174
UNIT 2
Covering, Support, and Movement of the Body
6
in sites that are subjected to both pressure and stretch, such as
the padlike cartilages (menisci) of the knee and the discs be-
tween vertebrae, colored red in Figure 6.1.
Growth of Cartilage
Unlike bone, which has a hard matrix, cartilage has a flexible
matrix which can accommodate mitosis. It is the ideal tissue to
use to rapidly lay down the embryonic skeleton and to provide
for new skeletal growth.
Cartilage grows in two ways. In
appositional growth
(ap
0
o-
zish
9
un-al; “growth from outside”), cartilage-forming cells in
the surrounding perichondrium secrete new matrix against
the external face of the existing cartilage tissue. In
interstitial
growth
(in
0
ter-stish
9
al; “growth from inside”), the lacunae-
bound chondrocytes divide and secrete new matrix, expanding
the cartilage from within. Typically, cartilage growth ends dur-
ing adolescence when the skeleton stops growing.
Under certain conditions—during normal bone growth in
youth and during old age, for example—cartilage can become
calcified (hardened due to deposit of calcium salts). Note, how-
ever, that calcified cartilage is not bone; cartilage and bone are
always distinct tissues.
Check Your Understanding
1.
Which type of cartilage is most plentiful in the adult body?
2.
What two body structures contain flexible elastic cartilage?
3.
Cartilage grows by interstitial growth. What does this mean?
For answers, see Appendix H.
Classification of Bones
Name the major regions of the skeleton and describe their
relative functions.
Compare and contrast the four bone classes and provide
examples of each class.
±e 206 named bones of the human skeleton are divided into
two groups: axial and appendicular.
±e
axial
skeleton
forms the long axis of the body and in-
cludes the bones of the skull, vertebral column, and rib cage,
shown in orange in Figure 6.1. Generally speaking these bones
protect, support, or carry other body parts.
±e
appendicular
skeleton
(ap
0
en-dik
9
u-lar) consists of the
bones of the upper and lower limbs and the girdles (shoulder
bones and hip bones) that attach the limbs to the axial skeleton
(colored gold in Figure 6.1). Bones of the limbs help us move from
place to place (locomotion) and manipulate our environment.
Bones come in many sizes and shapes. For example, the pisi-
form bone of the wrist is the size and shape of a pea, whereas the
femur (thigh bone) is nearly 2 feet long in some people and has
a large, ball-shaped head. ±e unique shape of each bone fulfills
a particular need. ±e femur, for example, withstands great pres-
sure, and its hollow-cylinder design provides maximum strength
with minimum weight to accommodate our upright posture.
±e human skeleton is initially made up of cartilages and fi-
brous membranes, but bone soon replaces most of these early
supports. ±e few cartilages that remain in adults are found
mainly in regions where flexible skeletal tissue is needed.
Basic Structure, Types, and Locations
A
skeletal cartilage
is made of some variety of
cartilage tissue
molded to fit its body location and function. Cartilage con-
sists primarily of water, which accounts for its resilience, that
is, its ability to spring back to its original shape a²er being
compressed.
±e cartilage, which contains no nerves or blood vessels, is
surrounded by a layer of dense irregular connective tissue, the
perichondrium
(per
0
ĭ-kon
9
dre-um; “around the cartilage”). ±e
perichondrium acts like a girdle to resist outward expansion
when the cartilage is compressed. Additionally, the perichon-
drium contains the blood vessels from which nutrients diffuse
through the matrix to reach the cartilage cells internally. ±is
mode of nutrient delivery limits cartilage thickness.
As we described in Chapter 4, there are three types of carti-
lage tissue in the body: hyaline, elastic, and fibrocartilage. ±e
skeletal cartilages include examples from all three. All three
types have the same basic components—cells called
chondro-
cytes
, encased in small cavities (lacunae) within an
extracellular
matrix
containing a jellylike ground substance and fibers.
Hyaline Cartilages
Hyaline cartilages
, which look like frosted glass when freshly
exposed, provide support with flexibility and resilience. ±ey
are the most abundant skeletal cartilages. ±eir chondrocytes
are spherical (see Figure 4.8g), and the only fiber type in their
matrix is fine collagen fibers (which are undetectable micro-
scopically). Colored blue in
Figure 6.1
, skeletal hyaline carti-
lages include
Articular cartilages
, which cover the ends of most bones at
movable joints
Costal cartilages
, which connect the ribs to the sternum
(breastbone)
Respiratory cartilages
, which form the skeleton of the
larynx
(
voicebox
) and reinforce other respiratory passageways
Nasal cartilages
, which support the external nose
Elastic Cartilages
Elastic cartilages
resemble hyaline cartilages (see Figure 4.8h),
but they contain more stretchy elastic fibers and so are better
able to stand up to repeated bending. ±ey are found in only
two skeletal locations, shown in green in Figure 6.1—the exter-
nal ear and the epiglottis (the flap that bends to cover the open-
ing of the larynx each time we swallow).
Fibrocartilages
Highly compressible with great tensile strength,
fibrocartilages
consist of roughly parallel rows of chondrocytes alternating
with thick collagen fibers (see Figure 4.8i). Fibrocartilages occur
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