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Cartilaginous tissue

Synonyms: Cartilago (Latin), Textus cartilagineus, Chondros (Greek)
German: Knorpelgewebe, Knorpel

1 Definition

Cartilaginous tissue, or simply cartilage, is a type of connective tissue formed by specialized cells (chondrocytes) and extracellular basic substance.

2 Prevalence

Cartilaginous tissue is present in many locations of the human body. As articular cartilage, it covers the articular surfaces of all real joints (diarthroses), eg. the knee joint and the hip joint. Here, it provides low-friction-mobility to the joint terminals. Also the intervertebral disks and menisci consist of cartilaginous tissue. In the growing organism, cartilaginous tissue can be found in the epiphyseal plates.

Moreover, in humans, you can find cartilage in the following regions:

Region Cartilage
Ear Auricular cartilage
Meatal cartilage
auditory tube cartilage
Nose Nasal cartilages
Larynx Laryngeal cartilages
Airways Tracheal cartilages
Thorax Rib cartilage

3 Histology

Cartilaginous tissue consists of cartilage cells, the chondrocytes, and extracellular cartilaginous matrix (ECM). This consists of an unstructured basic substance with an embedded organized net of collagen fibers. Individual chondrocytes also can divide and form isogenic groups in the cartilaginous tissue (3-5 cells). These cell complexes are surrounded by a strongly basophilic ECM, which is also called "cartilage territory". An isogenic group with a cartilage territory is called chondron. The cartilaginous matrix near the chondrocytes is called "territorial matrix", the areas more distant to the cells are called "interstitial matrix" or "interterritorial zone". The substance between the individual chondrocytes within an isogenic group is called "pericellular matrix".

3.1 Chondrocytes

The round chondrocytes maintain the matrix, where they are embedded in the form of small cell nests. They derive from chondroblasts, which are responsible for the active construction of the cartilaginous tissue. The degeneration of cartilage is also controlled by specialized cells, the chondroclasts.

3.2 Cartilaginous matrix

The cartilaginous matrix has a high proportion of water of up to 70%, and it has almost no blood vessel and nerves. Its main components are collagens, elastin, and proteoglycanes, whose total proportion and exact composition differs according to the type of cartilage. The proteoglycanes have numerous negatively charged molecular components, which is why they attract sodium ions. These, in turn, bind water in the cartilaginous tissue.

3.3 Cartilage types

According to consistency and composition of the cartilaginous matrix, you can differentiate between three types of cartilage:

4 Biomechanics

Cartilaginous tissue is an elastic tissue stable against bending, which is characterized by a high resistance against tearing and a great elasticity of compression. In case of the hyaline cartilage, this elasticity of compression is due to the formation of so-called proteoglycane aggreagtes (aggrecan with hyaluronate), which then are accumulated at the collagen fibrils (mostly collagen type II).

Hyaluronate (or hyaluronic acid) has a high affinity to water due to its anionic character, and it takes up a high volume in aqueous solution. Due to the connection with the collagen fibrils, hyaluronate is compressed to and maintained at 1/5 of its normal volume - against its natural expansion tendency. This molecular construction is comparable to elastic springs that do not completely fulfill their expansion tendency. When the cartilage is under mechanical strain, it gives in to its compression limits, by having water streaming out of the ECM. When the pressure on the cartilage subsides again, the expansion tendency of the compressed "elastic springs" becomes effective, and the cartilage is decompressed. This function can decline with old age because the composition of the proteoglycanes and collagen fibrils changes.

5 Clinical presentation

In nutritional disorders of the cartilage, you can observe a degeneration of the cartilaginous matrix and an apoptosis of cartilage cells. Then, you can see the phenomenon of the "unmasking" of the collagen fibers, i.e. the collagen fibers normally embedded in the basic substance come to the surface. This process clinically presents as osteoarthrosis, among others.

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