Covering, Support, and Movement of the Body
the tendency to accumulate bone mass during early life and a
person’s risk of osteoporosis later in life.
Beginning in the fourth decade of life, bone mass decreases
with age. Te only exception appears to be in bones of the skull.
Among young adults, skeletal mass is generally greater in males
than in females. Age-related bone loss is faster in whites than in
blacks (who have greater bone density to begin with) and faster
in females than in males.
Qualitative changes also occur: More osteons remain incom-
pletely formed, mineralization is less complete, and the amount
of nonviable bone increases, reﬂecting a diminished blood sup-
ply to the bones in old age. Tese age-related changes are also
bad news because fractures heal more slowly in old people.
Electrical stimulation of fracture sites and daily ultrasound
treatments hasten repair and healing. Presumably electrical
ﬁelds inhibit P±H stimulation of osteoclasts and induce forma-
tion of growth factors that stimulate osteoblasts.
Check Your Understanding
What is the status of bone structure at birth?
The decrease in bone mass that begins in the fourth decade
of life affects nearly all bones. What are the exceptions?
For answers, see Appendix H.
Tis chapter has examined skeletal cartilages and bones—their
architecture, composition, and dynamic nature. We have also dis-
cussed the role of bones in maintaining overall body homeostasis,
as summarized in
. Now we are ready to look
at the individual bones of the skeleton and how they contribute to
its functions, both collectively and individually.
Fetal primary ossiﬁcation centers at 12 weeks.
The darker areas indicate primary ossiﬁcation centers in the skeleton
of a 12-week-old fetus.
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Basic Structure, Types, and Locations
A skeletal cartilage exhibits chondrocytes housed in lacunae
(cavities) within the extracellular matrix (ground substance and
ﬁbers). It contains large amounts of water (which accounts for its
resilience), lacks nerve ﬁbers, is avascular, and is surrounded by a
ﬁbrous perichondrium that resists expansion.
Hyaline cartilages appear glassy; the ﬁbers are collagenous. Tey
provide support with ﬂexibility and resilience and are the most
abundant skeletal cartilages, accounting for the articular, costal,
respiratory, and nasal cartilages.
Elastic cartilages contain abundant elastic ﬁbers, in addition to
collagen ﬁbers, and are more ﬂexible than hyaline cartilages. Tey
support the outer ear and epiglottis.
Fibrocartilages, which contain thick collagen ﬁbers, are the
most compressible cartilages and resist stretching. Tey form
intervertebral discs and knee joint cartilages.
Growth of Cartilage
Cartilages grow from within (interstitial growth) and by adding
new cartilage tissue at the periphery (appositional growth).
Classiﬁcation of Bones
Bones are classiﬁed as long, short, ﬂat, or irregular on the basis of
their shape and their proportion of compact or spongy bone.
Functions of Bones
Bones give the body shape; protect and support body organs;
provide levers for muscles to pull on; store calcium and other
minerals; store growth factors and triglyceride; and are the site of
blood cell and osteocalcin production.