Cartilage

Cartilage is a type of connective tissue derived from embryonic mesenchyme that is responsible for structural support, resilience, and the smoothness of physical actions. Perichondrium (connective tissue membrane surrounding cartilage) compensates for the absence of vasculature in cartilage by providing nutrition and support. The abundant ground substance contains large amounts of chondroitin sulfate, hyaluronic acid, and water (80% of cartilage is water). All types of cartilage contain type II collagen produced by chondrocytes. Elastic cartilage additionally contains elastic fibers, whereas fibrocartilage also contains dense connective tissue (type 1 collagen).

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Overview

Definition

Cartilage is a type of connective tissue that forms structural components of the human skeleton and provides support to various organs.

Composition

Chondrocytes:

  • The major cell type of the cartilage tissue
  • Synthesize extracellular matrix (ECM) components and become embedded in them (lacunae)
  • Young chondrocytes retain the ability to divide; daughter cells secrete new ECM and become surrounded by the new lacunae.
  • Have low oxygen needs; nutrition is provided by perichondrium since cartilage is lacking blood vessels and innervation
  • Stimulated by somatotropins from the pituitary, inducing synthesis of sulfated glycosaminoglycans (GAGs) and secretion of proteoglycans

Chondroblasts:

  • Perichondrial cells that are located at the periphery of the cartilage and develop into chondrocytes 
  • Conversion of chondroblasts to chondrocytes occurs when chondroblasts become surrounded by the new matrix they produce.

Extracellular matrix:

  • Produced by chondrocytes 
  • Abundant, usually of firm consistency and resistant to compression
  • Composed of collagen (type Ⅱ most prevalent), proteoglycans, and glycoproteins
  • Chondroitin sulfate (aggrecan) is the most abundant proteoglycan in the matrix of hyaline cartilage

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Chondrogenesis

Embryonic development

  • Cartilage develops from embryonic mesenchyme.
  • Rounding up mesenchymal osteoprogenitor cells indicates the beginning of the process.
  • Chondroblasts are formed and begin secreting ECM consisting mostly of aggrecan and type Ⅱ collagen.
  • Chondroblasts are converted to chondrocytes.
  • Both cells are rich with rough endoplasmic reticulum (RER) for collagen synthesis.
  • Cartilage growth:
    • Interstitial growth: mitotic division of pre-existing chondrocytes → ECM formation
    • Appositional growth: formation of chondroblasts from progenitor cells in the perichondrium
    • Isogenous cells are formed by the mitotic division of chondroblasts.

Skeletal growth during childhood

  • Embryonic skeleton is composed primarily of cartilage.
  • Cartilage gradually calcifies and is replaced by bone as chondrocytes are replaced by osteocytes.
  • Epiphyseal plate is a thin layer of cartilage that persists at the ends of long bones after birth and allows for longitudinal bone growth.
  • Once the epiphyseal plate is completely ossified, no further longitudinal growth occurs.

Regeneration and repair

  • Since perichondrium loses the ability to form new cells, the regeneration of cartilage is almost nonexistent.
  • Perichondrial cells will produce mostly dense connective tissue.
  • Repair is also limited since chondrocytes are locked up in lacunae and cannot travel to damaged areas.

Types of Cartilage

There are 3 main types of cartilage tissue:

  • Hyaline cartilage (most abundant)
  • Elastic cartilage
  • Fibrocartilage
Table: Characteristics of 3 types of cartilage
Hyaline cartilageElastic (yellow) cartilageFibrocartilage
Composition of extracellular matrix
  • Type II collagen (randomly oriented fibrils)
  • Aggrecan
  • Type II collagen
  • Aggrecan
  • Elastic fibers (yellow)
  • Type II collagen (parallel fibrils)
  • Type I collagen
Major cells
  • Chondrocytes
  • Chondroblasts
  • Chondrocytes
  • Chondroblasts
  • Chondrocytes
  • Fibroblasts
Arrangement of chondrocytesIsolated or in small isogenous groupsUsually in small isogenous groupsIsolated or in isogenous groups arranged axially
Presence of perichondriumYes (except at epiphysis and articular cartilage)YesNo
Locations
  • Upper respiratory tract
  • Articular ends and epiphyseal plates of long bones
  • Fetal skeleton
  • External ear
  • External acoustic meatus
  • Auditory tube
  • Epigottis and larynx
  • Intervertebral discs
  • Pubic symphysis
  • Meniscus and certain other joints
  • Insertions of tendons
Functions
  • Provides smooth, low-friction surfaces in joints
  • Structural support for respiratory tract
  • Provides flexible shape
  • Support of soft tissues
  • Provides cushioning
  • Tensile strength
  • Resistance to tearing and compression

Clinical Relevance

  • Osteoarthritis: a disease caused by degeneration of articular (hyaline) cartilage. The process of degradation and faulty repair is mediated by chondrocytes. Risk factors are age, obesity, female gender, and joint trauma. 
  • Osteochondritis dissecans: a joint disorder characterized by focal aseptic necrosis of the articular cartilage and subchondral bone. This condition is usually associated with the detachment of a bone-cartilage fragment, which becomes displaced in the joint space. Causes pain and swelling of the affected joint, which catches and locks during movement. 
  • Chondrosarcoma: malignant bone tumor of chondrocytes that most commonly affects pelvic and long bones. Most common in older adults. Usually presents as a slow-growing mass associated with dull achy pain. Diagnosed with imaging and biopsy, and treatment involves wide surgical excision.
  • Enchondroma: a benign bone tumor originating in cartilage. This condition rarely causes pain or other symptoms, and is most commonly picked up incidentally on imaging. Usually small in size (< 5 cm) and does not require treatment.

References

  1. Fawcett D.W. Cartilage. Retrieved 30 May 2021, from https://www.britannica.com/science/connective-tissue/Cartilage#ref470898
  2. Fawcett D.W. (1994). A Textbook of Histology, 12th ed., chapter 5, pp. 133–169.

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