Cartilage

Cartilage is a type of connective tissue that adapts to the pushing and pulling required for mechanical movement. It is composed of chondrocytes (cartilage cells) and a specialized extracellular matrix (ECM). There are three types of cartilage: hyaline cartilage (the most predominant type, e.g., in the nasal septum), fibrocartilage (e.g., in intervertebral discs), and elastic cartilage (e.g., in the auricle). Cartilage does not contain vessels and receives nutrients via diffusion by a cartilaginous membrane (perichondrium) or joint fluid (synovial fluid). During embryonic skeletal development, numerous bones are initially generated as cartilage models, which then ossify in utero or after birth.

Cartilage cells

• Chondroblasts
  • Develop from mesenchymal progenitor cells
  • Are capable of cell division: multiply to produce chondrocytes
  • Secrete extracellular matrix (ECM)
• Chondrocytes
  • Develop from chondroblasts
  • Produce and maintain the ECM
  • Are embedded in cavities (lacunae) of the ECM
  • Are no longer capable of cell division in their mature stage
  • Play an important role in endochondral ossification, which is essential for:
    • Development of the fetal skeletal system
    • Longitudinal bone growth
    • Bone healing after a fracture

Extracellular matrix (ECM) of cartilage

• Produced by: chondroblasts and chondrocytes
• Components: The composition of the ECM determines the characteristics of the different cartilage types.
  • Different types of collagen
  • Proteoglycan molecules with side chains of glycosaminoglycans
  • Glycoproteins, e.g., chondronectin, which establishes a connection between the ECM and chondrocytes

Perichondrium

• Definition: connective tissue composed of an outer fibrous layer and an inner chondrogenic layer, which surrounds elastic and extra-articular cartilage
  • The outer fibrous layer contains blood vessels, lymphatics, and nerves, all of which provide nutrients to, and drain, cartilage.
    • Cartilage that is not surrounded by perichondrium (e.g., articular cartilage) is supplied by synovial fluid and a subchondral vascular network.
  • The inner chondrogenic layer contains progenitor cells, which regenerate cartilage.

Articular cartilage and fibrocartilage are not surrounded by perichondrium.References:^[1][2][3]

All types of cartilage have high resilience to compression and contain type II collagen fibers (type II cartwolage.)

Chronic wear and tear decreases proteoglycan synthesis, making articular cartilage less elastic and more friable. Progressive cartilage degeneration results in joint space narrowing and reactive subchondral bone sclerosis (thickening), which occurs in the initial stages of osteoarthritis. Osteophyte formation is caused by insufficient cartilage repair and is a characteristic of moderate to severe osteoarthritis.

References:^[1][4]

Chondrogenesis

Chondrogenesis is a process of development, growth, and regeneration of cartilage. It is triggered by mechanical stimuli (e.g., load and hydrostatic pressure changes) and/or chemical stimuli (e.g., fibroblast growth factors, thyroid hormones, cytokines). The following sequence of steps can occur during chondrogenesis.

1. Mesenchymal osteoprogenitor cells accumulate and differentiate into chondroblasts.
2. Chondroblasts proliferate and produce the ECM of the cartilage. They are increasingly pushed apart as a result of ECM formation (located between the chondroblasts).
3. The cartilage model is formed by chondrocyte proliferation and ECM secretion (interstitial growth).
4. Mesenchymal cells at the periphery of the cartilage model differentiate into fibroblasts, which form a connective tissue capsule (perichondrium or periosteum).
5. Undifferentiated cells remain in the inner layer of the perichondrium and can differentiate into chondroblasts.
6. Cartilage growth can occur via new layers of cartilage tissue produced by osteoprogenitor cells in the perichondrium (appositional growth) or via the regeneration of damaged chondrocytes.

Regeneration

• Cartilage is regenerated from undifferentiated cartilage precursor cells of the perichondrium.
• Therefore, articular cartilage and fibrocartilage, which do not possess a perichondrium, cannot regenerate.
• In late adolescence, chondrocytes are no longer capable of cell division and cartilage stops growing; thus, cartilage regeneration in adults is poor.

In individuals with achondroplasia, chondrocytes fail to proliferate particularly at the epiphyseal growth plates of long bones, resulting in impaired endochondral ossification and short limbs. Intermembranous ossification, which occurs in the frontal and parietal bones, is not affected, resulting in a large head relative to the limbs.

• Bone healing
• Osteoarthritis
• Chondroblastoma
• Chondrosarcoma
• Lumbar disk herniation
• Achondroplasia
• Meniscus tear
• Costochondritis