Biomedical Engineering Reference
In-Depth Information
are calcium salts, so their formation can be generically classified as calcification.
Representative examples of beneficial calcification include the formation of teeth
and bone, while harmful calcification includes, in addition to stone formation,
the buildup of atherosclerotic plaques, the calcification of brain tissue, and the
accumulation of fine calcium salt crystals that often accompanies the presence of
cancerous tissues.
Among the calcium salts formed by biomineralization, calcium phosphates,
represented by apatite, have by far received the most attention and in-depth study.
This is probably because calcium phosphate salts are not only integral to tissue
formation but are also believed to be very deeply involved in life processes and
in the evolution of species [ 1 ]. While there have been many debates on this topic, a
consensus of opinions has not yet been reached. For instance, one enduring question
is why vertebrates, as represented by humans, use apatite (a calcium phosphate
salt) to produce hard tissues in the body while invertebrates such as shellfish
use calcium carbonate salts such as aragonite and calcite to produce hard tissues.
One explanation is that invertebrates have a unique immune mechanism. And while
there are intriguing theories related to apatite and the origins of life, the supporting
evidence is inconclusive. For example, the surface of apatite has extremely strong
affinity for the substances constituting living organisms such as amino acids,
proteins, sugars, and lipids. Moreover, amino acids arrange themselves with high
regularity on apatite surfaces and are able to initiate condensation reactions, a key
factor in the origin of life [ 2 ].
Biomineralization is a typical interdisciplinary topic that is related to physics,
chemistry, and biology (including medical science), and the subject of interest
depends on the perspective taken. When considered from the viewpoint of crystal
growth, the important points are likely to include the growth mechanisms of
biominerals themselves, the actual nature of the organic-inorganic interactions that
are the essence of the biomineralization process, and the products in the environment
with a complex chemical composition such as organisms.
As an example of biomineralization, these sections will describe and explain
calcium phosphate salts—the most important biominerals in humans—with par-
ticular focus on apatite. In this chapter, the mechanisms of apatite crystal growth
will be introduced. Apatite will be considered as purely an inorganic material,
and developments in the understanding of crystal growth mechanisms in simple
solution systems will be reviewed. In Chap. 4, examples of research focusing on
the relationship between proteins and apatite will be described from the perspective
of organic-inorganic interactions. The process used by proteins to control the
formation of minerals will be explained, and the control of biomineralization using
protein engineering will be described. In Chap. 5, the current state of research
on the tooth enamel, as a representative example of biomineralization within
an organism, will be reviewed. The relationship between the concept of apatite
formation mechanisms with clusters as growth units (described in this chapter) and
the roles of amorphous calcium phosphate (ACP) and octacalcium phosphate in the
formation of enamel, which is attracting much attention, will be described.
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