The biology of cartilage
Hyaline Cartilage

has a biomechanic function

is localized on the articular
surfaces of the joints.
Cartilage function

Shock absorbant:
although it is at most only a few
millimeters thick, it has
surprising stiffness to
compression and resilience; it
also has an exceptional ability
to distribute loads, thereby
minimizing peak stresses on
subchondral bone.

Most important
characteristic: its
durability. In most
people it provides normal joint
function for eighty years or
more. No synthetic material
approaches this level of
performance.
Cartilage histology

it’s a skeletal connective
tissue

it has a mesodermal origin

it has a peculiar
organization.
Cartilage histology
Adult articular cartilage appears to
be a simple inert tissue. Light
microscopy shows that it consists
primarily of ECM, with only one type
of cell, the chondrocyte.
It lacks blood vessels, lymphatic
vessels and nerves.
Compared with tissues such as
muscle or bone, cartilage has a low
level of metabolic activity and is
less responsive to changes in
loading and to injury.
Cartilage histology

elastic cartilage

fibrous cartilage

hyaline cartilage
Cartilage histology

elastic cartilage

fibrous cartilage

hyaline cartilage
Cartilage histology

elastic cartilage

fibrous cartilage

hyaline cartilage
Hyaline cartilage components:

cellular component : chondrocytes (about
1% of the total volume)

extracellular matrix : collagen type II, HA,
glycosaminoglycans, proteoglycans, water.
As in other connective tissues, articular cartilage derives its
form and mechanical properties from its matrix
The chondrocytes
The chondrocytes
Chondrocytes surround themselves with their
ECM and do not form cell-to-cell contacts.
Chondrocyte characteristics:
• Spheroidal shape
• Synthesis of type II collagen, large
aggregating proteoglycans and specific
non-collagenous proteins
• Formation of these molecules into the
ECM.
• High metabolic activity (the total
metabolic activity of the tissue is low
due to the low cell density)
The chondrocytes
Maintenance of the articular surface
requires turnover of the matrix
macromolecules and alterations in the
macromolecular framework of the
matrix in response to use of the joint.
To accomplish these activities, the
cells must sense changes in the
composition of the matrix that are due
to degradation of macromolecules, as
well as changes in the demands
placed on the articular surface.
This sensing function is probably
performed by short cilia extending from
the cell into the matrix.
The chondrocytes
In adult animals, chondrocytes derive
their nutrition from nutrients in the
synovial fluid, which, to reach the cell,
must pass through a double diffusion
barrier:
• synovial tissue and synovial fluid
• cartilage matrix.
The nature of this system leaves
chondrocytes with a low concentration
of oxygen relative to most other
tissues; therefore, they depend
primarily on anaerobic metabolism.
The extracellular matrix
Collagen fibers
Chondrocite
Proteoglican
The extracellular matrix
It’s made of tissue fluid and the
framework of structural
macromolecules that give the tissue
its form and stability.
Tissue fluid:
• water: 80% of wet weight of articular
cartilage
• gases, small proteins, metabolites and
a high concentration of cations to
balance the negatively charged
proteoglycans.
The extracellular matrix
Structural macromolecules (20-40% of the
wet weight of the tissue):
• collagens (60% of dry weight of
cartilage)
• proteoglycans (25-35% of dry weight)
• glycoproteins (15-20% of dry weight)
Collagens are distributed uniformly throughout the
depth of the cartilage and provide the tissue with
its form and tensile strength.
Proteoglycans and glycoproteins bind to the
collagen meshwork and help in keeping water
trapped inside.
The collagens
Articular cartilage contains multiple
genetically distinct collagen types:
• type II
• type VI
• type IX
• type X
• type XI
Collagens type II, IX and XI form the cross-banded
fibrils seen with electron microscopy.
Collagen type II accounts for 90 to 95% of the
collagen in articular cartilage.
The proteoglycans
Consist of a protein core and one
or more glycosaminoglycan
chains (long unbranched polysaccharide
chains consisting of repeating
disaccharides).
Two classes in articular cartilage:
• Large aggregating
proteoglycan monomers or
aggrecans.
• Small proteoglycans
(decorin, fibromodulin, etc.)
Proteoglycan aggregates
Proteoglycan aggregates
Most aggregans are linked with hyaluronic acid to form
proteoglycan aggregates.
These large molecules have a central backbone of
hyaluronan that can range in length from several hundred to
more than 10,000 nanometers. Large aggregates may have
more than 300 associated aggrecan molecules.
Hyaluronan
• It’s one of the critical molecules
for the maintenance of the
physico-chemicals
characteristics of the
extracellular matrix of articular
cartilage.