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

1230 Chapter 22: Stem Cells and Tissue Renewal
osteogenic cell
(osteoblast
precursor)
osteoblast
osteoid
(new, uncalcified
bone matrix)
old, calcified bone
matrix
Figure 22–14 Deposition of bone matrix
by osteoblasts. Osteoblasts lining the
surface of bone secrete the organic matrix
of bone (osteoid) and are converted into
osteocytes as they become embedded in
this matrix. The matrix calcifies soon after
it has been deposited. The osteoblasts
themselves are thought to derive from
osteogenic stem cells that are closely
related to fibroblasts.
osteocyte
10 µm
it is not isolated from its fellows. Tiny channels, or canaliculi, radiate from each
lacuna and contain cell processes from the resident osteocyte, enabling it to form
gap junctions with adjacent osteocytes (Figure 22–14). Blood vessels and nerves
run through the tissue, keeping the bone cells alive and reacting when the bone
is damaged.
A mature bone has a complex and beautiful architecture, in which dense plates
of compact bone tissue enclose spaces spanned by light frameworks of trabecular
bone—a filigree of delicate shafts and flying buttresses of bone tissue, with soft
marrow in the interstices (Figure 22–15). The creation, maintenance, and repair
of this structure depend not only on the cells of the connective-tissue family
MBoC6 m23.55/22.14
that synthesize matrix, but also on a separate class of cells called osteoclasts that
degrade it, as we explain below.
Bone Is Continually Remodeled by the Cells Within It
For all its rigidity, bone is by no means a permanent and immutable tissue.
Running through the hard extracellular matrix are channels and cavities occupied
by living cells, which account for about 15% of the weight of compact bone.
These cells are engaged in an unceasing process of remodeling: while osteoblasts
deposit new bone matrix, osteoclasts demolish old bone matrix. This mechanism
provides for continuous turnover and replacement of the matrix in the interior of
the bone.
Osteoclasts (Figure 22–16) are large, multinucleated cells that originate, like
macrophages, from hematopoietic stem cells in the bone marrow (discussed later
trabecular bone
(A)
2 mm
(B)
compact bone
Figure 22–15 Trabecular and compact
bone. (A) Low-magnification scanning
electron micrograph of trabecular bone in a
vertebra of an adult man. The soft marrow
tissue has been dissolved away. (B) A
slice through the head of the femur, with
bone marrow and other soft tissue likewise
dissolved away, reveals the compact bone
of the shaft and the trabecular bone in
the interior. Because of the way in which
bone tissue remodels itself in response to
mechanical load, the trabeculae become
oriented along the principle axes of stress
within the bone. (A, courtesy of Alan Boyde;
B, from J.B. Kerr, Atlas of Functional
Histology. Mosby, 1999.)