134
UNIT 1
Organization of the Body
4
Chondroblasts
, the predominant cell type in growing carti-
lage, produce new matrix until the skeleton stops growing at the
end of adolescence. Te firmness of the cartilage matrix prevents
the cells from becoming widely separated, so
chondrocytes
, or
mature cartilage cells, are typically found in small groups within
cavities called
lacunae
(lah-ku
9
ne; “pits”).
Homeostatic Imbalance
4.2
Because cartilage is avascular and aging cartilage cells lose their
ability to divide, injured cartilages heal slowly. Tis phenom-
enon is excruciatingly familiar to those who have experienced
sports injuries. During later life, cartilages tend to calcify or
even ossify (become bony). In such cases, the chondrocytes are
poorly nourished and die.
Tere are three varieties of cartilage:
hyaline cartilage
,
elas-
tic cartilage
, and
fibrocartilage
, each dominated by a particular
fiber type.
Hyaline Cartilage
Hyaline cartilage
(hi
9
ah-līn), or
gristle
, is
the most abundant cartilage in the body. Although it contains
large numbers of collagen fibers, they are not apparent and the
matrix appears glassy (
hyal
5
glass, transparent) blue-white
when viewed by the unaided eye. Chondrocytes account for
only 1–10% of the cartilage volume (Figure 4.8g).
Hyaline cartilage provides firm support with some pliability.
It covers the ends of long bones as
articular cartilage
, providing
springy pads that absorb compression at joints. Hyaline carti-
lage also supports the tip of the nose, connects the ribs to the
sternum, and supports most of the respiratory system passages.
Most of the embryonic skeleton consists of hyaline cartilage be-
fore bone forms. Skeletal hyaline cartilage persists during child-
hood as the
epiphyseal plates
(e
0
pĭ-fis
9
e-ul), actively growing
regions near the ends of long bones.
Elastic Cartilage
Histologically,
elastic cartilage
(Figure 4.8h) is
nearly identical to hyaline cartilage. However, elastic cartilage has
many more elastic fibers. Found where strength and exceptional
stretchability are needed, elastic cartilage forms the “skeletons” of
the external ear (the pinna) and the epiglottis (the flap that covers
the opening to the respiratory passageway when we swallow).
Fibrocartilage
Structurally,
fibrocartilage
is intermediate be-
tween hyaline cartilage and dense regular connective tissues. Its
rows of chondrocytes (a cartilage feature) alternate with rows of
thick collagen fibers (characteristic of dense regular connective
tissue) (Figure 4.8i). Because it is compressible and resists ten-
sion well, fibrocartilage is found where strong support and the
ability to withstand heavy pressure are required: for example,
the intervertebral discs (resilient cushions between the bony
vertebrae) and the spongy cartilages of the knee (menisci) (see
Figure 6.1, p. 175).
(f)
Connective tissue proper: dense connective tissue, elastic
Description:
Dense regular connective tissue
containing a high proportion of elastic fibers.
Function:
Allows tissue to recoil after
stretching; maintains pulsatile flow of blood
through arteries; aids passive recoil of lungs
following inspiration.
Location:
Walls of large arteries; within
certain ligaments associated with the vertebral
column; within the walls of the bronchial tubes.
Elastic fibers
Aorta
Heart
Photomicrograph:
Elastic connective tissue in the wall of the
aorta (250
m
).
Figure 4.8
(continued)
Connective tissues. (f)
Connective tissue proper. (For a related image,
see
A Brief Atlas of the Human Body
, Plate 16.)
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