Chemistry Reference
In-Depth Information
of differentiation. It will suffice to say here that the cells of the IEE form a continuous
layer or sheet in which the preameloblasts have a cuboidal appearance. As they
differentiate to their mature form the preameloblasts elongate and their nuclei recede
proximally away from the dentino-enamel junction (DEJ) producing highly polarized
secretory cells. At the DEJ, the ameloblasts form a secretory structure similar to dentinal
tubules in function but not shape. These asymmetric, tapered cell extensions, called the
Tomes' processes, define spaces into which a number of specialized proteins and the
mineral ions are secreted. The enamel mineral is nucleated and the crystals grown within
in these limited chambers defined by the surfaces of adjacent Tomes' processes at the
apical ends of the ameloblasts. Mineralization is initiated at the DEJ, and the mineral
forms as thin ribbon-like hydroxyapatite crystals in small bundles called prisms. As the
crystals grow in length and the enamel layer thickens the ameloblasts recede from the
DEJ, however, each cell retains the closed Tome's process space and the same bundle of
apatite crystals, the same prisms, continue to elongate in the direction of the outer enamel
surface. Each enamel prism, made up of thin ribbon-like crystals is thus the product of a
single secretory cell. The initial ribbons grow in length and may be continuous from the
DEJ to the tooth outer surface. When the enamel has reached its full thickness, the
ameloblasts cease their secretory production of enamel and enter a post-secretory,
resorptive phase, cleaning up in a sense, the enamel surface before dying and sloughing
off the tooth surface. The ameloblasts, and hence the prisms, follow a sinuous path to the
outer tooth surface and the prisms form interlocking structures that contribute to the
strength of the enamel, but this structure is dependent on the location within the tooth, the
tooth function, and the species of animal producing the enamel.
Enamel matrix proteins
The mature ameloblasts secrete a complex mixture of proteins, glycoproteins and
proteolytic enzymes into the extracellular Tomes' process compartments, and these
proteins control the growth and form of the mineral phase. At the early stage of apatite
development the majority of the proteins are amelogenins, essentially hydrophobic
proteins that aggregate and fill the inter-Tomes' process spaces. There are, however,
other proteins and proteases present. The enamel matrix proteins, as noted earlier, fit into
a phylogenetic clade rising by gene duplication from the same primordial ancestor gene
as the SIBLING proteins. Ameloblastin and enamelin are closely grouped on human
chromosome 4q13 while amelogenin, more distant in structure and function, resides on
the X and Y chromosomes (Kawasaki and Weiss 2003). The genes for amelogenin
(AMEL), ameloblastin (AMBN) and enamelin (ENAM) have all been sequenced in a
variety of species and found to be different, but highly conserved across species and the
amino acid sequences have been determined from protein data as well. It is important to
note that all of these proteins are essentially removed during the formation of the enamel,
so that the mature enamel is only 1-2% protein. Thus, the mineralizing compartments are
undoubtedly changing in content during the entire process of mineral development. This
is clearly reflected in the amelogenin content. When isolated from teeth undergoing
mineralization, the amelogenin fraction is very heterogeneous, so that many amelogenin-
related peptides can be seen on SDS-gel electrophoresis. Most of these are degradation
products of the full-length protein. However, the amelogenin gene is subject to alternative
splicing. In the mouse up to nine peptides corresponding to specific gene splice products
have been identified (Hu et al. 1997). AMBN and ENAM proteins have also been found
to be degraded during their existence in the mineralizing enamel matrix.
The amelogenin-related peptides account for more than 90% of the initial enamel
matrix. Crystal growth begins in this amelogenin-rich milieu. Full-length amelogenins
have between 173 and 210 amino acids, depending on the species, with a molecular
