Cartilage is a type of connective tissue that forms structural components of the human skeleton and provides support to various organs.
- 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
- 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.
- 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
- 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
|Hyaline cartilage||Elastic (yellow) cartilage||Fibrocartilage|
|Composition of extracellular matrix|
|Arrangement of chondrocytes||Isolated or in small isogenous groups||Usually in small isogenous groups||Isolated or in isogenous groups arranged axially|
|Presence of perichondrium||Yes (except at epiphysis and articular cartilage)||Yes||No|
- 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.
- Fawcett D.W. Cartilage. Retrieved 30 May 2021, from https://www.britannica.com/science/connective-tissue/Cartilage#ref470898
- Fawcett D.W. (1994). A Textbook of Histology, 12th ed., chapter 5, pp. 133–169.