C h a p t e r 4 2
Structure and Function of the Skeletal System
1067
When appropriately stimulated by bone morphogenic
proteins (BMPs) and transforming growth factors, they
undergo cell division and differentiate into osteoblasts.
Osteoblasts.
Osteoblasts are cells that secrete bone
matrix. Bone formation occurs in two stages: ossification
and calcification. Ossification involves the formation of
the initial unmineralized bone, or osteoid (also called
prebone). Osteoblasts secrete both type I collagen (which
constitutes 90% of the protein of bone) and bone matrix
proteins, forming the osteoid. They also are responsible
for calcification of the bone matrix. The calcification
process appears to be initiated by the osteoblast through
the secretion of the enzyme
alkaline phosphatase
, which
is thought to act locally in bone tissue to raise calcium
and phosphate levels to the point at which precipitation
occurs. The activity of the osteoblasts undoubtedly con-
tributes to the increase in serum levels of alkaline phos-
phatase that follows bone injury and fractures.
Osteocytes.
Osteocytes are mature bone cells, derived
from osteoblasts, which are housed in
lacunae
within
the calcified bony matrix (see Fig. 42-4B). Extracellular
fluid-filled passageways permeate the calcified matrix and
connect with the lacunae of adjacent osteocytes. These
passageways are called
canaliculi.
Because diffusion does
not occur through the calcified matrix of bone, the cana-
liculi serve as communicating channels for the exchange
of nutrients and metabolites between the osteocytes and
the blood vessels on the surface of the bone layer.
Osteocytes are the cells responsible for maintain-
ing the bone matrix. They can synthesize new matrix,
as well as participate in matrix degradation. Death of
osteocytes through trauma, cell senescence, or apopto-
sis, results in resorption of the bone matrix by osteo-
clasts, followed by repair or remodeling of the bone
tissue by osteoblasts.
Osteoclasts.
Osteoclasts are large multinucleated
“bone-chewing” cells that function in the resorption of
bone, removing its mineral content and organic matrix.
They are derived from the same mononuclear hemato-
poietic progenitor cells that give rise to blood monocytes
and tissue macrophages. Osteoclast formation occurs in
close association with the stromal cells of the bone mar-
row. These cells secrete essential cytokines that promote
Osteocyte
Canaliculi
Lamella
Lacuna
Haversian canal
Compact bone
B
A
Inner circumferential
lamellae
Spongy bone
Blood vessel
into marrow
Volkmann canal
Haversian canal
Vessel of haversian canal
Outer
circumferential
lamellae
Haversian
system
Periosteum
FIGURE 42-4.
(A)
Haversian
systems as seen in a wedge
of compact bone tissue.The
periosteum has been peeled back
to show a blood vessel entering
a Volkmann canal.
(B)
Osteocytes
lying within lacunae; canaliculi
permit interstitial fluid to reach
each lacuna.
