Identification of important interactions between subchondral bone ...

2.2.4 Articular cartilage
CHAPTER 2: Introduction
In a normal knee joint, the epiphyses of femur and tibia are covered with a layer of articular
cartilage, which functions as shock-absorber and provides a friction-free surface that enables the
bones to move smoothly against each other. Articular cartilage is avascular and is not innervated.
In this way, nutrients from the synovial fluid are transported to the chondrocytes (the only cell
type in cartilage), and waste products are transported back to the synovial fluid by diffusion.
Chondrocytes are cartilage specific and comprise 2-5% of the total cartilage 5,11,14,49 . Their function
is to produce and maintain the surrounding matrix. Hence, the physical condition of cartilage
ultimately depends on the health of these cells. Similar to osteoblasts, chondrocytes derive from
the mesenchymal stem cells 19,50 , and they differentiate into pre-hypertrophic and hypertrophic
chondrocytes before they undergo apoptosis. Even though chondrocytes are metabolically very
active, they normally do not divide after adolescence. Only small defects associated with minimal
loss of matrix components are regenerated. More extensive defects exceed the repair capacity,
and consequently the damage becomes permanent 51,52,53 .
Cartilage tissue has unique viscoelastic and compressive properties provided by the
extracellular matrix (ECM) (fig. 6), which consists mainly of collagen type II (15%) and forms a
fibrous network that entraps proteoglycans (10%), of which aggrecan is the predominant
molecule. The aggrecan molecule is a protein that covalently binds sulphated glycosaminoglycan
(sGAG) chains to the core protein, which is non-covalently attached to hyaluronic acid. The
aggrecans attract water molecules, due to the high negatively charged sGAGs, and allow the
tissue to withstand compressive forces (water comprises approximately 70% of
cartilage) 5,14,49,54,55,56 . The ECM also contains a number of less abundant molecules such as minor
collagens, growth factors and adhesive molecules, which are out of the scope of this thesis 14 .
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Fig. 6. Cartilage components.
The articular cartilage is mainly
composed of collagen type II,
chondrocytes and aggrecan
attached to hyaluronic acid. The
figure was produced by
Madsen, S.H.