Chemistry Reference
In-Depth Information
first at multiple independent sites within the collagen fibril as well as on the surface of the fibrils. It continues into
the zones between collagen molecules, displacing water molecules as it goes, to leave a final water content in bone
of up to 25% of the volume. This residual water may help to stabilise collagen by forming inter- and intra-
hydrogen bonds with hydrophilic residues, like the hydroxyl group of hydroxyproline and other polar side chains
(
Figure 19.18
)
.
FIGURE 19.18
Water helps to stabilise collagen by forming inter- and intra-hydrogen bonds with hydrophilic residues. (From
Nyman et al.
The enamel of mammalian teeth is much more heavily mineralised than bone, which makes it much harder. In
addition, it does not contain collagen, although in its final mature state it does contain small amounts of specialised
matrix proteins. Early tooth development is a classical illustration of the interaction between two tissue types
(epithelial cells and mesenchymal cells
3
)
, whereby a number of signaling molecules are involved in orchestrating
reciprocal interactions between the two types of tissue.
Enamel formation is thought to involve the following steps in its assembly: (1) stimulation of cells called
ameloblasts, derived from the epithelium, which are responsible for the secretion of enamel matrix proteins and
the carbonated apatite of the enamel; (2) self-assembly of matrix proteins, notably amelogenin, which assembles
into nanospheres to form “ribbons” of matrix; (3) secretion of saturating levels of Ca
2
þ
and of PO
4
; (4) nucleation
of crystal formation; (5) regulation of crystallite growth by the matrix; and (6) proteolytic degradation of the
matrix and rapid filling in with carbonated apatite crystallite. The principal water-soluble protein of enamel is
amelogenin, which is degraded as the enamel matures
transient degradation intermediates are found in
developing enamel. This 180 residue hydrophobic protein self-assembles into nanospheres in vitro, which
resemble the matrix ribbons found in vivo, and it interacts with carbonated apatite in vitro to limit crystallite
growth just as it is thought to act in vivo to channel accumulation of the mineral crystallite into rods. However,
there are other enamel-specific proteins which have been isolated, studied and in some cases cloned. And, to make
e
3. Mesenchymal tissue is immature, unspecialised tissue, found in the early embryo of animals, whereas epithelial cells are parenchymal cells
which line an internal cavity or tube.
