Histology of Cartilage

Cartilage

• Cartilage is a type of connective tissue with a firm consistency of the extracellular matrix.
• There are three types of cartilage: hyaline cartilage, yellow elastic fibrocartilage, and white fibrocartilage.

Hyaline Cartilage

• Appearance: translucent pale blue in fresh state
• Consistency: firm or rigid with some degree of flexibility
• Structure:
  • Formed of perichondrium, cartilage cells (chondrocytes), and cartilage matrix
• Sites:
  • Constitutes the majority of the foetal skeleton
  • Articular surface of bones
  • Costal cartilage in the thoracic cage
  • Nose, trachea, and bronchi

Perichondrium

• A vascular connective tissue membrane that surrounds the cartilage except at the articular surfaces of the joints
• Formed of two layers: outer fibrous layer and inner chondrogenic layer
• Function:
  • Acts as a source of O2 and nutrients to the cartilage cells
  • The inner chondrogenic layer is responsible for new cartilage formation

Cartilage Cells (Chondrocytes)

• There are two types of chondrocytes: young chondrocytes and mature "old" chondrocytes
• Young chondrocytes:
  • Arise from the inner chondrogenic layer
  • Flat cells with open-face nuclei and pale basophilic cytoplasm
  • Present singly in their lacunae at the periphery of the cartilage and parallel to the perichondrium
• Mature "old" chondrocytes:
  • Site: deep in the cartilage in the form of rows perpendicular to the perichondrium
  • Shape: oval or rounded when single, and triangular or semicircular when in groups
  • Have rounded and open-face nuclei and granular, basophilic cytoplasm rich in glycogen, fat, and phosphatase enzyme
• Function:
  • Synthesize and secrete the components of the cartilage matrix
  • Can divide, responsible for the growth of the cartilage

Cartilage Matrix

• General characters:
  • Rubbery in consistency
  • Non-vascular
• Staining reaction:
  • Appears homogenous basophilic by Hx and E due to the presence of sulphated glycosaminoglycans
  • Shows strong PAS positivity
• Components:
  • Collagen fibers (Type II)
  • Proteoglycans (chondritin 4 sulphate and 6 sulphate)
  • Glycoprotein (chondronectin)
  • Protein (chondrocalcin)

Yellow Elastic Fibrocartilage

• Appearance: yellow in color due to the presence of elastic fibers
• Consistency: elastic and flexible due to the presence of elastic fibers
• Structure:
  • The matrix contains an abundant network of elastic fibers
  • Can be stained by standard elastic stains (e.g. Verhoeff's stains)
• Sites:
  • Auricle of the ear
  • External auditory meatus
  • Eustachian tube
  • Epiglottis

White Fibrocartilage

• Appearance: white in fresh state
• Consistency: tense and resists stretch due to the presence of collagen fibers
• Structure:
  • Differs from hyaline cartilage in:
    • Not covered by perichondrium but surrounded by dense C.T. rich in blood vessels
    • Chondrocytes are arranged in rows or columns
    • Matrix contains bundles of coarse type I collagen fibers in between rows of chondrocytes
• Sites:
  • Intervertebral discs
  • Semilunar cartilage of knee joints
  • Symphysis pubis
  • Lips of glenoid cavity
  • Terminal parts of tendons