06 September 2006
Cartilage and Bone
Dr. Robert Crissman, Ph.D.



Cartilage


  • Supportive CT
  • CT that makes up skeletal framework of body
  • Consists of cartilage and bone
    • Provide support for other soft tissues

Cartilage

  • Three type: hyaline, elastic, fibrocartilage
    • Names based on the predominant fiber present in the extracellular matrix
  • Hyaline cartilage
    • Made of type II cartilage
    • Found where needed for weight bearing but some flexibility required
  • Elastic cartilage
    • Made of elastic fibers
  • Fibrocartilage
    • Made of type I collagen

Hyaline Cartilage


  • Most common type of cartilage
  • Condrocytes
    • Diagnositic Characteristic: Large round cells well separated from other cells, avascular
    • Lots of extracellular matrix
    • Surrounded by perichondrium – dense irregular CT

Chondrocytes

  • Reside in lacunae
    • Artifact: Cell shrinks away from matrix, leaving a “lake” during slide making; normally, cells are right up against matrix
  • Maintains extracellular matrix
  • Does not divide

Chondroblasts

  • Two cells in one lacunae
    • Diagnostic Characteristic: Divides
  • Produces additional new matrix
    • Two cells in one lacunae push each other apart as new matrix is being laid down

Cartilage Growth


  • Interstitial growth
    • “Growth from within”
    • Add new matrix within cartilage because extracellular matrix is flexible enough
  • Appositional growth
    • Growth on the perimeter or sides of cartilage

Perichondrium

  • Two layers
    • Fibrous Layer
      • Outer layer of dense irregular CT
      • Vascular
    • Chondrogenic layer
      • Differentiating into chondroblasts
      • Appositional growth
  • Lots of proteoglycans so that H2O diffusion can occur
    • However, prevents wandering cells from going into cartilage

Ground Substance


  • Abundant, amorphous gel
  • Rich in proteoglycans
  • Territorial matrix (a.k.a. capsular matrix)
    • Immediately around cell
    • Darker staining due to chondroitin sulfate
    • Few collagen fibrils
  • Interterritorial matrix
    • Between cells
    • Lighter staining matrix between cells because of lower condroitin sulfate content

Proteoglycans

  • Aggrecan aggregates are what provides cartilage its characteristic structure and gel-like character
  • More organized than CT proper
  • Aggrecan aggregate is a hyaluronic acid with 100 aggrecan molecules attached
  • Diffusion of nutrients and H2O
  • Resists compression
  • Slippery
    • Mother nature’s Teflon

Glycoproteins

  • Adhesion molecules
  • Chondronectin
    • Attaches cell membrane to ECM
  • Chondrocalon
    • Binds Ca2+ to the ECM

Extracellular Fibers


  • Type II collagen doesn’t aggregate together to form fibrils or fibers
    • Cannot be seen at light microscopy level
    • Fine microfibers at electromicroscope level
      • Pericellular capsule visible adjacent to cell
        • Thought to protect cells form mechanic stress

Hyaline Cartilage Location


  • Costal cartilage
  • Trachea, bronchi of lungs
  • Larynx, nasal cartilage
  • Articular cartilage of synovial joints
  • Fetal skeleton
    • Allow flexibility during birth
    • Cartilage grows faster than bone
      • Has both appositional and interstional growth (bone is only appositional growth)

Functions of Hyaline Cartilage


  • Provide structural support
    • Elastic firmness that is resilient yet fairly rigid
  • Withstand pressure and shear forces
    • Acts as shock absorber
    • Due to proteoglycan aggregates being full of water
  • Slippery
    • 80% of weight is water
    • Due to proteoglycans
  • Capable of rapid growth

Cartilage Formation


  • Can be formed directly from Mesenchyme
    • Areas of low O2 content causes formation of chondroblasts
      • In areas of high O2 content causes differentiation to bone
    • Called procartillage
    • Chondroblasts secretes matrix interstitially, pushing away from each other to form cartilage

Elastic Cartilage

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  • Chondrocytes
    • Large round cells, widely scattered
  • Principal fiber
    • Elastic fibers
    • Collagen Type II
    • Both fibers secreted by condrocyte
      • Special stain required to visualize elastic fibers
  • Ground Substance
    • Same as hyaline cartilage
    • Lots of aggrecan aggregates
  • Location
    • Auricle and epiglottis
  • Function
    • Resilient support
  • Diagnositic Characteristics: large, round chondrocytes, widely scattered cells, dark stained elastic fibers on light microscope
    • Has perichondrium and chondrogenic layer with few elastic fibers

