Chapter 7. Cartilage
Chapter 7 Cartilage Prof Abdel-Majeed
Safer Histology & Hisctochemistry
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Cartilage: Introduction
Cartilage is characterized by an extracellular matrix
(ECM) enriched with glycosaminoglycans and
proteoglycans, macromolecules that interact with
collagen and elastic fibers.
Variations in the composition of these matrix
components produce three types of cartilage adapted
to local biomechanical needs.
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Cartilage is a specialized form of connective tissue in
which the firm consistency of the ECM allows the tissue
to bear mechanical stresses without permanent
distortion.
In the respiratory system cartilage forms a framework
supporting soft tissues. Because it is smooth-surfaced
and resilient,
cartilage provides a shock-absorbing and sliding area for
joints and facilitates bone movements.
Cartilage is essential for the development and growth of
long bones, both before and after birth.
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Cartilage consists of cells (chondrocytes ),
An extensive extracellular matrix composed of
fibers and ground substance.
Chondrocytes synthesize and secrete the ECM and
the cells themselves are located in matrix cavities
called lacunae.
Collagen, hyaluronic acid, proteoglycans, and small
amounts of several glycoproteins are the principal
macromolecules present in all types of cartilage
matrix.
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Note:
Because collagen and elastin are flexible, the firm
gel-like consistency of cartilage depends on
electrostatic bonds between collagen fibers and the
glycosaminoglycan side chains of matrix
proteoglycans.
It also depends on the binding of water (solvation
water) to the negatively charged glycosaminoglycan
chains that extend from the proteoglycan core
proteins.
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As a consequence of different functional
requirements,:
Three forms of cartilage have evolved.
Each one exhibiting variation in matrix composition.
In the matrix of hyaline cartilage, the most common
form, type II collagen is the principal collagen type
The more pliable and distensible elastic cartilage
possesses, in addition to collagen type II, an
abundance of elastic fibers within its matrix.
Fibrocartilage, present in regions of the body
subjected to pulling forces, is characterized by a
matrix containing a dense network of coarse type I
collagen fibers.
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Fig. 7-1
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Fig. 7-1
Distribution of cartilage in adults.
(a):three types of adult cartilage distributed in many
areas of the skeleton, particularly in joints and where
pliable support is useful, as in the ribs, ears, and
nose. Cartilage support of other tissues throughout
the respiratory system is also prominent.
The photomicrographs show the main features of (b)
hyaline cartilage, (c) fibrocartilage, and (d) elastic
cartilage.
Chapter 7 Cartilage Prof Abdel-Majeed
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avascular
Cartilage is avascular and is nourished by the
diffusion of nutrients from capillaries in adjacent
connective tissue (perichondrium) or from synovial
fluid in joint cavities.
chondrocytes exhibit low metabolic activity.
Cartilage also lacks lymphatic vessels and nerves.
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The perichondrium harbors the vascular supply for
the avascular cartilage and also contains nerves and
lymphatic vessels.
Articular cartilage, which covers the surfaces of the
bones in movable joints, is devoid of perichondrium
and is sustained by the diffusion of oxygen and
nutrients from the synovial fluid.
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Fig. 7-2
Chapter 7 Cartilage Prof Abdel-Majeed
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Hyaline Cartilage
Hyaline cartilage is the most common and best studied .
Fresh hyaline cartilage is bluish-white and translucent.
In the embryo, it serves as a temporary skeleton until it is
gradually replaced by bone.
In adult mammals,
hyaline cartilage is located in the articular surfaces of the
movable joints, in the walls of larger respiratory passages
(nose, larynx, trachea, bronchi), in the ventral ends of
ribs, where they articulate with the sternum.
in the epiphyseal plate, where it is responsible for the
longitudinal growth of bone.
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Fig. 7-2
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Safer Histology & Hisctochemistry
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Matrix
Forty percent of the dry weight of hyaline cartilage
consists of collagen embedded in a firm, hydrated gel of
proteoglycans and structural glycoproteins.
In routine histology preparations, the collagen is
indiscernible for two reasons:
the collagen is in the form of fibrils which have
submicroscopic dimensions and
the refractive index of the fibrils is almost the same
as that of the surrounding substances.
