CARTILAGE - Islamic University of Gaza

CARTILAGE Dr. Emad I Shaqoura

M.D, M.Sc. Anatomy

Faculty of Medicine,

Islamic University-Gaza

October, 2015

Introduction

Hyaline Cartilage

Elastic Cartilage

Fibrocartilage

Cartilage Formation, Growth, & Repair

CARTILAGE, DR. EMAD I SHAQOURA, IUG FACULTY OF MEDICINE


Cartilage is a tough, flexible form of connective tissue.

It is composed of:

1. Chondrocytes: that synthesize and maintain ECM components

and are located in matrix cavities called lacunae.

2. An extracellular matrix (ECM) rich in GAGs and proteoglycans,

which interact with collagen and elastic fibers.


The physical properties of cartilage depend on electrostatic

bonds between the collagen and elastin fibers and the GAGs.

Its semi-rigid consistency is attributable to water bound to

the negatively charged sulfated GAGs.

Cartilage is avascular and receives nutrients by diffusion from

capillaries in adjacent connective tissue (perichondrium), so,

chondrocytes exhibit low metabolic activity.

Cartilage also lacks lymphatic vessels and nerves.


Copyright © McGraw-Hill Companies

FIGURE 7-1

Support of soft tissue e.g., respiratory tract, ear & nose.

Cartilage provides shock absorbing and sliding regions within

joints and facilitates bone movements.

Cartilage also guides development and growth of long bones,

both before and after birth.


Hyaline Cartilage

Elastic Cartilage

Fibrocartilage



Hyaline (Gr. hyalos , glassy) cartilage, is the most common

of the three forms.

It is homogeneous and semitransparent in the fresh state.


Sites 1. Articular surfaces of movable joints.

2. Walls of larger respiratory passages (nose, larynx, trachea, bronchi).

3. Costal cartilages.

4. Epiphyseal plates of long bones.

4. Temporary skeleton of the embryo.


Osteoarthritis, a chronic condition that commonly occurs

during aging, involves the gradual loss or changed physical properties of

the hyaline cartilage that lines the articular ends of bones in joints.

Joints that are weight bearing (knees, hips) or heavily used (wrist,

fingers) are most prone to cartilage degeneration.

Fragments released by wear-and-tear to the articular cartilage trigger

secretion of matrix metalloproteinases and other factors from

macrophages in adjacent tissues, which exacerbate damage and cause

pain and inflammation within the joint.


The dry weight of hyaline cartilage is 40% collagen (mainly

type II) embedded in a firm, hydrated gel of proteoglycans and

structural glycoproteins.

In routine histology preparations, the proteoglycans cause the

matrix to be generally basophilic and the thin collagen fibrils are

barely discernible.

Aggrecan (250 kD), with approximately 150 GAG side chains of

chondroitin sulfate and keratan sulfate, is the most abundant

proteoglycan of hyaline cartilage.


Hundreds of aggrecan proteoglycans are bound non-

covalently by link proteins to long polymers of hyaluronic

acid.

These proteoglycan complexes bind further to the surface

of type II collagen fibrils.

Water bound to GAGs in the proteoglycans constitutes up

60%-80% of the weight of fresh hyaline cartilage.



FIGURE 7-2

Chondronectin is the main glycoprotein of cartilage that

binds specifically to GAGs, collagen type II, and integrins,

mediating the adherence of chondrocytes to the ECM.

Staining variations within the matrix reflect local

differences in its molecular composition.

Immediately surrounding each chondrocyte, the territorial

matrix (richer in GAGs) stains differently from the intervening

areas of interterritorial matrix (more collagen).



FIGURE 7-2

Cells occupy relatively little of the hyaline cartilage mass.

Two cell types are present:

1. Young chondrocytes (chondroblasts):

Present at the periphery of the cartilage.

Have an elliptic shape, with the long axis parallel to the surface.

2. Mature Chondrocytes:

Deeper in the cartilage.

They are round.

They may appear in groups of up to eight cells that originate from mitotic divisions of

a single chondrocyte and are called isogenous aggregates.



FIGURE 7-2


FIGURE 7-3

As the chondrocytes become more active in secreting

collagens and other ECM components, the aggregated cells

are pushed 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.

In living tissue, and in properly prepared sections, the

chondrocytes fill the lacunae completely.


Because cartilage is devoid of blood capillaries,

chondrocytes respire under low-oxygen tension.

