200
UNIT TWO
Factors Affecting Bone Development,
Growth, and Repair
A number of factors infl
uence bone development, growth,
and repair. These include nutrition, exposure to sunlight,
hormonal secretions, and physical exercise. For example,
vitamin D is necessary for proper absorption of calcium in
the small intestine. In the absence of this vitamin, calcium
is poorly absorbed, and the inorganic salt portion of bone
matrix lacks calcium, softening and thereby deforming
bones. In children, this condition is called
rickets,
and in
adults, it is called
osteomalacia.
Vitamin D is scarce in natural foods, except for eggs, but
it is readily available in milk and other dairy products forti-
±
ed with vitamin D. Vitamin D also forms from dehydrocho-
lesterol, produced by cells in the digestive tract or obtained
in the diet. The blood carries dehydrocholesterol to the skin,
where exposure to ultraviolet light from the sun converts it
to vitamin D.
Vitamins A and C are also required for normal bone
development and growth. Vitamin A is necessary for osteo-
blast and osteoclast activity during normal development.
This is why de± ciency of vitamin A may retard bone devel-
opment. Vitamin C is required for collagen synthesis, so its
lack may also inhibit bone development. In this case, osteo-
blasts cannot produce enough collagen in the extracellular
matrix of the bone tissue. As a result, bones are abnormally
slender and fragile.
A long bone continues to lengthen while the cartilagi-
nous cells of the epiphyseal plates are active. However, once
the ossi±
cation centers of the diaphysis and epiphyses meet
and the epiphyseal plates ossify, lengthening is no longer
possible in that end of the bone.
A developing bone thickens as compact bone is deposited
on the outside, just beneath the periosteum. As this compact
bone forms on the surface, osteoclasts erode other bone tissue
on the inside
(f g. 7.10)
. The resulting space becomes the med-
ullary cavity of the diaphysis, which later ± lls with marrow.
The bone in the central regions of the epiphyses and dia-
physis remains spongy, and hyaline cartilage on the ends of
the epiphyses persists throughout life as articular cartilage.
Table 7.2
lists the ages at which various bones ossify.
If a child’s long bones are still growing, a radiograph will reveal epi-
physeal plates
(F
g. 7.11)
. If a plate is damaged as a result of a frac-
ture before it ossiF
es, elongation of that long bone may prematurely
cease, or if growth continues, it may be uneven. ±or this reason, inju-
ries to the epiphyses of a young person’s bones are of special con-
cern. Surgery is used on an epiphysis to equalize growth of bones
developing at very di²
erent rates.
PRACTICE
6
Describe the development of an intramembranous bone.
7
Explain how an endochondral bone develops.
8
List the steps in the growth of a long bone.
Homeostasis of Bone Tissue
After the intramembranous and endochondral bones form,
the actions of osteoclasts and osteoblasts continually remodel
them.
Bone remodeling
occurs throughout life as osteoclasts
resorb bone tissue, and osteoblasts replace the bone. These
opposing processes of
resorption
and
deposition
are highly
regulated so that the total mass of bone tissue in an adult
skeleton normally remains nearly constant, even though 3%
to 5% of bone calcium is exchanged each year.
FIGURE 7.10
Micrograph of a bone-resorbing osteoclast (800×).
TABLE
7.2
|
Ossif
cation Timetable
Age
Occurrence
Age
Occurrence
Third month of prenatal
development
OssiF
cation in long bones begins.
15 to 18 years (females)
17 to 20 years (males)
Bones of the upper limbs and scapulae completely
ossiF
ed.
±ourth month of prenatal
development
Most primary ossiF
cation centers have
appeared in the diaphyses of bones.
16 to 21 years (females)
18 to 23 years (males)
Bones of the lower limbs and hip bones completely
ossiF
ed.
Birth to 5 years
Secondary ossiF
cation centers appear in
the epiphyses.
21 to 23 years (females)
23 to 25 years (males)
Bones of the sternum, clavicles, and vertebrae
completely ossiF
ed.
5 to 12 years (females)
5 to 14 years (males)
OssiF
cation rapidly spreads from the
ossiF
cation centers.
By 23 years (females)
By 25 years (males)
Nearly all bones completely ossiF
ed.
Developing
medullary
cavity
Osteoclast
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