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U N I T 1 2
Musculoskeletal Function
to cartilage, the extracellular matrix of bone is mineral-
ized, producing a hard tissue capable of providing sup-
port for the body and protection for its vital structures.
Cartilage
Cartilage is an avascular tissue that consists of cells,
called
chondrocytes
, and an extensive extracellular
matrix composed of fibers and ground substance. The
chondrocytes are sparse, but are essential to the produc-
tion and maintenance of the matrix. A connective tissue
sheath, called the
perichondrium
, overlies the free sur-
faces of most cartilage. The perichondrium is vascular
and its blood vessels supply nutrients to the chrondro-
cytes. In areas where the cartilage has no perichon-
drium, such as the articular surfaces of moving joints,
the chondrocytes receive their nourishment from the
synovial fluid that bathes the joint surfaces.
Types of Cartilage
There are three types of cartilage, each exhibiting varia-
tions in matrix composition: elastic, fibrocartilage, and
hyaline.
Elastic cartilage
has a dense network of elastic
fibers scattered throughout its matrix. It is found in areas
where flexibility is important, such as in the auricle of
the ear, the auditory canal, the epiglottis, and the larynx.
Fibrocartilage
is characterized by a matrix containing a
combination of chrondrocytes and dense fibrous tissue.
It is found in areas subjected to pulling forces such as
attachments of ligaments to the cartilaginous surface of
bones, the symphysis pubis, and intervertebral disks.
Hyaline cartilage
is the most abundant and the best
studied of the three types. It forms the articulating sur-
faces of the moveable joints in the body, costal carti-
lages that join the ribs to the sternum and vertebrae, and
many of the cartilages of the respiratory tract. Hyaline
cartilage is also essential for growth before and after
birth. In the embryo, most of the axial and appendicular
skeleton is formed first as a cartilage model and then
is replaced by bone. In postnatal life, hyaline cartilage
continues to play an essential role in the growth of long
bones and persists as articular cartilage in the adult.
The matrix of hyaline cartilage is composed of col-
lagen fibers embedded in a firm, hydrated gel of struc-
tural glycoproteins and proteoglycans, which have a
high affinity for water. This high degree of hydration in
the cartilage matrix contributes to the weight-bearing
properties of cartilage and allows the cartilage matrix
to respond to varying pressure loads. The multiadhesive
glycoproteins influence interactions between the chon-
drocytes and matrix molecules and have clinical value as
markers of cartilage turnover and degeneration.
Repair of Hyaline Cartilage
Hyaline cartilage can tolerate considerable amounts of
intense and repetitive stress, but manifests a striking
inability to heal from even the most minor injuries. This
lack of response to injury is attributable to the avascular
nature of cartilage, the immobility of chondrocytes, and
the limited ability of chondrocytes to proliferate. Some
repair can occur, but only if the defect involves the peri-
chondrium, in which case chondrogenic cells from the
perichondrium enter the defect and form new cartilage.
If the defect is large, the cells form dense connective tis-
sue to repair the injury. Hyaline cartilage is also subject
to calcification, a process in which calcium phosphate
crystals become embedded in the cartilage matrix. In
most situations, given sufficient time, cartilage that cal-
cifies will be replaced by bone. Chondrocytes normally
derive all of their nutrients and dispose of waste materi-
als through the extracellular matrix. When the matrix
becomes heavily calcified, diffusion is impeded and the
chondrocytes swell and die. The ultimate consequence
of this event is removal of the calcified matrix and its
replacement by bone.
Bones and Bone Structure
Bones are the major component of the skeletal system.
Besides contributing to body shape and form, bones
perform several other important functions. They pro-
vide the hard framework that supports the body and
provides protection for its delicate soft tissues and
organs. For example, the bones of lower limbs act as
pillars to support the body trunk when we stand, and
Axial skeleton
Appendicular
skeleton
FIGURE 42-1.
The axial (blue) and appendicular (uncolored)
skeleton.
