Types of Cartilage

CARTILAGE

• is a specialised type of connective tissue.
• consists, like other connective tissues, of cells and extracellular components.
• does, unlike other connective tissues, not contain vessels or nerves.
• is surrounded by a layer of dense connective tissue, the perichondrium.

Cartilage is rather rare in the adult humans, but it is very important during development because of its firmness and its ability to grow rapidly. In developing humans, most of the bones of the skeleton are preceded by a temporary cartilage "model". Cartilage is also formed very early during the repair of bone fractures.
 

Types of Cartilage

Hyaline Cartilage

Hyaline cartilage develops, like other types of connective tissue, from mesenchymal cells. From about the fifth foetal week precursor cells become rounded and form densely packed cellular masses, chondrification centres. The cartilage-forming cells, chondroblasts, begin to secrete the components of the extracellular matrix of cartilage. The extracellular matrix consists of, ground substance (hyaluronan, chondroitin sulfates and keratan sulfate) and tropocollagen, which polymerises extracellularly into fine collagen fibres.

Tropocollagen type II is the dominant form in collagen fibres of almost all types of cartilage.

As the amount of matrix increases the chondroblasts become separated from each other and are, from this time on, located isolated in small cavities within the matrix, the lacunae. Concurrently the cells differentiate into mature cartilage cells, chondrocytes.

Growth occurs by two mechanisms

• Interstitial growth - Chondroblasts within the existing cartilage divide and form small groups of cells, isogenous groups, which produce matrix to become separated from each other by a thin partition of matrix. Interstitial growth occurs mainly in immature cartilage.
• Appositional growth - Mesenchymal cells surrounding the cartilage in the deep part of the perichondrium (or the chondrogenic layer) differentiate into chondroblasts. Appositional growth occurs also in mature cartilage.

Like all protein-producing cells, chondroblasts contain plenty of rough endoplasmatic reticulum while they produce matrix. The amount of rough endoplasmatic reticulum decreases as the chondroblasts mature into chondrocytes. Chondrocytes fill out the lacunae in the living cartilage.

The matrix appears structureless because the collagen fibres are too fine to be resolved by light microscopy (~20nm), and because they have about the same refractive index as the ground substance. Collagen accounts for ~ 40% of the dry weight of the matrix.

The matrix near the isogenous groups of chondrocytes contains larger amounts and different types of glycosaminoglycans than the matrix further away from the isogenous groups. This part of the matrix is also termed territorial matrix or capsule. In H&E stained sections the territorial matrix is more basophilic, i.e. it stains darker. The remainder of the matrix is called the interterritorial matrix. Fresh cartilage contains about 75% water which forms a gel with the components of the ground substance. Cartilage is nourished by diffusion of gases and nutrients through this gel.

 
Suitable Slides

sections of the trachea or larynx - H&E, van Gieson

Trachea, cat, H&E and Trachea, cat, van Gieson
Both stains are equally well suited to look at the organisation of hyaline cartilage. The van Gieson method stains collagen red. The cartilage appears as a wide red zone underneath the epithelium and loose connective tissue, which line the lumen of the trachea. The staining may appear a little lighter close to the lacunae. This lighter stained zone defines the territorial matrix surrounding the lacunae and chondrocytes. Colour intensities appear reversed in the H&E stained section. The two compartments of the matrix are usually better defined than in van Gieson stained sections. The interterritorial matrix appears very light; the territorial matrix is somewhat darker. Groups of chondrocytes surrounded by these lighter (van Gieson) or darker (H&E) staining zones belong to the same isogenous group. A layer of dense connective tissue surrounding the cartilage and blending with it is the perichondrium.
The isogenous groups may form small "squares" (e.g. four chondrocytes, separated by thin cartilage membranes, in a 2x2 arrangement) or short columns (e.g. four chondrocytes in a 1x4 arrangement).
Draw a small section of the cartilage and identify in your drawing territorial matrix, interterritorial matrix, isogenous groups, and chondrocytes. Think about how the spatial arrangement of chondrocytes in the isogenous group may reflect patterns of cell divisions.

Title: Types of Cartilage
by:om at 2013-02-15T02:56:00+07:00
Rating: 4.8 of 5 Reviews
Types of Cartilage