Corpus: Cartilage

Cartilaginous tissue, or simply cartilage, is a form of connective tissue composed of specialised cells (chondrocytes) and extracellular ground substance.

Cartilage tissue is found in many parts of the human body. As articular cartilage (cartilago articularis), it covers the joint surfaces of all true joints (diarthroses), e.g. the knee joint and the hip joint. Here it ensures the low-friction mobility of the joint ends. The intervertebral discs (disci intervertebrales) and menisci also consist of cartilage tissue. In the growing organism, cartilage tissue is also found in the epiphyseal joints.

In humans, cartilage is also found in the following regions:

Cartilage tissue consists of cartilage cells, the chondrocytes, and the extracellular cartilage matrix (ECM). This is made up of an unstructured basic substance with an organised network of collagen fibres embedded in it.

The cell complexes of chondrocytes are surrounded by a highly basophilic ECM, which is referred to as the "territory", "territorial matrix" or "cartilage courtyard". The cartilage matrix that lies further away from the cells is the "interstitial matrix" or "interterritorial zone". The substance between the individual chondrocytes within an isogenic group is called the "pericellular matrix".

3.1. Cartilage cells

The roundish chondrocytes maintain the matrix and are arranged in the form of small cell nests of 3-5 cells, which all develop from a common chondroblast. These cell nests are also known as isogenic groups. An isogenic group with its territory is called a chondron.

The chondroblasts develop from undifferentiated mesenchymal cells and secrete the ECM. After cell division has ceased, the chondroblasts are called chondrocytes. Chondrocytes still have the ability to divide during cartilage development, but lose this ability in mature cartilage.

Specialised cells, the chondroclasts, also break down the cartilage. Due to the insular position of the cartilage cells in the cartilage matrix, cartilage damage cannot be compensated for by the immigration of neighbouring cells.

3.2. Cartilage matrix

The cartilage matrix has a water content of up to 70% and has almost no blood vessels, nerves or nociceptors. The exchange of substances in the cartilage takes place by diffusion. Its main components are collagens, elastin and proteoglycans, the total proportion and exact composition of which differs between the various types of cartilage. The proteoglycans have numerous negatively charged molecular components and therefore attract sodium ions. They in turn bind water in the cartilage tissue.

3.3. Cartilage types

Depending on the nature and structure of the cartilage matrix, a distinction is made between three forms of cartilage:

• Hyaline cartilage
• Elastic cartilage
• Fibrous cartilage

Cartilage is an elastic, flexurally stable tissue that has a high resistance to tearing and elasticity under pressure. In the case of hyaline cartilage, this compressive elasticity comes about because so-called proteoglycan aggregates (aggrecan with hyaluronan) are formed and then attached to the collagen fibrils (usually type II collagen).

Due to its anionic character, hyaluronan (or hyaluronic acid) has a high affinity for water and takes up a large volume in aqueous solution. By combining with the collagen fibrils, hyaluronan - contrary to its nature to expand - is compressed and held at 1/5 of its normal volume. This molecular structure can be compared to springs, which do not fully fulfil their tendency to expand. If the cartilage is subjected to mechanical stress, it yields up to its compressibility limit as water flows out of the ECM. If the pressure on the cartilage decreases again, the expansion tendency of the compressed "springs" takes effect and decompression occurs. This function can diminish with age because the composition of the proteoglycan and collagen fibrils changes.

Nutritional disorders of the cartilage lead to degeneration of the cartilage matrix and the death of cartilage cells. This leads to so-called "unmasking" of the collagen fibres, i.e. the collagen fibres normally embedded in the basic substance come to the surface. This process manifests itself clinically as osteoarthritis, among other things. As osteoarthritis leads to considerable loss of function in the long term, cartilage regeneration is an important subject of clinical research.