Elastic, Fibrous, Articular Cartilage

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.

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 discsor articular discs - H&E, van Gieson

Fibrous Cartilage, Intervertebral Disc, sheep, H&Eand 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.