Layers and Composition of Cartilage

Courtesy: Prof Nabil Ebraheim, University of Toledo, Ohio, USA

 

Layers of Articular Cartilage

• The articular cartilage is avascular, meaning that it does not have a blood supply and does not have nerves.
• It is a shock absorbing structure.
• Bone is made first, so bone contains collagen Type I.
• Cartilage is made after the bone, so cartilage contains collagen Type Il.

In this lecture, we will highlight three things:
1.Composition of the cartilage.
2.Layers of the cartilage.
3If the cartilage is injured, can it heal?

Cartilage is made of water, collagen Type II and proteoglycans.

• Water (about 74%)
• Collagen (about 15%)
• Proteoglycans (about 1%)
• Other material (less than 1%)

 

• The cartilage needs water for lubrication and for transport of nutrients.
• The cartilage will need collagen to give the cartilage the tensile strength and stiffness.
• The collagen makes a mesh work that is both flexible and tough, but entraps the proteoglycans and the cartilage cells.
• The cartilage cells (chondrocytes) is about 1% and it is responsible for the synthesis, the maintenance, and the hemostasis of cartilage.

SOX9 is the master switch for differentiation of cells of chondrocyte lineage.

 

• The other important structure is the proteoglycan.

ProteoGlycan Vs GlycosaminoGlycan

• The cartilage proteoglycans are large, long chains of negative charged molecules.
• The glycosaminoglycans are linear polysaccharides such as keratin sulfate and chondroitin sulfate
• The proteoglycan is a core protein
• glycosaminoglycan chains: consist of chondroitin sulfate and the keratin sulfate, and this is called proteoglycan monomer.

Chondroitin sulfate is the most prevalent glycosaminoglycan 

• It becomes a proteoglycan aggregate when it gets connected to the hyaluronic acid by a link protein.
• The cartilage proteoglycan is multiple glycosaminoglycans bound to a core protein which is bound to hyaluronic acid through a link protein.

The link protein connects the proteoglycan monomer to the hyaluronic acid backbone.

• Proteoglycans can be associated with up to 50% of its weight in water (the proteoglycan swells).
• It is responsible for the swelling pressure of the cartilage and the collagen restrains that swelling.
• The proteoglycans attract water because of the long chains of negative charges
• That will increase the swelling and that fluid pressure provides strength in compression.
• The swelling pressure in the cartilage is predominantly due to the association of its changeable water with the aggrecan.

Aggrecan is associated with 50 times its weight in water.

• The aggrecan aggregates on hyaluranic acid with a link protein.
• It has a longer core protein with multiple keratin sulfate and chondroitin sulfate chains.