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		School of Anatomy and Human Biology - The University of Western Australia
		Blue Histology - Skeletal Tissues - 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

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.

 
Elastic Cartilage

• occurs in the epiglottic cartilage, the corniculate and cuneiform cartilage of the larynx, the cartilage of the external ear and the auditory tube.
• corresponds histologically to hyaline cartilage, but, in addition, elastic cartilage contains a dense network of delicately branched elastic fibres.

 
Suitable Slides

Sections of the epiglottis - elastin

Epiglottis, human, elastin
Preparations of the epiglottis are usually dominated by the cartilage surrounded by varying amounts of connective tissue and epithelia. The appearance of the cartilage (in this preparation a blue-green colour) will depend on the method used to show tissue components other than elastic fibres. Although the matrix appears blue-green, the typical organisation of cartilage is readily visible. Within the green matrix you can see the fine elastic fibres which give this cartilage its elastic properties. The elastic fibres may form dense masses in which individual fibres are difficult to distinguish. The staining of these masses of fibres may appear more reddish than dark-violet.
A change of the colour of the stain in intensely stained tissue areas is called "metachromatic staining".
Draw and label a small section of elastic cartilage.

Depending on the quality of tissue preservation on your slide, it may be possible to identify the types of epithelia present in the section. It wouldn't hurt trying.

 
Fibrous Cartilage

• is a form of connective tissue transitional between dense connective tissue and hyaline cartilage. Chondrocytes may lie singly or in pairs, but most often they form short rows between dense bundles of collagen fibres. In contrast to other cartilage types, collagen type I is dominant in fibrous cartilage.
• is typically found in relation to joints (forming intra-articular lips, discs and menisci) and is the main component of the intervertebral discs.
• merges imperceptibly into the neighbouring tissues, typically tendons or articular hyaline cartilage. It is difficult to define the perichondrium because of the fibrous appearance of the cartilage and the gradual transition to surrounding tissue types.

 
Suitable Slides

sections of intervertebral discs or articular discs - H&E, van Gieson

Fibrous Cartilage, Intervertebral Disc, sheep, H&E and Articular Disc, rabbit, H&E
The fibrous cartilage forming the intervertebral discs varies in appearance from the center of the disc (the nucleus pulposus) the the periphery of the disc (the annulus fibrosus). Centrally, the fibrous matrix is very loose. The jelly-like consistency of the central part allows the intervertebral discs to function as a shock absorber. Towards the periphery, the fibrous matrix is organised into layers. It is often visible that the fibres of different layers are oriented at angles to each other - similar to the orientation of the thread in radial tires. Chondrocytes are very flattened in the periphery and may be difficult to find.
Midway between periphery and center of the intervertebral disc, chondrocytes are scattered singly or in small isogenous groups in the dense fibrous matrix of the cartilage. If you take a close look at the cells you will see that their appearance actually resembles that of chondrocytes in other types of cartilage - their characteristic appearance distinguishes fibrous cartilage preparations from connective tissues. The very regular arrangement of the fibres in the articular disc may initially let you guess at dense regular connective tissue. Isogenous groups of chondrocytes again identify the tissue as fibrous cartilage.
Draw a small section of the fibrous cartilage, including (if possible) a group of chondrocytes.

 
Articular Cartilage

• is a specialised form of hyaline cartilage.
• transforms the articulating ends of the bones into lubricated, wear-proof, slightly compressible surfaces, which exhibit very little friction.
• is not surrounded by a perichondrium and is partly vascularised.
• is, depending on the arrangement of chondrocytes and collagenous fibres, divided into several zones:

  
  Tangential layer

  Chondrocytes are rather small and flattened parallel to the surface. The most superficial part (lamina splendens) is devoid of cells. Collagen fibres in the matrix of the tangential layer are very fine. They run parallel to the surface of the cartilage.

  Similar to the collagen fibres of the skin, the general orientation of collagen fibres in articular cartilage is determined by tensile and compressive forces at the articulating surfaces.

  Transitional zone

  The chondrocytes are slightly larger, are round and occur both singly and in isogenous groups. Collagen fibres take an oblique course through the matrix of the transitional zone.

  Radial zone

  Fairly large chondrocytes form radial columns, i.e. the stacks of cells are oriented perpendicular to the articulating surface. The course of the collagen fibres follows the orientation of the chondrocyte columns.

  Calcified cartilage layer

  It rests on the underlying cortex of the bone. The matrix of the calcified cartilage layer stains slightly darker (H&E) than the matrix of the other layers.

The main source of nourishment for articular cartilage is the synovial fluid, which fills the joint cavity. Additional small amounts of nutrients are derived from blood vessels that course through the calcified cartilage close to the bone.
Living chondrocytes have been found in small pieces of cartilage floating in the joint cavity after damage to the articular cartilage.

Osteoarthritis, the slow progressive degeneration of articular cartilage, is the most common joint disease. It may be caused by persistent and abnormally high loads on the joint surfaces, which initially result in the loss of proteoglycans and chondrocytes from the articulating surface of the cartilage. Subsequently, the cartilage may crack (fibrillate), erode and expose the underlying bone.

 
Suitable Slides

Sections of large joints - H&E
Layers are difficult to identify in the articular cartilage of small joints.

Articular Cartilage, bovine, H&E
The layers of articular cartilage are easiest to identify in large joints. Note the changing orientations of the lacunae and isogenous groups at different depth in the cartilage. The changing orientations of chondrocytes and isogenous groups reflect the orientations of the collagen fibres in the matrix. The fibres are not visible in the slide. The darker hue of the cartilage close to the bone is caused by the calcification of the cartilage.
Draw the articular cartilage at low magnification. Indicate in your drawing the preferred orientations of lacunae and isogenous groups and the expected orientation of collagen fibres.

Degeneration and Regeneration of Cartilage

Due to the fairly poor access of nutrients to the chondrocytes they may atrophy in deep parts of thick cartilage. Water content decreases and small cavities arise in the matrix, which often leads to the calcification of the cartilage. This further compromises nutrition. The chondrocytes may eventually die, and the cartilage is gradually transformed into bone.

Chondrogenic activity of the perichondrium is limited to the period of active growth before adulthood. Although chondrocytes are able to produce matrix components throughout life, their production can not keep pace with the repair requirements after acute damage to hyaline or articular cartilage. If these cartilages are injured after the period of active growth, the defects are usually filled by connective tissue or fibrous cartilage. The extracellular matrix of these "repair tissues" is only poorly integrated with the matrix of the damaged cartilage.

Fortunately, cartilage is rather well suited for transplantation - the metabolism of the chondrocytes is rather slow, the antigenic power of cartilage is low, and it is difficult, if not impossible, for antibodies or cells of the immune system to diffuse through the matrix into the cartilage.

page content and construction: Lutz Slomianka
last updated: 6/08/09