Biomedical Engineering Reference
In-Depth Information
higher crystallinity than bone ones and show a sharp peak when measured by XRD
analysis. Enamel is formed from large and highly crystallized HAP crystals that are
densely packed.
Enamel has a low organic content. In the early stage of enamel formation, the
enamel consists of 30% mineral and 20% organic matrix, primarily amelogenin,
while the remaining 50% consists of water. During the maturation process, the
amelogenin is degraded by proteolytic enzymes and disappears from the enamel.
This results in a final enamel composition of 95-97% inorganic content and less
than 1% organic content [
4
]. Thus, enamel is an extremely hard tissue that consists
almost entirely of inorganic substances (the elastic modulus of enamel is 80 GPa,
[
5
] whereas that of bone is 10 GPa [
6
]). However, enamel does not possess as much
flexibility as bone (the tensile strengths of enamel are 10 or 40 MPa according to
direction [
7
], whereas that of bone is 100 MPa [
8
]).
The proteins secreted into the extracellular matrices play a key role in the
mineralization of bone and teeth. Recent observations have shown that, in the early
development of both bone and enamel, the minerals comprising them consist of
a large amount of amorphous calcium phosphate (ACP) [
9
-
11
]. Eventually, HAP
crystals form from the ACP and assemble into aggregates, which have structures
specific to bone and teeth functions. Therefore, to understand the biomineralization
of bone and teeth, it is necessary to know how proteins in the extracellular
matrices are involved in the deposition of ACP, the nucleation of nanocrystals,
and in the growth, transformation, and formation of hierarchical structures made
by comprising aggregates of nanocrystals.
In this chapter, we introduce results from recently published work focusing on
the structure and function of proteins that are involved in HAP crystal formation
in the body. We also summarize the challenges to regulating crystal growth and
elucidating the mechanisms of crystal formation using artificial proteins, which are
not attained by using only naturally occurring proteins.
4.2
Proteins Involved in HAP Crystal Formation
The proteins involved in HAP crystal formation can be categorized into three groups
on the basis of their functions. Those in one group form the insoluble organic
matrix and determine the contours of the hard tissues. Type 1 collagen is the organic
matrix for bone, dentin, and cementum, while amelogenin is the matrix for enamel.
Those in the second group produce mineral deposits in specific areas. They bind
to collagen and accelerate HAP crystallization. Those in the third group inhibit
deposition of minerals in inappropriate areas. Since biological fluid is supersaturated
with respect to HAP, it is not surprising that depositions take place ubiquitously
in the body. Proteins with inhibitory function against non-specific deposition are
therefore important for normal tissue formation.