Hyaline cartilage cells metabolize glucose mainly by

anaerobic glycolysis to produce lactic acid as the end

product.

Chondrocyte synthesis of sulfated GAGs and secretion of

proteoglycans are accelerated by many hormones and growth

factors e.g., GH.


Cells of cartilage can give rise to either benign (chondroma)

or slow-growing, malignant (chondrosarcoma) tumors in which

cells produce normal matrix components.

Chondrosarcomas seldom metastasize and are generally

removed surgically.


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.

The perichondrium consists largely of collagen type I fibers and

fibroblasts.

Among these fibroblasts in the inner layer of the perichondrium are

progenitor cells for chondroblasts that divide and differentiate into

chondrocytes.



FIGURE 7-2


FIGURE 7-3

Nutrients from the blood diffuse from the perichondrium to

reach the deeper chondrocytes.

Transport of water and solutes in the matrix is promoted by

the pumping action of intermittent cartilage compression and

decompression.

Because of the limits of diffusion, the maximum thickness of

the hyaline cartilage is limited and it usually exists as small,

thin plates.


The inability of cartilage to regenerate or to be repaired

fully may be attributed to the chondrocytes’ immobility, low

metabolic and mitotic rates, and avascularity.

If a cartilage injury involves the perichondrium, new

chondroblasts and fibroblasts may be mobilized and limited

repair can occur, but most of the new tissue produced is

dense connective tissue and normal function of the cartilage

is often impaired.



Elastic cartilage is similar to hyaline cartilage

except that it contains an abundant network of elastic fibers

in addition to collagen type II, which give fresh

elastic cartilage a yellowish color.

Demonstration of the elastic fibers usually requires stains

such as orcein or resorcin fuchsin.



FIGURE 7-4

Elastic cartilage is found in:

1. The auricle of the ear.

2. The walls of the external auditory canals.

3. The auditory (eustachian) tubes.

4. The epiglottis, and the cuneiform cartilage in the larynx.

Elastic cartilage in these locations includes a perichondrium

similar to that of most hyaline cartilage.



Fibrocartilage is essentially a combination of hyaline

cartilage and dense connective tissue with gradual transitions

between these tissues.

It is found in intervertebral discs, in attachments

of certain ligaments, and in the pubic symphysis.

Chondrocytes of fibrocartilage occur singly and in

aligned isogenous aggregates and produce matrix containing

type II collagen.



FIGURE 7-5

Regions with chondrocytes and hyaline matrix are separated

by other regions containing bundles of type I collagen and

scattered fibroblasts.

The relative scarcity of proteoglycans makes the matrix of

fibrocartilage more acidophilic than that of hyaline or elastic

cartilage.

There is no distinct surrounding perichondrium in

fibrocartilage.



All cartilage forms from 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 become more densely packed together.

The dividing cells are typically called chondroblasts and

chondrocytes when proliferation has ceased; both have

basophilic cytoplasm rich in RER for collagen synthesis.



FIGURE 7-6


FIGURE 7-7

Production of the ECM encloses the cells in their lacunae

and then gradually separates chondroblasts from one another.

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 forms the perichondrium.


The cartilage tissue enlarges by:

1. Interstitial growth, resulting from the mitotic division of

preexisting chondroblasts.

2. Appositional growth, which involves differentiation of new

chondroblasts from the perichondrium.

In both cases, the synthesis of matrix contributes greatly to

the growth of the cartilage.


Appositional growth of cartilage is more important during

postnatal development, although interstitial growth in the

articular cartilage and epiphyseal plates of long bones is

important in increasing the length of long bones.

In articular cartilage, cells and matrix near the articulating

surface are gradually worn away and must be replaced from

within, because there is no perichondrium to add cells by

appositional growth.


Damaged cartilage undergoes slow and often incomplete

repair, except in young children.

It occurs primarily by activity of cells in the perichondrium,

which invade the injured area and produce new cartilage.

In extensively damaged areas the perichondrium produces a

scar of dense connective tissue instead of forming new cartilage.

The poor capacity of cartilage for repair or regeneration is due

in part to the avascularity of this tissue.


In contrast to other forms of cartilage and other tissues,

hyaline cartilage is susceptible to calcification during aging.

Calcification of the hyaline matrix, accompanied by

degenerative changes in the chondrocytes, is a common

part of the aging process.

It resembles endochondral ossification by which bone is

formed in many respects .




Documents

CARTILAGE - Islamic University of Gaza