Chapter 6
Bones and Skeletal Tissues
193
6
along with reduced mineralization, causes a spotty weaken-
ing of the bones. Late in the disease, osteoclast activity wanes,
but osteoblasts continue to work, oFen forming irregular bone
thickenings or filling the marrow cavity with Pagetic bone.
Paget’s disease may affect any part of the skeleton, but it is
usually a localized condition. Te spine, pelvis, femur, and skull
are most oFen involved and become increasingly deformed and
painful. It rarely occurs before age 40, and it affects about 3% of
North American elderly people. Its cause is unknown, but a vi-
rus may trigger it. Drug therapies include calcitonin (adminis-
tered by a nasal inhaler), and the newer bisphosphonates, which
have shown success in preventing bone breakdown.
Check Your Understanding
23.
Which bone disorder is characterized by excessive deposit of
weak, poorly mineralized bone?
24.
What are three measures that may help to maintain healthy
bone density?
25.
What name is given to “adult rickets”?
For answers, see Appendix H.
Developmental Aspects
of Bones: Timing of Events
Describe the timing and cause of changes in bone
architecture and bone mass throughout life.
Bones are on a precise schedule from the time they form until
death. Te mesoderm germ layer gives rise to embryonic mesen-
chymal cells, which in turn produce the membranes and cartilages
that form the embryonic skeleton. Tese structures then ossify
according to an amazingly predictable timetable that allows fetal
age to be determined easily from either X rays or sonograms. Al-
though each bone has its own developmental schedule, most long
bones begin ossifying by 8 weeks aFer conception and have well-
developed primary ossification centers by 12 weeks
(Figure 6.17)
.
Birth to Young Adulthood
At birth, most long bones of the skeleton are well ossified except
for their epiphyses. AFer birth, secondary ossification centers
develop in a predictable sequence. Te epiphyseal plates persist
and provide for long bone growth all through childhood and the
sex hormone–mediated growth spurt at adolescence. By age 25,
nearly all bones are completely ossified and skeletal growth ceases.
Age-Related Changes in Bone
In children and adolescents, bone formation exceeds bone re-
sorption. In young adults, these processes are in balance, and
in old age, resorption predominates. Despite the environmental
factors (discussed earlier) that influence bone density, genet-
ics still plays the major role in determining how much a per-
son’s bone density will change over a lifetime. A single gene that
codes for vitamin D’s cellular docking site helps determine both
and estrogen deficiency is strongly implicated in osteoporosis
in older women.
Several other factors can contribute to osteoporosis:
Petite body form
Insufficient exercise to stress the bones
A diet poor in calcium and protein
Abnormal vitamin D receptors
Smoking (which reduces estrogen levels)
Hormone-related conditions such as hyperthyroidism, low
blood levels of thyroid-stimulating hormone, and diabetes
mellitus
Osteoporosis can develop at any age as a result of immobility.
It can also occur in males with prostate cancer who are being
treated with androgen-suppressing drugs.
Treating Osteoporosis
Osteoporosis has traditionally been treated with calcium and vi-
tamin D supplements, weight-bearing exercise, and
hormone
(
es-
trogen
)
replacement therapy
(
HRT
). ±rustratingly, HR² slows the
loss of bone but does not reverse it. Additionally, because of the
increased risk of heart attack, stroke, and breast cancer associated
with estrogen replacement therapy, it is a controversial treatment
these days. Although not a substitute for HR², estrogenic com-
pounds in soy products (principally the isoflavones daidzein and
genistein) offer a good addition or adjunct for some patients.
Newer drugs are available. Bisphosphonates decrease oste-
oclast activity and number, and partially reverse osteoporosis
in the spine. Selective estrogen receptor modulators (SERMs),
such as raloxifene, dubbed “estrogen light,” mimic estrogen’s
beneficial bone-sparing properties without targeting the uterus
or breast. Additionally,
statins
, drugs used to lower cholesterol
levels, have an unexpected side effect of increasing bone min-
eral density up to 8% over four years. Te monoclonal antibody
drug
denosumab
significantly reduces fractures in men fighting
prostate cancer and improves bone density in the elderly.
Preventing Osteoporosis
How can osteoporosis be prevented (or at least delayed)? Te
first requirement is to get enough calcium while your bones are
still increasing in density (bones reach their peak density during
early adulthood). Second, keep in mind that excessive intake of
carbonated beverages and alcohol leaches minerals from bone
and decreases bone density. ±inally, get plenty of weight-bearing
exercise (walking, jogging, tennis, etc.) throughout life. Tis will
increase bone mass above normal values and provide a greater
buffer against age-related bone loss.
Paget’s Disease
OFen discovered by accident when X rays are taken for some
other reason,
Paget’s disease
(paj
9
ets) is characterized by ex-
cessive and haphazard bone deposit and resorption. Te newly
formed bone, called
Pagetic bone
, is hastily made and has an
abnormally high ratio of spongy bone to compact bone. Tis,
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