Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

FIBROBLASTS AND THEIR TRANSFORMATIONS: THE CONNECTIVE-TISSUE CELL FAMILY
1231
lysosomes
multiple
nuclei
osteoclast
bone matrix
tight seal
to matrix
(A)
bone
matrix
ruffled border
of osteoclast
10 µm
(B)
Figure 22–16 Osteoclasts. (A) Drawing of an osteoclast in cross section. This giant, multinucleated cell erodes bone matrix. The
“ruffled border” is a site of secretion of acids (to dissolve the bone minerals) and hydrolases (to digest the organic components of
the matrix). Osteoclasts vary in shape, are motile, and often send out processes to resorb bone at multiple sites. They develop
from monocytes and can be viewed as specialized macrophages. (B) An osteoclast on bone matrix, seen by scanning electron
MBoC6 m23.59/22.16
microscopy. The osteoclast has been crawling over the matrix, eating it away, and leaving a trail of pits where it has done so.
(A, from R.V. Krsti ć, Ultrastructure of the Mammalian Cell: An Atlas. Berlin: Springer-Verlag, 1979; B, courtesy of Alan Boyde.)
in this chapter). The precursor cells are released into the bloodstream and collect
at sites of bone resorption, where they fuse to form the multinucleated osteoclasts,
which cling to surfaces of the bone matrix and eat it away. Osteoclasts are
capable of tunneling deep into the substance of compact bone, forming cavities
that are then invaded by other cells. A blood capillary grows down the center of
such a tunnel, and the walls of the tunnel become lined with a layer of osteoblasts
(Figure 22–17). These osteoblasts lay down concentric layers of new matrix,
which gradually fill the cavity, leaving only a narrow canal surrounding the new
blood vessel. At the same time as some tunnels are filling up with bone, others are
being bored by osteoclasts, cutting through older concentric systems.
quiescent osteoblast
(bone-lining cell)
small blood vessel
endothelial cell
fibroblast
osteocyte
osteoblast about to
lay down new bone
to fill in the
excavated tunnel
100 µm
new bone
new bone matrix
not yet calcified
old bone
loose connective
tissue
inward-growing
capillary sprout
osteoclast
excavating
tunnel through
old bone
Figure 22–17 The remodeling of
compact bone. Osteoclasts acting
together in a small group excavate a tunnel
through the old bone, advancing at a rate
of about 50 μm per day. Osteoblasts enter
the tunnel behind them, line its walls, and
begin to form new bone, depositing layers
of matrix at a rate of 1–2 μm per day. At
the same time, a capillary sprouts down the
center of the tunnel. The tunnel eventually
becomes filled with concentric layers of
new bone, with only a narrow central
canal remaining. Each such canal, besides
providing a route of access for osteoclasts
and osteoblasts, contains one or more
blood vessels that transport the nutrients
the bone cells require for survival. Typically,
about 5–10% of the bone in a healthy adult
mammal is replaced in this way each year.
(After Z.F.G. Jaworski, B. Duck and
G. Sekaly, J. Anat. 133:397–405, 1981.
With permission from Blackwell Publishing.)