Type II collagen
Hyaline cartilage contains primarily type II collagen,
although small amounts of collagen types VI and IX are
Chapter 7 Cartilage Prof Abdel-Majeed
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also present.
Safer Histology & Hisctochemistry
Cartilage proteoglycans contain chondroitin 4-sulfate,
chondroitin 6-sulfate, and keratan sulfate, covalently
linked to core proteins.
Proteoglycans are bound noncovalently to long molecules
of hyaluronic acid by link proteins, forming very large
proteoglycan aggregates such as aggrecan that interact
with collagen.
Structurally, proteoglycans resemble bottle brushes, the
protein core being the stem and the radiating
glycosaminoglycan (GAG) chains the bristles.
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Fig. 7-3
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solvation water
The high content of solvation water bound to the negative
charges of the GAGs acts as a shock absorber or
biomechanical spring; this is of great functional
importance, especially in articular cartilages.
Structural multiadhesive glycoprotein chondronectin:
In addition to type II collagen and proteoglycan, an
important component of cartilage matrix is the structural
multiadhesive glycoprotein chondronectin. Like fibronectin
in connective tissue, this macromolecule binds specifically
to GAGs, collagen type II and integrins, mediating the
adherence of chondrocytes to the ECM.
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Cartilage matrix is generally basophilic due to the
high concentration of sulfated GAGs and staining
variations within the matrix reflect differences in the
molecular composition.
Immediately surrounding each chondrocyte the
ECM is richer in GAGs and poor in collagen.
These areas comprise the territorial matrix and
usually stain differently from the rest of the matrix.
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Fig. 7-2
Chapter 7 Cartilage Prof Abdel-Majeed
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Cells:
Chondrocytes
At the periphery of hyaline cartilage, young
chondrocytes have an elliptic shape, with the long
axis parallel to the surface. Farther in, they are
round and may appear in groups of up to eight cells
originating from mitotic divisions of a single
chondrocyte.
These groups are called isogenous aggregates (Gr.
isos, equal, + genos, family).
Chondrocytes synthesize collagens and the other
matrix molecules.
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As matrix is produced, cells in the aggregates are
moved apart and occupy separate lacunae.
Cartilage cells and the matrix often shrink during
routine histologic preparation,
resulting in both the irregular shape of the
chondrocytes and their retraction from the matrix.
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Chondrocytes respire under low oxygen tension.
Hyaline cartilage cells metabolize glucose mainly by
anaerobic glycolysis to produce lactic acid as the
end product.
Nutrients from the blood diffuse through the
perichondrium to reach the more deeply placed
cartilage cells.
Transport of water and solutes is promoted by the
pumping action of intermittent cartilage
compression and decompression.
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Chondrocyte function is hormone dependent.
Synthesis of sulfated GAGs is
accelerated by:
growth hormone,
thyroxin,
testosterone
slowed by:
cortisone,
hydrocortisone,
estradiol.
.
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Cartilage growth depends mainly on the pituitaryderived growth hormone somatotropin. This hormone
does not act on cartilage cells directly but promotes the
endocrine release in the liver of insulin-like growth
factor-1 (IGF-1), sometimes called somatomedin C. IGF-1
acts directly on cartilage cells, promoting their growth
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Perichondrium
Except in the articular cartilage of joints, all hyaline
cartilage is covered by a layer of dense connective
tissue, the perichondrium, which is essential for the
growth and maintenance of cartilage.
It consists largely of collagen type I fibers and
contains numerous fibroblasts.
Although cells in the inner layer of the
perichondrium resemble fibroblasts, they are
precursors for chondroblasts which divide and
differentiate into chondrocytes.
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Fig. 7-2
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Elastic Cartilage
Elastic cartilage is essentially very similar to hyaline
cartilage except that it contains an abundant network of
fine elastic fibers in addition to collagen type II fibrils
Fresh elastic cartilage has a yellowish color owing to the
presence of elastin in the elastic fibers.
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Fig. 7-4
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Elastic cartilage is frequently found to be gradually
continuous with hyaline cartilage.
Elastic cartilage possesses a perichondrium.
Elastic cartilage is found in the auricle of the ear, the walls
of the external auditory canals, the auditory (eustachian)
tubes, the epiglottis, and the cuneiform cartilage in the
larynx.
