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Articular Cartilage

- See:
       - Chondral and Osteochondral Injuries of the KneeArticular Cartilage: Injury and Potential for Repair: / Osteoarthritis 
       - Enchondral Ossification 

- Discussion:
    - hyaline cartilage caps ends of bones that form synovial joints; 
    - in other hyaline cartilage structures, surrounding perichondrium contains both capillaries for nutrition and the cells that become
           involved in appositional growth
    - thickness of articular cartilage varies from joint to joint, and in humans it is thickest over ends of femur & tibia, ranging
           from 2-4 mm; 
    - cartilage contains predominantly type II collagens w/ lesser amounts of type IX and type XI; 
    - function:
           - it provides both a cushion & slick surface for movement; (see lubrication)
           - consistency of the extracellular matrix allows the tissue to bear mechanical stresses without permanent distortion;
           - shock-absorbing because it is resilient;
           - smooth surface allows sliding against it
    - synovial fluid
           - nourishment is supplied synovial fluid that bathes cartilage;
           - withdrawal of synovial fluid often leads to rapid deterioration of cartilage; 
           - ref: Synovial fluid depletion: successful arthrodesis without operative cartilage removal 
    - perichondrium:
          - articular cartilage has no more than a peripheral rim of perichondrium on its free surface, and calcified cartilage abutting
                   bone limits diffusion from blood vessels supplying subchondral bone; 
          - free surfaces of most hyaline cartilage (but not articular cartilage) are covered by a layer of fibrous connective tissue,
          - deep portion of perichondrium is composed of chondroblasts;
          - external portion is less cellular and more densely fibrous; 
    - extracellular matrix
          - filler material of cartilage is composed of proteoglycan aggregates w/ chondroitin sulfate & keratan sulfate as
                   chief glycosaminoglycans
           - approx 10% of wet weight of cartilage is collagen;
           - approx 75% of matrix is water;
           - remainder is a nonfibrous filler material;
           - these entities together form stiff sol; 
           - containes fibers, ground substance
           - collagen, hyaluronic acid, proteoglycans, glycoproteins, elastic (in elastic cartilage)
           - macromolecules, water, fibers bind together and producing function properties and flexibility;
           - no blood, nerve supply
           - low metabolic rate;
    - collagen alignment and function:
           - type II collagen is the primary type of collagen in articular cartilage; 
           - leaf-like arcades of collagen fibers within which the chondrocytes are oriented perpendicular to the articular surface in
                    the deep zone
           - arch through the middle zone
           - become horizontal and parallel to the articular surface in the superficial tangential zone;
           - functions of collagen fibers w/ in cartilage: 
                  - provides tensile strength to tissue and resist movement of interstitial water & proteoglycans from the cartilage, esp. while it
                         sustains compressional loading;
                  - to anchor ground substance of articular cartilage to subchondral bone; 
                  - references:
                         - Quantitative structural organization of normal adult human articular cartilage.
                         - Three-dimensional collagen architecture in bovine articular cartilage.
                         - The ultrastructure of mouse articular cartilage: collagen orientation and implications for tissue functionality. A polarised light and scanning electron microscope study and review.

- Histology:
     - chondrocytes, occupy lacunae generously distributed through the matrix; 
     - young chondrocytes & chondroblasts have rounded nuclei (or double nuclei); 
     - cytoplasm contains elongated mitochondria, well-developed Golgi apparatus, varying amounts of glycogen, & lipid droplets; 
             - each peripheral lacuna typically houses a single chondrocyte; 
             - deeper lacunae may contain two or more chondrocytes; 
             - surrounding each cell is a territorial matrix w/ a higher concentration of proteoglycans
     - references:
             - Histology of Cartilage and Bone

- 4 Zones of artiuclar cartilage:
     - superficial layer (tangential zone);
           - makes up 10% of cartilage;
           - consists of 2 sub-zones:
                  - fibrilar sheet / lamina splendens is the more superficial layer;
                        - clear film consisting of a sheet of small fibrils with little polysaccharide and no cells;
                  - cellular layer w/ flattened chondrocytes;
                        - flat chondrocytes and collagen fibers are arranged tangentially to the articular surface;
           - thinnest layer, with the highest content of collagen and the lowest concentration of proteoglycans;
                  - collagen (type IX) is arranged at right angles to adjacent bundles and parallel to the articular surface;                          
           - subsequently has greatest ability to resist shear stresses and serves as a gluiding surface for joint;
           - may also function to limit passage of large molecules between synovial fluid and cartilage;
           - superficial zone is the first to show changes of osteoarthritis;
    - transitional layer
           - this zone is involves transition between the shearing forces of surface layer to compression forces in the cartilage layers;
           - composed almost entirely of proteoglycans
           - spherical chondrocytes
           - less strongly bound;
    - deep radial layer
           - largest part of the articular cartilage
           - it distributes loads and resists compression;
           - collagen fibers and chondrocytes are perpendicular to the subchondral plate;
    - calcified cartilage layer
           - contains the tidemark layer;
                   - tidemark is basophilic line which stradles the boundry between calcified and uncalcified cartilage;
           - separates hyaline cartilage from subchondral bone;
           - type X collagen is present mainly in the calcified cartilage layer and in hypertrophic zone of the growth plate; 
           - much of hyaline cartilage in non articular areas of the body ultimately calcifies w/ maturation;
                   - when calcification occurs, chondrocytes die & matrix disintegrates;
                   - articular cartilage is unique in that its more superficial zones do not calcify, except in states such as pseudogout 
           - references:
                   - Calcification of cartilage formed in vitro on calcium polyphosphate bone substitutes is regulated by inorganic polyphosphate.
                   - The basic science of the subchondral bone.
                   - Histomorphometric analysis of adult articular calcified cartilage zone.
                   - [Study on shape and structure of calcified cartilage zone in normal human knee joint]
                   - Chondro-osseous junctional region: evidence for contact of uncalcified cartilage with subchondral bone and marrow spaces.
                   - The tidemark of the chondro-osseous junction of the normal human knee joint.
                   - Type X collagen, a natural component of mouse articular cartilage: association with growth, aging, and osteoarthritis.

  - References:

The effect of antibiotics on the destruction of cartilage in experimental infectious arthritis.
The deleterious effects of drying on articular cartilage.
The effects of exposure of articular cartilage to air. A histochemical and ultrastructural investigation
Periodic rewetting enhances the viability of chondrocytes in human articular cartilage exposed to air. 
The potential for regeneration of articular cartilage in defects created by chondral shaving and subchondral abrasion. An experimental investigation in rabbits.
Cell origin and differentiation in the repair of full-thickness defects of articular cartilage.
An ultrastructural study of normal young adult human articular cartilage.  
The Strings That Hold Us Together
Chondroid bone, secondyar cartilage, and metaplasia
Cartilage Slide Show:
Hyaline Cartilage Slide Show
Cartilage Slide Show 2
Alternatives to Total Knee Replacement: Autologous Hamstring Resurfacing Arthroplasty