Biology Reference
In-Depth Information
Taken together these findings indicate differences among osteoclasts and in a
recent paper we have proposed that each bone site has its own specific set of
osteoclasts (Everts et al.
2009
).
8.2.6.6 Osteoclasts Prefer “Old” Bone
It is of considerable interest that osteoclasts seem to prefer to resorb “older” bone
(Henriksen et al.
2009
). Such a preference appears to make sense in the remodelling
of bone; old bone is removed and replaced by new bone. The mechanisms behind
this preference are not elucidated yet, but it could be related to the level of mineral
in bone of different ages. Since it is well now that the amount of mineral increases
age-dependently and osteoclasts prefer to attach to mineralized substrates, the level
of mineral may thus modulate the activity of the osteoclast.
8.2.6.7 Differences in the Composition of Bone at Different Skeletal Sites
Recently it was shown that considerable differences exist between the composition
of bones at different skeletal parts. Van den Bos et al. (
2008
) compared calvaria and
long bone and noted differences in the amount of collagen (higher in calvaria), non-
collagenous proteins and the level of collagen cross-links (higher in calvaria)
(Fig.
8.11
).
These differences can provide an explanation for the differences in osteoclasts
and their activity; a different substrate needs another cellular activity to resorb such
substrate. Whether the difference in substrate does indeed modulate the osteoclast
has not been investigated yet.
Recently it was demonstrated that for dentin a difference in osteoclastic activity
may be induced by the substrate. Dentin obtained from permanent teeth was
compared with dentin from deciduous teeth and the former dentin proved to inhibit
osteoclastic activity (Sriarj et al.
2009
).
8.2.7 Modulation of Resorption by Cytokines, Growth Factors
and Hormones
Osteoclastic bone resorption is modulated by a wide variety of compounds like the
cytokines IL-1, IL-6 and TNF
and the hormones PTH and
calcitonin (Chambers
2000
). With the exception of calcitonin, these compounds do
not directly affect the activity of the osteoclasts. The osteoblast-like bone lining cell
that is localized next to the osteoclast responds to the compounds and then signals
the osteoclast. For a review on the role of cytokines in osteoclastic bone resorption,
the reader is referred to (Teitelbaum
2007
).
, the growth factor TGF
a
b
