195
CHAPTER SEVEN
Skeletal System
areas, and a specialized type of soft connective tissue called
marrow
(mar
o) F lls them. The two forms of marrow, red and
yellow, are described later in this chapter (see also F g. 7.2).
Microscopic Structure
Recall from chapter 5 (p. 160) that bone cells called
osteo-
cytes
(os
te-o-sı¯tz) are in tiny, bony chambers called
lacu-
nae,
which form concentric circles around
central canals
(Haversian canals). Osteocytes transport nutrients and
wastes to and from nearby cells by means of cellular pro-
cesses passing through
canaliculi.
The extracellular matrix of
bone tissue is largely collagen and inorganic salts. Collagen
gives bone its strength and resilience, and inorganic salts
make it hard and resistant to crushing.
Compact Bone
In compact bone, the osteocytes and layers of extracellular
matrix concentrically clustered around a central canal form
a cylinder-shaped unit called an
osteon
(os
te-on), some-
times called an Haversian system
(f gs. 7.4
and
7.5)
. Many
of these units cemented together form the substance of com-
pact bone. The orientation of the osteons resists compressive
forces.
Each central canal contains blood vessels and nerve
F bers surrounded by loose connective tissue. Blood in these
vessels nourishes bone cells associated with the central canal
via gap junctions between osteocytes.
Central canals extend longitudinally through bone tis-
sue, and transverse
perforating canals
(Volkmann’s canals)
connect them. Perforating canals contain larger blood ves-
sels and nerves by which the smaller blood vessels and nerve
F bers in central canals communicate with the surface of the
bone and the medullary cavity (see F
g. 7.4).
Spongy Bone
Spongy bone is also composed of osteocytes and extracel-
lular matrix, but the bone cells do not aggregate around cen-
tral canals. Instead, the cells lie within the trabeculae and
get nutrients from substances diffusing into the canaliculi
that lead to the surfaces of these thin, bony plates.
Severe bone pain is one symptom of sickle cell disease, which is
inherited. Under low oxygen conditions, abnormal hemoglobin (an
oxygen-carrying protein) bends the red blood cells that contain it
into sickle shapes, obstructing circulation. Radiographs can reveal
blocked arterial blood F
ow in bones of sickle cell disease patients.
PRACTICE
2
Explain how bones are classi±
ed.
3
List ±
ve major parts of a long bone.
4
How do compact and spongy bone di²
er in structure?
5
Describe the microscopic structure of compact bone.
Spongy
bone
(a)
Compact
bone
Remnant of
epiphyseal plate
Spongy bone
Compact bone
(b)
Spongy
bone
Compact
bone
(c)
FIGURE 7.3
Compact bone and spongy bone. (
a
) In a femur, the wall
of the diaphysis consists mostly of compact bone. (
b
) The epiphyses of
the femur contain spongy bone enclosed by a thin layer of compact
bone. (
c
) This skull bone contains a layer of spongy bone sandwiched
between plates of compact bone.
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