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bottom of the pit from matrix remnants left by the osteoclast (Everts et al.
2002
).
The bone lining cells are attracted to the resorption site and it has been suggested
that TGF
released from the bone matrix during resorption plays a role in this
process. Alternatively, it has been proposed that TRACP attracts the bone lining
cells (Perez-Amodio et al.
2006b
). When the osteoclast leaves the resorption pit a
high level of TRACP is found left behind in the pit and this enzyme may attract the
bone lining cells after which the enzyme is endocytosed and subsequently blocked
in its activity.
Following entering the pit, the bone lining cell starts cleaning the bottom. The
cleaning involves engulfment of the demineralized collagen fibrils protruding
from the bottom and subsequent digestion of these fibrils (Everts et al.
2002
).
This degradation is mediated by MMPs, probably MMP-13 (Zhao et al.
1999
)or
MMP-14 (Andersen et al.
2004
), both enzymes being highly expressed by bone
lining cells. In line with the view that MMP-14 may be important in the digestion of
non-mineralized collagen protruding from the bone surface is the very high amount
of non-digested fibrillar collagen enclosed in vacuoles in the bone lining cells as
found in bone samples of mice deficient for MMP-14 (Fig.
8.12
). This observation
suggests that the cell can engulf the collagen but digestion is hampered, comparable
to the digestion of connective tissue collagen by the phagocytic activity of fibro-
blasts. In the absence of MMP-14 huge amounts of non-digested ingested fibrillar
collagen are present in fibroblasts (Beertsen et al.
2002
).
The cleaning of the bottom is followed by the deposition of a thin layer of
collagen and osteopontin, the cement line (McKee and Nanci
1996
). Following
this activity osteoblasts deposit new bone in the lacuna (Mulari et al.
2004
).
b
BLC
Fig. 8.12 Non-mineralized
collagen fibrils protruding
from the bone surface are
engulfed by a bone lining cell
(BLC). Some of these fibrils
are indicated with an
arrow
.
The bone (calvaria) was
obtained from a mouse
deficient for MMP-14
(Holmbeck et al.
1999
). Note
the high number of collagen
fibrils engulfed by the bone
lining cell
bone
