Biomedical Engineering Reference
In-Depth Information
initiated, these proteins are degraded into smaller fractions, change the interaction
with forming crystals and inorganic ions, and are gradually removed from the
matrix, providing space for enamel crystals to grow. The chapter ends with a
discussion of unique hard tissues with unusual mineral phases in vertebrate and
invertebrates.
Keywords
Tooth
enamel
•
Apatite
•
Octacalcium
phosphate
•
Amorphous
calcium phosphate • Amelogenin
5.1
Introduction
Tooth enamel, the hardest tissue found in animals, has extremely long crystals
with high crystallinity in parallel alignment. How the crystals are formed has
been a controversial issue since the 1960s. In this chapter, enamel crystals and
their surrounding environment are overviewed, and work aimed at clarifying the
mechanism of tooth enamel biomineralization is discussed.
Abbreviations used for apatite-type compounds are:
Calcium hydroxyapatite: H-Ap
Calcium chlorapatite: Cl-Ap
Calcium fluorapatite: F-Ap
Carbonate-containing calcium hydroxyapatite: CO
3
-HAp
Fluoride-containing calcium hydroxyapatite: F-HAp
CO
3
, F, Cl-containing calcium apatite: CO
3
,F,Cl-Ap
Biological apatite and calcium hydroxyapatite containing impurity ions, vacancies,
and structural water: HAP
5.2
Enamel Crystals and Surrounding Environment
5.2.1
Overview of Tooth Enamel and its Formation
Tooth enamel is a highly mineralized tissue, with a composition approximately 95%
HAP. Enamel crystals have an extremely elongated morphology from the dentin-
enamel junction (DEJ) to the tooth surface, and they align parallel to each other
(Fig.
5.1
). The crystallinity of enamel HAP is almost equivalent to that of mineral
apatites. An apatite with properties comparable to those of enamel HAP has not
been obtained in vitro under physiological conditions.
Tooth enamel is formed by ameloblasts, which secrete an enamel matrix
composed of water, proteins, and inorganic components. After the first layer of
dentin has formed, the ameloblasts enter their formative stage. Moving away from
