Biomedical Engineering Reference
In-Depth Information
4.1
Overview of Bone and Teeth Biomineralization
The crystalline phase of calcium phosphate that participates in bone formation is
the hydroxyapatite (HAP) phase, the most stable calcium phosphate phase under
physiological conditions. Bone contains collagen—an organic substance—at a dry
weight content of 30-40%. Bone is thus an inorganic-organic hybrid material and is
characterized by great elasticity and strength compared to pure inorganic substances.
Bone HAP nanocrystals measure 30-50 nm in length, 10-50 nm in width, and
3-5 nm in thickness. HAP crystallizes on the collagen fibrils along the direction
in which they extend [ 1 ].
Bone is synthesized by osteoblasts, which secrete type I collagen into the
extracellular matrix. Collagen molecules self-assemble into insoluble fibers through
multiple processes. Since biological fluid is supersaturated with respect to HAP,
it is not surprising that calcification occurs ubiquitously in the human body [ 2 ].
When calcification first begins, minerals are formed specifically on the collagen
fibrils. Two mechanisms have been proposed to explain this phenomenon. The first
is that some molecules in biological fluid inhibit non-specific mineral deposition.
It has been shown that acidic molecules containing carboxyl, phosphate, or sulfate
groups inhibit deposition of calcium phosphate in vitro. The second mechanism is
owing to the acidic proteins that possess affinity to collagen. Osteoblasts secrete
several types of proteins in addition to collagen. Some are acidic, containing many
acidic residues such as aspartic acid (Asp), glutamic acid (Glu), and phosphorylated
serine, and have the ability to bind to collagen. The binding of acidic proteins
to collagen apparently concentrates the calcium ions by using negatively charged
groups, which elevates the degree of supersaturation around collagen. This induces
calcium phosphate precipitation on the collagen molecules. These processes occur
in the early stages of biomineralization during bone formation. As calcification
progresses, osteoblasts are buried in the surrounding minerals. Cells imbedded in
the minerals no longer secrete proteins and become osteocytes.
Since the details of tooth formation are discussed in Chap. 5, the process is
summarized only briefly here. A tooth consists of three layers of mineralized
tissue. Enamel forms the outer layer, dentin forms the middle layer, and cementum
forms the inner layer, and they are synthesized by ameloblasts, odontoblasts, and
cementoblasts, respectively. Tooth calcification is regulated by proteins secreted into
the extracellular matrices. Like osteoblasts, odontoblasts and cementoblasts secrete
type I collagen, while ameloblasts secrete mainly amelogenin, but not collagen.
More than 90% of the organic phase of enamel consists of amelogenin. Amelogenin
is believed to regulate the formation of enamel HAP crystals and their assembly into
highly organized structures [ 3 ].
Enamel HAP crystals differ from those found in bone, dentin, and cementum in
their morphology and size. Enamel HAP crystals do not have a platelet structure
but rather forms needle-shaped crystals that extend along the c -axis. Enamel HAP
crystals are much larger than bone ones, measuring 100-1,000 nm in length,
30-60 nm in width, and 10-30 nm in thickness. Enamel HAP crystals have a
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