Chapter 7 Cartilage Prof Abdel-Majeed
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Fibrocartilage
Intermediate between dense connective tissue and
hyaline cartilage.
It is found in intervertebral disks, in attachments of
certain ligaments, and in the pubic symphysis.
Fibrocartilage is always associated with dense
connective tissue and the border between these two
tissues is not clear-cut, showing a gradual transition.
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Chapter 7 Cartilage Prof Abdel-Majeed
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Fibrocartilage contains chondrocytes, either singly
or in isogenous aggregates, usually arranged axially,
in long rows separated by coarse collagen type I
fibers and less proteoglycans than other forms of
cartilage.
Because it is richer in collagen type I, the
fibrocartilage matrix is more acidophilic.
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Fig. 7-5
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In fibrocartilage dense collagen fibers can form either
irregular or parallel bundles between the axial
aggregates of chondrocytes .
The general orientation of the collagen depends on the
stresses on fibrocartilage,
since the collagen bundles take up a direction parallel to
those stresses. There is no distinct perichondrium in
fibrocartilage.
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Fig. 7-5
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Intervertebral disks are composed of fibrocartilage
primarily.
Between the vertebrae and are held to them by
ligaments.
Each disk has two major histological components:
the peripheral annulus fibrosus rich in bundles of
type I collagen
and the central nucleus pulposus with a gel-like
matrix rich in hyaluronic acid.
Intervertebral disks act as lubricated cushions and
shock absorbers preventing adjacent vertebrae from
being damaged by abrasive forces or impact during
movement of the spinal column.
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Cartilage Formation,Growth and Repair
All cartilage derives from the embryonic mesenchyme in
the process of chondrogenesis.
The first indication of cell differentiation is the rounding
up of the mesenchymal cells,
which retract their extensions, multiply rapidly, and
form cellular condensations.
The cells formed by this direct differentiation of
mesenchymal cells, now called chondroblasts,
have a ribosome-rich basophilic cytoplasm.
Synthesis and deposition of the matrix then begin to
separate the chondroblasts from one another.
Chapter 7 Cartilage Prof Abdel-Majeed
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Chapter 7 Cartilage Prof Abdel-Majeed
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Chapter 7 Cartilage Prof Abdel-Majeed
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• During embryonic development, the differentiation
of cartilage takes place primarily from the center
outward;
• therefore, the more central cells have the
characteristics of chondrocytes,
• whereas the peripheral cells are typical
chondroblasts.
• The superficial mesenchyme develops into the
perichondrium.
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Fig. 7-6
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Fig. 7-7
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Further growth of cartilage is attributable to two
processes:
interstitial growth, resulting from the mitotic division
of preexisting chondrocytes;
appositional growth, resulting from the differentiation
of perichondrial cells.
In both cases, the synthesis of matrix contributes
greatly to the growth of the cartilage. Interstitial
growth is the less important of the two processes
postnatally.
It occurs during the early phases of cartilage formation,
when it increases tissue mass by expanding the
cartilage matrix from
within
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Interstitial growth also occurs in the epiphyseal plates of
long bones and within articular cartilage.
In the epiphyseal plates, interstitial growth is important
in increasing the length of long bones.
In articular cartilage, as the cells and matrix near the
articulating surface are gradually worn away, the
cartilage must be replaced from within, since there is
no perichondrium there to add cells by appositional
growth.
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In cartilage found elsewhere in the body,
interstitial growth becomes less pronounced, as the
matrix becomes increasingly rigid from the cross-linking
of matrix molecules.
Cartilage then increases in girth only by appositional
growth.
Chondroblasts differentiate in the inner layers of the
perichondrium, proliferate, and become chondrocytes
once they have surrounded themselves with
cartilaginous matrix and are incorporated into the
existing cartilage.,
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Fig. 7-2
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regeneration
Except in young children, damaged cartilage
undergoes slow and often incomplete regeneration,
by activity of cells in the perichondrium which invade
the injured area and generate new cartilage.
In extensively damaged areas—and occasionally in
small areas—the perichondrium produces a scar of
dense connective tissue instead of forming new
cartilage.
The poor regenerative capacity of cartilage is due in
part to the avascularity
of this tissue.
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End of chapter 7
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