Fibrocartilage


  • Collagen Type I, densely packed
    • Almost looks like dense regular CT
      • Diagnostic Characteristic: Has large round chondrocytes
  • Not a lot of ground substance, replaced with type I collagen
  • Chondrocytes secrete collagen and ground fibers
    • Don’t have to be epithelium to secrete
  • Functions:
    • Pubic symphysis
    • Junction at attachment between tendon and bone

Intervertebral discs

  • Fibrocartilage functions as a shock absorber
    • Annulus fibrosis – fibrocartilage
    • Nucleus pulposis – gelatinous matrix in the center
  • Herniated disk
    • Fibrocartilage fails and nucleus pulposis is extruded, putting pressure on spinal cords and causing pain
    • Treated by keeping patient off his/her feet so that nucleus pulposis will suck back in
      • Wait for fibrocartilage to reform

Joints


  • Function in articulation
  • Definition
    • Junction between two or more bones
    • Typically allows movement between bones

Functional Classificaiton

  • Based on degree of movement
  • Synarthroses – immovable
  • Amphiarthroses – slightly movable
  • Diathroses – freely moveable, most common

Structural Classification

  • Based on type of tissue between bones
  • Fibrous joints
    • Think layer of fibrous CT between bones
    • No cavity
      • Sutures – immovable bones in skull, synarthroses
      • Syndesmoses – articulating bones separated by dense fibrous tissue, amphiarthroses
      • Gomphoses – cone shaped socket; articulating bones separated by periodontal ligament, synarthroses
  • Cartilagenous joints
    • United by cartilage
    • No cavity
      • Synchondroses – connected by hyaline cartilage, synarthroses
      • Symphyses – connecting material is a broad, flat disk of fibrocartilage, amphiarthroses
  • Synovial joints
    • Ends of bone are covered by articular cartilage
    • Have a cavity
      • Gliding, hinge, pivot, ellipsoidal, saddle, ball-and-socket joints

Synovial Joints


  • Five common characteristics
    • Synovial cavity
    • Articular cartilage lines ends of bones
    • Have articular capsule
    • Synovial membrane
    • Synovial fluid is present within synovial membrane
      • Contains hyaluronic acid, lubricant
      • Nutrient supply to articular cartilage

Articular Cartilage

  • Modified hyaline cartilage
  • Functions to promote smooth movement between bones
    • Structurally designed to do so by lacking perichondrium on the surface
      • Naked cartilage on naked cartilage
      • Nutrients from synovial fluid produced by the synovial membrane that lines the cavity
        • Not really efficient at diffusing nutrients, but it does, slowly
    • Teflon function
  • Made of type II collagen that does form arched fibers like canes
    • Help resists shear stress forces
  • Additional support: cartilage near bone, away from articular cartilage is calcified
  • Subchondral bone layer – increases strength

Repair of Joint Cartilage


  • Difficult to do:
    • Nutrient supply is poor
    • Articular cartilage doesn’t undergo mitosis
  • To repair:
    • Allow condroprogenitor cells to enter fracture
      • Inject or drill holes to bone marrow
    • Move joint continuously to stimulate cartilage formation
      • Strap you to a machine that slowly moves you all the time
    • Repaired tissue is fibrocartilage
      • Not as smooth as before
    • Other forms of repair:
      • Subchondral bone grafts
      • Autologous bone grafts
      • Autologous tissue culture grafts
      • Injection of hyaluronic acid

Osteoarthritis


  • Degenerative arthritis affecting synovial joints
  • Most common type of arthrisis
  • Non-inflammatory and progressive deterioration of articular cartilage
  • Characterized by:
    • Fibrillation of articular cartilage
      • Loss of ground substance and retention of fibers
    • Proliferation of cartilage at periphery
    • Replacement of cartilage spur with bone tissue to form bone spurs which reduce degree of movement

Rheumatoid Arthrisis


  • Inflammatory autoimmune disease
  • Antibodies attack own joint cartilage and synovial membrane
  • Primary inflammation of synovial membrane
    • Joints will be swollen
  • Synovial membrane thickens and synovial fluid secretion increases
    • Joint swell up even more
  • Pressure due to increased amounts of synovial fluid causes pain and tenderness
  • Pannus of tissue forms across cartilage and erodes away cartilage
  • Bone exposed and eventually fuses across joint