Cartilage is a flexible but strong supportive connective tissue.  Unlike bone and all other connective tissue types, cartilage is avascular,  lacking blood vessels.  For this reason alone, cartilage does not possess the regenerative capacity of bone or the other connective tissue types.  Remember, nutrient delivery is essential for tissue repair.  Blood vessels provide the nutrient delivery to most tissues.

Cartilage contains a gelatinous ground substance called chondroitin sulfate.  Embedded within the ground substance are collagen and elastic protein fibers.  Together, these components form a matrix that is flexible, yet very durable and also resistant to compression forces.  Cartilage therefore, is found in many body locales where support, flexibility, and resistance to compression are important.

There are three varieties of cartilage, hyaline, elastic, and fibrocartilage.  The most abundant type is hyaline, found as supportive tissues in the nose, ears, trachea, larynx, and smaller respiratory tubes.  As articular cartilage, hyaline is found covering the articular surfaces of bones in synovial joints.  Here, hyaline cartilage reduces friction and acts as shock-absorbing  tissue.  Hyaline cartilage also forms the costal cartilages where ribs attach to the sternum and is the precursor to bone in most of the embryonic skeleton.

Cartilage is formed by chondroblasts.  These are cellular progeny of mesenchyme cells.  As a result, chondroblasts are typically found along the edges of cartilage plates just under the perichondreum where new appositional growth occurs.  Cartilage can also expand via interstitial growth.  In epiphyseal plates, chondrocytes enlarge and divide during maturation to form single or multicellular lacunae arranged in linear stacks.  Take a look now at cartilage in an epiphyseal plate!  You've seen this before as a body locale where endochondral bone formation occurs!  Use this image to review the following key points.

• cartilage possess lacunae that contain much larger cells than those of bone.

• to the left of the stacks of chondrocytes, the extracellular matrix of hyaline cartilage is very uniform in appearance in contrast to that of bone.  

• cartilage typically appears blue or light purple because extracellular components of the matrix attract basic stains.  Bone on the other hand typically appears pink or red in most stained specimens.

• bone always forms first as cancellous bone.  Notice the trabeculae with bone marrow cells between them(dark blue granular areas).

Hyaline Cartilage

The type of protein fiber embedded within the matrix of cartilage determines the cartilage type.  In hyaline cartilage protein fibers are large and predominantly collagen.  The optical density of these fibers is the same as the ground substance surrounding them and as a result, they are not visible within the extracellular matrix.  Hyaline cartilage subsequently appears as a very uniform, glossy type tissue with evenly dispersed chondrocytes in lacunae.  Typically, perichondreum is found around hyaline cartilage.

• Here is a good example of hyaline cartilage!

• Here is hyaline cartilage with perichondreum!  Chondroblasts would be found as flattened, elongate cells between the perichondreum and cartilage.

Elastic Cartilage

Elastic cartilage has a preponderance of dark-staining elastic fibers embedded in ground substance.  These fibers are clearly visible and this trait is the single, best identifier to be used for differentiating elastic cartilage from hyaline.  Perichondreum is also typically found around elastic cartilage.  Elastic cartilage is found in the pharyngotympanic(eusatachian) tubes, epiglottis, and ear lobes where needs dictate supportive tissues possess elasticity.

• Compare this elastic cartilage to the hyaline you just viewed!  Sometimes the fibers are fine.  Do you see lacunae?

• In this closer view of elastic cartilage, the fibers are more visible!  Note the lacunae!

Fibrocartilage

Fibrocartilage(fibrous) is a type of cartilage that contains fine collagen fibers arranged in layered arrays.  In contrast to the very uniform appearance of hyaline cartilage, fibrocartilage possesses a more open or spongey architecture with gaps between lacunae and collagen fiber bundles.  It is this open spongey structure that makes fibrocartilage a good shock-absorbing material in the pubic symphysis and intervertebral disks.  It can appear quite different in these two locales.  Most textbooks show images of fibrocartilage from the intervertebral disks where it is very open and loose.  In the pubic symphysis, it can be much tighter in construction, appearing like a dense connective tissue with lacunae.  

• Here is tighter fibrocartilage from a pubic symphysis!  Note the small gaps in the matrix and cells in lacunae!

• Here is fibrocartilage that is much more open and loose!  Note the small collagen fiber bundles that run in different directions.  This is a more obvious spongey, shock absorbing type of tissue!

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