The skeletal system of the human body is composed of bones and special connective tissue structures such as cartilage, ligaments, and tendons. To meet the mechanical requirements of movement, cartilage serves as an elastic tissue that adapts to pushing and pulling. It is composed of chondrocytes (cartilage cells) and a specialized extracellular matrix (ECM). There are three types of cartilage: hyaline cartilage (most predominant type, e.g., in the nasal septum), fibrocartilage (e.g., in intervertebral discs), and elastic cartilage (e.g., in the auricle). Cartilage is free of vessels and is nourished through diffusion by its cartilaginous membrane (perichondrium) or joint fluid (synovial fluid). In skeletal development, numerous bones are initially generated as cartilage models, which then ossify in utero or after birth.
- Produced by: chondroblasts and chondrocytes
- Components: The exact composition differs in the various cartilage types and determines their specific characteristics.
- Definition: Connective tissue surrounding elastic and extra-articular cartilage that consists of an outer fibrous layer and an inner chondrogenic layer.
Overview of cartilage types
|Hyaline cartilage||Elastic cartilage||Articular cartilage (specialized hyaline cartilage)||Fibrocartilage|
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|Perichondrium|| || || || |
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- All cartilage types contain type II collagen fibers
Chronic wear and tear may decrease proteoglycan synthesis, which makes articular cartilage less elastic and more friable. Degradation of cartilage results in joint space narrowing and thickening of the subchondral bone (i.e., subchondral sclerosis). Insufficient attempts of reparation result in osteophytes (also: bone spurs) and inflammation. These changes are known as osteoarthritis.
- Mesenchymal accumulate and differentiate into chondroblasts.
- Chondroblasts proliferate and produce the ECM of the cartilage and are increasingly pushed apart as a result of ECM formation (located between the chondroblasts).
- The chondroblast proliferation and ECM formation (interstitial growth). grows through
- After completion of proliferation or cartilage growth, chondrocytes develop from chondroblasts and are bundled into isogenic groups.
- Mesenchymal cells at the periphery of the cartilage model differentiate into fibroblasts, which form a connective tissue capsule (perichondrium or periosteum).
- Undifferentiated cells remain at the inner layer of the perichondrium and can differentiate into chondroblasts.
- The cartilage can either grow through new layers of cartilage tissue produced by osteoprogenitor cells in the perichondrium (appositional growth) or by regeneration of damaged chondrocytes.
In patients with achondroplasia, chondrocytes fail to proliferate particularly at the epiphyseal plates of long bones near the joints, which results in impaired endochondral ossification, short arms, and legs. Intermembranous ossification, which occurs in the frontal and parietal bones, is not affected, resulting in a large head relative to the limbs.