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distinct from the absence of a change in crystallinity after age 30 in
these samples [583]. In addition, other changes, like an increase of
Ca
, occur in bone mineral with
age [662-665]. Both the crystal sizes and carbonate content were
found to increase during aging in rats and cows [663, 664]. The
increase in carbonate content with age has also been reported in
still other studies [666-668]. From a chemical point of view, these
changes indicate to a slow transformation of poorly crystallized
non-apatitic calcium orthophosphates into a better-crystallized ion-
substituted carbonate-containing CDHA. While there are still many
gaps in our knowledge, the researchers seem to be comfortable in
stating that in all but the youngest bone and dentine, the only phase
present is a highly disordered, highly substituted biological apatite.
2+
content and a decrease of HPO
4
2−
Figure 1.12
A schematic illustration of
mineralization of a collagen
fibril: top layer — calcium orthophosphate clusters (green)
form complexes with biopolymers (orange line), forming stable
mineral droplets; second top layer — mineral droplets bind
to a distinct region on the collagen fibers and enter the fibril;
second bottom layer — once inside the collagen, the mineral
in a liquid state diffuses through the interior of the fibril and
solidifies into a disordered phase of ACP (black); bottom layer
— finally, directed by the collagen, ACP is transformed into
oriented crystals of biological apatite (yellow). Reprinted from
Ref. [656] with permission.
in vivo
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