Synonyms in a broader sense
- Elastic cartilage
- Hyaline cartilage
Definition
Cartilage is a special form of connective tissue. A distinction is made between different forms of cartilage, which is adapted to the respective function. Forms of cartilage are:
- Hyaline cartilage
- Elsterian cartilage
- Fibrocartilage
Development of hyaline cartilage
Hyaline cartilage develops from the mesenchyme (form of connective tissue). At 45%, the proportion of collagen fibers is lower than in fibrous and elastic cartilage. The collagen fibrils are masked by the glycosaminoglycans present in the basic substance.
They are not visible in the light microscopic image, as their light refraction does not differ from that of the surrounding area due to the low fiber density. Hyaline cartilage, with the exception of articular cartilage, is covered by a cartilage skin (perichondrium). The innermost cell layer of the cartilage skin (stratum cellulare) retains the ability to form cartilage cells after growth is complete.
The outer layer (stratum fibrosum) consists mainly of collagen fibers that absorb tensile forces that arise when the cartilage body is bent. In this way, the cartilage retains a certain ability to regenerate even in adulthood. Nevertheless, the regenerative capacity of hyaline articular cartilage is in principle low.
New cartilage can only be formed from the perichondrium. If the cartilage skin is missing, functional cartilage can no longer be built up after destruction due to inflammatory and degenerative joint diseases. Chondrocytes (cartilage cells) recede from the cartilage substance (extracellular matrix) in the differentiated hyaline cartilage tissue, which is free of vessels and nerves, so that their volume proportion of cartilage cells is only between 1 and 10%.
Hyaline cartilage structure
Hyaline cartilage looks bluish milky when fresh and appears transparent in thin slices. The cartilage substance (extracellular matrix) of hyaline cartilage has a high water content of approx. 70%.
The dry substance of cartilage consists of (structure): Proteoglycans and type II collagen fibers make up the main mass with 45% each. As the main proteoglycan of hyaline cartilage, aggrecan together with hyaluronic acid forms the actual basic substance of cartilage tissue. Due to the high negative charge density of the glycosaminoglycan side chains, aggrecan has a high reversible water-binding capacity.
This is explained by the partially positive charge of the water molecule as a dipole. As a result, the water-loaded glycosaminoglycans repel each other and build up a tissue-specific internal pressure (swelling pressure of the cartilage), which is maintained by the tensile strength of the collagen fibers. In free aqueous solution, the rejection of the proteoaminoglycans would expand enormously.
The collagen fibers of the extracellular matrix retain the proteoaminoglycans. The proteoaminoglycans can be compared to springs that are restrained and compressed by the collagen fibrils. The high compressive elasticity is due to the fact that the proteoaminoglycans allow further compression, but immediately after compression they immediately expand again as far as the collagen fibrils allow.
At the same time, water is displaced during compression and reapplied during decompression. This movement of the joint cartilage is important for the nutrition of the cartilage. The function of the cartilage thus depends on the one hand on the quantitative and qualitative composition of the proteoglycans and their GAG chains and on the other hand on the ordered structure of the collagen fibrils and their structure. Both of these factors can become less effective with increasing age, which is particularly noticeable in articular cartilage in the form of symptoms in the joint.
- Proteoglycans
- Collagens
- Glycoproteins
- Lipidsand
- Electrolytes.
