160
UNIT ONE
(intervertebral discs) between the individual bones (ver-
tebrae) of the spinal column. It also cushions bones in the
knees and in the pelvic girdle.
Bone
Bone
(osseous tissue) is the most rigid connective tissue. Its
hardness is largely due to mineral salts, such as calcium phos-
phate and calcium carbonate, between cells. This extracellular
matrix also contains abundant collagenous F bers, which are
fl exible and reinforce the mineral components of bone.
Bone internally supports body structures. It protects
vital structures in the cranial and thoracic cavities and is
an attachment for muscles. Bone also contains red marrow,
which forms blood cells. It stores and releases inorganic
chemicals such as calcium and phosphorus.
Bone matrix is deposited by bone cells, called
osteoblasts,
in thin layers called
lamellae,
which form concentric pat-
terns around capillaries located within tiny longitudinal tubes
called
central,
or
Haversian, canals.
Once osteoblasts are in
lacunae surrounded by matrix, they are called osteocytes and
are rather evenly spaced within the lamellae. Consequently,
osteocytes also form concentric circles
(f g. 5.26)
.
lacks a direct blood supply, blood vessels are in the surround-
ing perichondrium. Cartilage cells near the perichondrium
obtain nutrients from these vessels by diffusion, aided by the
water in the extracellular matrix. This lack of a direct blood
supply is why torn cartilage heals slowly and why chondro-
cytes do not divide frequently. The three types of cartilage are
distinguished by their different types of extracellular matrix.
Hyaline cartilage
(f g. 5.23)
, the most common type, has
very F ne collagenous F bers in its extracellular matrix and looks
somewhat like white glass. It is found on the ends of bones in
many joints, in the soft part of the nose, and in the support-
ing rings of the respiratory passages. Parts of an embryo’s skel-
eton begin as hyaline cartilage “models” that bone gradually
replaces. Hyaline cartilage is also important in the develop-
ment and growth of most bones (see chapter 7, p. 197).
Elastic cartilage
(f g. 5.24)
is more fl
exible than hya-
line cartilage because its extracellular matrix has a dense
network of elastic F
bers. It provides the framework for the
external ears and parts of the larynx.
Fibrocartilage
(f
g. 5.25)
, a very tough tissue, has many
collagenous F bers. It is a shock absorber for structures sub-
jected to pressure. ±or example, F brocartilage forms pads
Chondrocyte
Elastic fibers
Nucleus
Extracellular
matrix
(a)
(b)
Lacuna
FIGURE 5.24
Elastic cartilage contains many elastic f
bers in its extracellular matrix (1,200×).
Chondrocyte
Nucleus
Extracellular
matrix
(a)
(b)
Lacuna
FIGURE 5.23
Cartilage cells (chondrocytes) are located in lacunae, in turn surrounded by extracellular matrix
containing very f
ne collagenous f
bers (400×). This is hyaline cartilage, the most common type.
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