystals in the early stage of maturation is
(Ca) 4.155 (Mg) 0.034 (Na) 0.191 (K) 0.004 (HPO 4 ) 0.35 (CO 3 ) 0.357 (PO 4 ) 2.384 (OH) 0 . 007 .
That for erupted mature enamel is (Ca) 4.568 (Mg) 0.032 (Na) 0.11 (K) 0.002 (HPO 4 ) 0.143
(CO 3 ) 0.256 (PO 4 ) 2.712 (OH) 0.378 .
The relatively large decrease in HPO 4 during enamel formation is ascribed to the
OCP-to-HAP phase transition [ 76 ]. The amount of Mg 2C substituted for the Ca 2C
is almost constant, 0.7-0.8% of the Ca 2C , among developing enamel crystals.
The solubility products ( K sp ) of developing enamel crystals at different stages
were determined on the basis of the chemical composition (Ca) 5-x (Mg) q (Na) u
(HPO 4 ) v (CO 3 ) w (PO 4 ) 3-y (OH) 1-z . The average ion activity product ( I APeq ) 1/ N ( N is
the sum of the valences for each ion comprising the solid phase; for details, see
1.3.1) had a level of 4.41
10 6
10 6
in the early secretory stage, 1.80
in the
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