186
UNIT 2
Covering, Support, and Movement of the Body
6
bone matrix on the external bone surface as osteoclasts on the
endosteal surface of the diaphysis remove bone (Figure 6.11).
However, normally there is slightly more building up than break-
ing down. Tis unequal process produces a thicker, stronger
bone but prevents it from becoming too heavy.
Check Your Understanding
15.
Bones don’t begin with bone tissue. What do they begin
with?
16.
When describing endochondral ossification, some say “bone
chases cartilage.” What does that mean?
17.
Where is the primary ossification center located in a long
bone? Where is (are) the secondary ossification center(s)
located?
18.
As a long bone grows in length, what is happening in the
hypertrophic zone of the epiphyseal plate?
For answers, see Appendix H.
Hormonal Regulation of Bone Growth
Te bone growth that occurs until young adulthood is exqui-
sitely controlled by a symphony of hormones. During infancy
and childhood, the single most important stimulus of epiphy-
seal plate activity is
growth hormone
released by the anterior
pituitary gland. Tyroid hormones modulate the activity of
growth hormone, ensuring that the skeleton has proper propor-
tions as it grows.
At puberty, male and female sex hormones (testosterone and
estrogens, respectively) are released in increasing amounts. Ini-
tially these sex hormones promote the growth spurt typical of
adolescence, as well as the masculinization or feminization of
specific parts of the skeleton. Later the hormones induce epi-
physeal closure, ending longitudinal bone growth.
We describe it in more detail later in conjunction with the
changes that occur in adult bones.
As adolescence ends, the chondroblasts of the epiphyseal
plates divide less o±en and the plates become thinner and thin-
ner until they are entirely replaced by bone tissue. Longitudinal
bone growth ends when the bone of the epiphysis and diaphysis
fuses. Tis process, called
epiphyseal plate closure
, happens at
about 18 years of age in females and 21 years of age in males.
Once this has occurred, only the articular cartilage remains in
bones. However, as noted earlier, an adult bone can still widen
by appositional growth if stressed by excessive muscle activity
or body weight.
Growth in Width (Thickness)
Growing bones widen as they lengthen. As with cartilages, bones
increase in thickness or, in the case of long bones, diameter, by
appositional growth. Osteoblasts beneath the periosteum secrete
Calcified
cartilage
spicule
Osseous
tissue
(bone)
covering
cartilage
spicules
Resting zone
Osteoblast
depositing
bone matrix
Proliferation
zone
Cartilage cells
undergo mitosis.
Hypertrophic
zone
Older cartilage cells
enlarge.
Ossification
zone
New bone forms.
Calcification
zone
Matrix calcifies;
cartilage cells die;
matrix begins
deteriorating; blood
vessels invade
cavity.
1
2
4
3
Figure 6.10
Growth in length of a long bone occurs at the
epiphyseal plate.
The side of the epiphyseal plate facing the
epiphysis contains resting cartilage cells. The cells of the epiphyseal
plate proximal to the resting cartilage area are arranged in four
zones—proliferation, hypertrophic, calcification, and ossification—
from the region of the earliest stage of growth
1
to the region
where bone is replacing the cartilage
4
(115
3
).
Bone growth
Bone remodeling
Articular cartilage
Epiphyseal plate
Cartilage
grows here.
Bone
replaces
cartilage
here.
Cartilage
grows here.
Bone that was
here has been
resorbed.
Bone that was
here has been
resorbed.
Appositional
growth adds
bone here.
Bone replaces
cartilage here.
Figure 6.11
Long bone growth and remodeling during
youth.
The events at the left depict endochondral ossification that
occurs at the articular cartilages and epiphyseal plates as the bone
lengthens. Events at the right show bone remodeling during long
bone growth to maintain proper bone proportions. The dashed red
outline matches the view on the left.
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