Cartilage

Basic characteristics
Cartilage is a type of a supporting connective tissue. It is avascular, firm and elastic. White or yellowish solid mass. It is characterized by a high concentration of glycosaminoglycans (GAGs) and proteoglycans. During embryonic and fetal development it becomes the basis for skeletal development. It diminishes with increasing age. It covers the articular surfaces of most joints. It forms the cartilaginous skeleton of many organs.


 * It's made up of:
 * Chondrocytes (unlike connective tissue, it does not contain any other cell types)
 * Base matter (intercellular mass)
 * Fibrous component - determines strength and flexibility
 * Amorphous component - utilized in nutrition
 * Cartilage is categorized into three types according to the composition of the underlying mass:
 * Hyaline
 * Elastic
 * Fibrous

Cartilage growth
Cartilage develops from the mesenchyme. The cells produce ECM by which they envelope, move further apart and remain embedded in the lacunae. Cartilage growth occurs in two ways:


 * Appositional growth = a system of regrowth of new cartilage from the inner layer of the perichondrium, which contains progenitor cells.
 * Interstitial growth = mitotic division of cartilage cells (proliferation).

In articular cartilage, the cells and intercellular mass must be replaced from the inside because apositional growth does not occur due to the absence of perichondrium on the surface. The regenerative capacity of cartilage is low, partly due to the fact that it is avascular and therefore with little metabolic activity. Regeneration is incomplete and often a scar of dense collagenous connective tissue forms.

Cartilage growth is indirectly influenced by the growth hormone somatotropin. It promotes the production of somatomedins, which then directly stimulate the hyaline cartilage cells to grow.

Hyaline cartilage

 * very difficult regeneration - there is no vascular system

Hyaline cartilage is the most common type of cartilage. It has a semi-transparent, glassy appearance and contains chondroitin sulfate A and C. Chondromucoprotein masks collagen fibres and fibrils.

The surface of the cartilage is covered by the perichondrium, which is highly vascularised and which therefore nourishes the cartilage - except for articular cartilage, which has no perichondrium and is nourished by synovial fluid.


 * location: nasal cartilages, laryngeal skelet, bronchi, trachea, suspension of ribs on sternum, articular cartilages, growth cartilages in long bones.

Chondrocytes
They are oval cells with a small spherical nucleus and light cytoplasm (rich in glycogen inclusions and fat droplets). Under the perichondrium are spindle-shaped which changes to a spherical shape towards the interior of the cartilage.

In the intercellular mass they are contained in the so-called lacunae, around which there is a basophilic sheath called the territorial matrix (basophilia conditioned by the increased occurrence of proteoglycans). Chondrocytes form isogenetic groups (in regions called territories or chondrons) or are deposited singly. Isogenetic groups are formed by mitotic division of the parental chondrocyte. Minimal proliferative activity is characteristic, and this cell type rarely divides. They are incapable of regeneration and proliferation.

Intercellular mass
The extracellular matrix of cartilage consists of type II collagen (collagen fibrils - not visible on the slide, masked by amorphous material), proteoglycans and multiadhesive glycoproteins. The most abundant proteoglycan is aggrecan, while chondronectin is an important glycoprotein. Most of the mass (40-60%) of cartilage consists of water bound to the GAG of proteoglycans.

Perichondrium
The connective tissue sheath forming the cartilage sheath that separates the cartilage from the surrounding tissues. It provides nourishment to the cartilage as it contains blood vessels and also small nerves. Strengthens cartilage and prevents excessive bending.


 * it consists of 2 components:
 * external = dense irregular collagenous connective tissue; strengthens cartilage; prevents cracking when bending;
 * internal = loose collagenous connective tissue rich in blood vessels; contains progenitor cells; nourishes cartilage.

Elastic cartilage
It is composed of collagen fibrils (type II collagen) and a net of elastic fibres, which give it a yellowish colour and ensure flexibility and elasticity. It contains numerous chondrocytes, which are evenly dispersed but form small isogenetic groups (2 to 3 chondrocytes each).

Staining with Weigert's resorcin-fuchsin, orcein, aldehyde fuchsin - on the slide we see a fibrous component that forms networks around the chondrocytes.


 * location: ear lobe, Eustachian tube (only partially), epiglottis - laryngeal flap, small laryngeal cartilages, nasal septum

Fibrocartilage
It represents the transition between dense irregular collagen connective tissue and cartilage, in which the fibrous component predominates. It consists of thick collagen fibres (type I collagen). It contains few chondrocytes, which are isolated, i.e. do not form isogenetic groups. There is little amorphous material around the chondrocytes, therefore, due to the lower presence of proteoglycans, fibrous cartilage is more eosinophilic than the previous two types of cartilage.


 * location: intervertebral discs, symphysis pubis, some ligaments in temporomandibular joint, disci et menisci articulares.

Related articles

 * Bone
 * Tissue
 * Ossification
 * Chondrocyte