8.4 Articular cartilage composition, microstructure
- Typical total thickness: 2-4 mm
- Chondrocytes: cartilage cells
- Extracellular Matrix (ECM)
- Predominant component is water
- Type II collagen, Proteoglycans
- Charged molecules, ions
- Fractional composition varies by zone
- Avascular, aneural, low metabolic rate
[ ( slide credit: @Geeslin2016Cartilage ) ]
8.4.1 Chondrocytes
- Chondrocytes (cartilage cells)
- Sparsely distributed cells in articular cartilage
- 5% of wet weight; Less than 10% of tissue volume (water is largest by weight)
- Manufacture, secrete, and maintain organic component of extracellular matrix (ECM)
8.4.2 Chondrocyte Distribution in Articular Cartilage
@Jastifer2010Cartilage
- Superficial tangential zone (10-20%)
- Chondrocytes oblong, parallel to articular surface
- Middle Zone (40-60%)
- Chondrocytes round
- Deep Zone (30%)
- Calcified Zone
- Chondrocytes arranged in columnar fashion
- Subcondral Bone
- between calcified and non-calcified tissue
[ ( slide credit: @Jastifer2010Cartilage ) ]
8.4.3 Articular cartilage zones
@Geeslin2016Cartilage
@Ulrich-Vinther2003
[ ( slide credit: @Geeslin2016Cartilage ) ]
8.4.4 Organization of Cartilage
@Bartel2006
@Bartel2006
- Fig (a) Three phases of cartilage: porous matrix (made up of the proteoglycan aggrecan and collagen fibers), water, and charged ions associated with the proteoglycans.
- Fig (b) The molecular structure of the proteoglycan monomer (top and aggregate (bottom). (from @Mow1991)
8.4.5 Slightly simplified view of Extracellular (porous) matrix
- Organic matrix
- Composed of dense framework of type II collagen fibrils enmeshed in concentration of proteoglycans (Proteoglycan)
- Collagen (15-22% of wet weight)
- Proteoglycans (4-7% of wet weight)
- 60-85% water content, inorganic salts, other proteins, glycoproteins, and lipids
[ ( slide adapted: @Jastifer2010Cartilage ) ]
We’ll talk about most of these components
8.4.6 Collagen
The five most common types are:
- Type I: skin, tendon, vasculature, organs, bone (main component of the organic part of bone)
- Type II: cartilage (main collagenous component of cartilage)
- Type III: reticulate (main component of reticular fibers), commonly found alongside type I
- Type IV: forms basal lamina, the epithelium-secreted layer of the basement membrane
- Type V: cell surfaces, hair, and placenta
Note:
- Type II in Hyaline cartilage, Type I in Fibrocartilage
8.4.7 Collagen
- Most abundant protein in the body
- Think of it structurally as a rope
- Provides fibrous ultrastructure in cartilage
- Tropocollagen is basic biological unit of collagen
- Composed of 3 α chains coiled in left hand helices
- α chains coiled around each other in right hand triple helix
- Form tropocollagen molecules
- Cross links formed between tropocollagen molecules high tensile strength
@Jastifer2010Cartilage
[ ( slide credit: @Jastifer2010Cartilage ) ]
8.4.8 Collagen Structure
@Jastifer2010Cartilage
[ ( slide credit: @Jastifer2010Cartilage ) ]
8.4.9 Structure and Arrangement of Collagen in Articular Cartilage
Jazrawi et al. J Am Acad Orthop Surg 2011
- Collagen inhomogeniously distributed in three zones
- Superficial tangential zone (STZ)
- Zone with highest concentration of collagen
- Middle zone
- Collagen fibers randomly distributed and farther apart
- Deep zone
- Superficial tangential zone (STZ)
- Randomly layered fibrils of collagen to accommodate the high concentration of proteoglycans and water
- Pattern of collagen fibril arrangement related to tensile stiffness and strength characteristics
- Note correspondence between collagen and chondrocyte arrangement.
[ ( slide credit: @Jastifer2010Cartilage ) ]
8.4.10 Strength of Collagen
@Jastifer2010Cartilage
- Strong in tension
- Weak in compression (high slenderness ratio: length/width)
[ ( slide credit: @Jastifer2010Cartilage ) ]
8.4.11 Proteoglycans
- Proteoglycans another major component of extracellular matrix
- A compound consisting of a protein bonded to glycosaminoglycan groups, present especially in connective tissue.
- Complex macromolecules
Proteoglycans (Continued)
- Important for providing compressive strength
- Attract water, ions
- Building blocks
- Central protein core with polysaccharide chains
- Aggrecans: large aggregating proteoglycans with >100 sidechains
- Aggregate macromolecule: aggrecans bound to Hydroxy Appetite
[ ( slide credit: @Geeslin2016Cartilage ) ]
Proteoglycans (Continued)
Ulrich-Vinther et al. J Am Acad Orthop Surg 2003
- Large protein-polysaccharide molecules that exist as either monomers or as aggregates
- Proteoglycan aggregation promotes immobilization of the Proteoglycan’s within the collagen meshwork (adding structural rigidity to the extracellular matrix of articular cartilage)
[ ( slide credit: @Geeslin2016Cartilage ) ]
8.4.12 Proteoglycan Aggregate
Note the terms well used in supplement marketing!
@Jastifer2010Cartilage
[ ( slide credit: @Jastifer2010Cartilage ) ]
8.4.13 Water, ions
- Water
- Accounts for 70-90% of wet weight (most abundant component of cartilage)
- H2O allows movement of gases, waste products, nutrients to chondrocytes (in cartilage which is avascular)
- Ions
- Cations (Na\(^+\) and Ca\(^{2+}\)) are attracted to negatively charged proteoglycans for electrical neutrality
- Mechanical influence
- Fluid flow is important contribution
- Joint lubrication
[ ( slide credit: @Geeslin2016Cartilage ) ]
8.4.14 In summary:
- Collagen and Proteoglycans
- Form structural components that support mechanical stresses applied to cartilage
- Together with water determine biomechanical behavior of
cartilage
- Collagen and proteoglycans interact to form a porous composite fiber-reinforced organic solid matrix that is swollen with water
- Collagen-Proteoglycan solid matrix and interstitial fluid protect against high levels of stress and strain developing in the ECM when articular cartilage subjected to external loads
[ ( slide credit: @Jastifer2010Cartilage ) ]