Cartilage

CARTILAGE:

Cartilages are the specialized type of connective tissue composed of cells & extracellular matrix (fibers & ground substances).
• It is devoid of blood vessels, & nerve supply.
Thus, the healing of cartilage is quite slow.
 
Components: 
1) Cells
• Chondroblasts (immature)
• Chondrocytes (mature)
* Chondroblasts → responsible for synthesis of extracellular matrix.
2) Extracellular matrix
• Ground substance
  Mainly composed of

- Glycosaminoglycans (GAG) 
- Water & electrolyte
• Fibers (collagen & elastic fiber)

Fibrocartilage & Hyaline
→ Type-II collagen fibers
Elastic cartilage
→Elastic fibers


Peculiarities of cartilage
• Devoid of nerve supply
• Avascular, get nutrition from blood vessels of adjacent tissues by diffusion.
• Matrix is not calcified like in bone.
 
Functions of cartilage
• Supports the soft tissues of the body
• Shock absorber
• Sliding area for joints & facilitates bone movements.
• Growth & developments of long bones before & after birth.
 
 
Types of cartilage:
  1. Hyaline cartilage
  2. Fibrocartilage
  3. Elastic cartilage

 

Distribution of cartilage: 
Hyaline cartilage
• Articular cartilage (cartilage around articular surface of bone)
• Costal cartilage of ribs which articulate with the sternum
• Thyroid & cricoid cartilage of larynx
• Epiphyseal plate of Growing long bone
• In the walls respiratory tract i.e nose, larynx, trachea, bronchi


Fibrocartilage
• Annulus fibrosis of intervertebral disc
• Symphysis pubis
• Menisci (present in knee joint)
• Glenoid labrum (of shoulder joint socket)
• Acetabular labrum of the hip joint socket
• Articular discs of temporomandibular joint & wrist joint


Elastic cartilage
• Auricles or pinna of external ear
• Cartilaginous part of external auditory canal
• Auditory tube (Eustachian tube)
• Epiglottis, cuneiform & corniculate cartilage of larynx


 
Articular disc
It is the fibrocartilagenous structure, intervening between the articular surfaces in synovial joints.
Present in
• Temporo-mandibular joint (TMJ)
• Wrist Joint (Radio-carpel joint)
• Sternoclavicular joint
 
* Articular discs present in the wrist joint (radio-carpel joint) prevent ulna for making wrist joint.
* Articular discs of the Temporomandibular joint divide the joint cavity into two compartments.
 
 
Articular cartilage: 
It is a special type of hyaline cartilage that covers the articular surfaces of the bones of most synovial joints.
• In some cases, where the bone is ossified in membrane, the cartilage is fibrocartilage
• Absence of perichondrium in articular cartilage
• Acts as shock absorbers & ensures gliding movement
 
 
Intervertebral disc: 
The two adjacent vertebral bodies are united to each other by a special type of connective tissue called an intervertebral disc.
Two parts
• Annulus fibrosus (outer layer of fibrocartilage)
• Nucleus pulposus (inner mass of gelatinous materials, rich in hyaluronic acid)
 
Function
1. Allow certain movements between the vertebrae.
2. Acts as a shock absorber.
3. Protect the friction of the corresponding vertebrae.
 
• Annulus fibrosus, a type of fibrocartilage developed from mesenchyme.
• Nucleus pulposus developed from notochord.
Nucleus pulposus represents notochord.
 
 
Disc prolapse:
Dislocation or herniation of intervertebral disc from its position between the vertebrae.
• Due to a reduction in proteoglycan size within the nucleus pulposus diminishes its viscoelasticity leading to focal damage.
• Most common at L 4 & L 5 due to increased mechanical forces across this area.
 
 
Just to know:
Mode of nutrition of cartilage: 
• Cartilage is devoid of blood vessels
• Hyaline cartilage cells metabolize glucose mainly by anaerobic glycolysis to produce lactic acid.
• Hyaline & elastic cartilage → From vessels of perichondrium
• Fibrocartilage → From blood vessels of surrounding connective tissue
• Articular cartilage → From synovial fluid
 
Histology of Elastic cartilage: -
• Yellowish in appearance due to the presence of elastic fibers.
• Composed of-
1. Cells
• Chondrocytes &
• Chondroblasts
2. Extracellular matrix
• Fibers- Elastic fiber
• Ground substance

• Chondrocytes are present in the space called lacunae.
• Perichondrium present
• Elastic fibers form branching & anastomosing network
• It's composition is similar to hyaline cartilage except that it contains elastic fibers instead of collagen type-II fibers.
 
Histology of fibrocartilage: 
• Fibrocartilage is a tissue intermediate between dense connective tissue & hyaline cartilage.
• Cells lies in rows between collagen fiber bundles
• Long, parallel, wavy bundle of collagen fibers present
• No distinct perichondrium
 
Histology of Hyaline Cartilage
• Weakest cartilage of all
• Matrix around the cells → deeply stained called as territorial matrix.
• Inter-territorial matrix (lightly stained) → present between two groups of cells
• Chondrocytes lie in space called lacunae forming groups of cells called isogenous aggregates.
• Composed of-
Cells → Chondrocytes & Chondroblast
Fibers → Type-II collagen fibers
Ground substances
• Perichondrium present
• Chondrocytes appear in the space called lacunae.
 
 
Tips:
A lacunae contains either single cells or multiple cells.
Collection of lacunae group of cells called isogenous aggregates.
• Chondrocytes synthesize collagens & the other matrix molecules.
* Hyaline & Fibrocartilage contain Type-II type of collagen fibers but elastic cartilage contains elastic fibers.
* Perichondrium is absent in articular cartilage & Fibrocartilage.
 
 
 
Perichondrium:
• Its a sheath of dense connective tissue that surrounds the cartilage.
• Not found in articular cartilage & fibrocartilage
 
Two layers present
Outer fibrous layer (Vascular)
Inner chondrogenic layer (Cellular)
Composed of Type-I collagen fibers, fibroblasts & chondrocytes in inner layers.

 
Functions
• It harbors the vascular supply for avascular cartilage.
• Cells around the Perichondrium has the capacity to regenerate to some extent in case of cartilage damage.
• It contains nerves & lymphatic vessels.
 
 
Cartilage Formation/ Chondrogenesis:
• All cartilage derived from the embryonic mesenchyme.
• The mitotic proliferation of mesenchymal cells occurs which later differentiate into chondroblasts
• Chondroblasts secrete the components of extracellular matrix & form lacunae and become chondrocytes.
• Chondrocytes divide one or two times within lacunae to form isogenic groups.
• Each chondrocytes secrete their own matrix causing it to form its own lacunae and separate apart.
• The superficial mesenchyme develops into the Perichondrium.
• Synthesis of matrix contributes greatly to the growth of the cartilage.