203
CHAPTER SEVEN
Skeletal System
the skull, ribs, sternum, clavicles, vertebrae, and hip bones.
If the blood cell supply is deficient, some yellow marrow
may change back into red marrow and produce blood cells.
Chapter 14 (p. 524) discusses blood cell formation.
Inorganic Salt Storage
Recall that the extracellular matrix of bone tissue includes
collagen and inorganic mineral salts. The salts account for
about 70% of the extracellular matrix by weight and are
mostly small crystals of a type of calcium phosphate called
hydroxyapatite.
Clinical Application 7.2 discusses osteoporo-
sis, a condition that results from loss of bone mineral.
Marrow is a soft, netlike mass of connective tissue in
the medullary cavities of long bones, in the irregular spaces
of spongy bone, and in the larger central canals of compact
bone tissue. The two types of marrow are red and yellow.
Red marrow
functions in the formation of red blood cells
(erythrocytes), white blood cells (leukocytes), and blood
platelets. It is red because of the red, oxygen-carrying pig-
ment
hemoglobin
in red blood cells.
The distribution of marrow changes with age. In an infant,
red marrow occupies the cavities of most bones. With increas-
ing age, however, yellow marrow replaces much of it.
Yellow
marrow
stores fat and is inactive in blood cell production. In
an adult, red marrow is primarily found in the spongy bone of
af
ected area. Osteoclasts also appear and resorb
bone Fragments, aiding in “cleaning up” debris.
In time, ±
brocartilage ±
lls the gap between
the ends oF the broken bone. This mass, termed
a cartilaginous callus, is later replaced by bone
tissue in much the same way that the hyaline
cartilage oF a developing endochondral bone is
replaced. That is, the cartilaginous callus breaks
down, blood vessels and osteoblasts invade the
area, and a bony callus ±
lls the space.
Typically, more bone is produced at the site
oF a healing Fracture than is necessary to replace
the damaged tissues. Osteoclasts remove the
excess, and the result is a bone shaped much
like the original. ²igure 7B shows the steps in the
healing oF a Fracture.
IF the ends oF a broken bone are close
together, healing is Faster than iF they are Far
apart. Physicians can help the bone-healing
process. The First casts to immobilize Fractured
bones were introduced in Philadelphia in 1876,
and soon aFter, doctors began using screws and
plates internally to align healing bone parts.
Today, orthopedic surgeons also use rods, wires,
and nails. These devices have become lighter
and smaller; many are built oF titanium. A new
approach, called a hybrid Fixator, treats a bro-
ken leg using metal pins internally to align bone
pieces. The pins are anchored to a metal ring
device worn outside the leg.
Some bones naturally heal more rapidly
than others. The long bones oF the upper limbs,
For example, may heal in halF the time required by
the long bones oF the lower limbs, as Jacob was
unhappy to discover. However, his young age
would Favor quicker healing.
Compact
bone
Medullary
cavity
Hematoma
New blood
vessels
Fibrocartilage
Spongy bone
Compact bone
Medullary cavity
Periosteum
Bony callus
Blood escapes from ruptured blood
vessels and forms a hematoma.
Spongy bone forms in regions close to
developing blood vessels, and fibrocartilage
forms in more distant regions.
A bony callus replaces fibrocartilage.
Osteoclasts remove excess bony tissue,
restoring new bone structure much like
the original.
(a)
(b)
(c)
(d)
FIGURE 7B
Major steps (
a–d
) in the repair oF a Fracture.
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