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What is cartilage?

Anatomy, histology and definition of cartilage.

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Hey everyone! This is Nicole from Kenhub, and in this tutorial, we'll be answering the question, what is cartilage?

So, we've all come across cartilage in one way shape or form during our anatomy studies but what is cartilage? Well, cartilage is a type of connective tissue found in the body and it's a slightly translucent tissue consisting of cartilage cells or chondrocytes embedded in an extracellular matrix largely produced by the chondrocytes themselves. Sound complicated? Don’t worry, we're going to break it down to make it easier to understand.

Just before we get into the different types of cartilage, I want to talk a little bit about the extracellular matrix of cartilage – that is, the non-cellular component of substances created by the cells themselves in order to provide biochemical and structural support. The extracellular matrix of cartilage as I just mentioned is mainly produced by cartilage cells known as chondrocytes as we can see in this histological image of the elastic cartilage of the ear on the right.

The chondrocytes not only produce the amorphous ground substance and fibers that make up the extracellular matrix but they are also responsible for maintaining it. And, again, as you can see in the image, chondrocytes are loosely scattered throughout the extracellular matrix which is made up of collagen and are found in a cavity known as lacunae.

Cartilage also has a structure known as perichondrium – a fibrous sheath that envelops the cartilage. And the perichondrium has two layers – an outer fibrous layer that contains fibroblasts which you can see me pointing out just here with my arrow and this layer produces collagen primarily and the perichondrium also has a chondrogenic layer that contains chondroblasts and chondrocytes and which is the innermost layer. An important thing to note is the fact that perichondrium is not present within fibrocartilage, articular cartilage and hyaline cartilage under the skin.

And we've mentioned one of them already but, generally, there are three types of cartilage found in the human body namely, the hyaline cartilage as seen in the tracheal rings and articular cartilage, elastic cartilage as seen in the epiglottis, and fibrocartilage as seen in the temporomandibular joint.

Cartilage is a non-vascular tissue which means it does not contain blood vessels, therefore, making it ideal for articular surfaces. And it's important to note that the difference between the three types of cartilage is mainly due to the types of fibers present in the extracellular matrices as we'll see in the following slides.

So, first, let's have a little bit of a chat about hyaline cartilage. And hyaline cartilage is the most common type of cartilage found in the human body. The term hyaline is derived from the Greek word meaning transparent or glassy and as you can see from the non-highlighted part of the image, hyaline cartilage has a bluish white glassy appearance. In fact, hyaline cartilage serves many functions but is mainly associated with the skeletal system. Therefore, the first place we would normally find hyaline cartilage is in the cartilage usually found within the articular cartilage – the cartilage that covers the articulating surface of the bone. You can see an example of this in the image on the right which is of the first six costal cartilages. And it's this cartilage highlighted in green here connecting the anterior end of the rib to the sternum or the growth plate that is considered articular cartilage.

In addition, hyaline cartilage also occurs in extraskeletal sites such as the larynx, the cartilage rings of the trachea, the bronchi of the lungs and the tip of the nose. During fetal development, the hyaline cartilage in the embryo forms from the differentiation of mesenchymal cells that become crowded together in what are called chondrification centers. Unlike the other two types of cartilage, the extracellular matrix of hyaline cartilage is granular or homogenous and contains type two collagen and a high concentration of aggregating proteoglycans.

The second type of cartilage is known as elastic cartilage and this type of cartilage gives form to the external ear auricle, the epiglottis and the Eustachian or auditory tubes. And elastic cartilage differs from hyaline cartilage in that the extracellular matrix of elastic cartilage contains a substantial amount of elastic fibers which contributes to its elasticity and resilience although it also has some collagen fibers.

The third type of cartilage known as fibrocartilage is also associated with the skeletal system, and this type of cartilage is strong and rigid and is mostly commonly associated with regions that have dense connective tissue in the body. Fibrocartilage or fibrous cartilage is found in the meniscus of the knee, the annulus fibrosus of the intervertebral discs, the temporomandibular joint, the pubic symphysis and, generally, near the point of attachment of some large tendons to bone. It is composed primarily of type one collagen and has a lower proteoglycan content.

So, we've seen the three types of cartilage that are present in the human body and shown a little bit about the composition of the extracellular matrices, but what is the purpose or function of cartilage in general? Now, as we've seen, cartilage is a firm yet elastic tissue and it can also have a tough rubber-like characteristic that permits it to sustain a great amount of weight when covering the articular surfaces of bone. In addition, it can serve as a shock absorbing pad between the articulating bones in the spine and, finally, cartilage permits growth. And there are two types of cartilage that take place within cartilage – appositional growth and interstitial growth – and we're going to have a little bit of chat about both of these in the following slides.

So, our first slide is about appositional growth, and appositional growth or exogenous growth is where the osteoprogenitor cells in the surrounding perichondrium produce a new layer of cartilage cells from the outside inwards or, in other words, growth that takes place on the surface of the cartilage. And as we can see in our histological image of the hyaline cartilage of the tracheal rings, we have our chondrocytes on the outside here growing downwards into the cartilage to make a new matrix just like our image up top here. And appositional growth occurs mainly in mature cartilage.

Interstitial growth is also known as endogenous growth where a growth occurs from within the cartilage or in the middle of the tissue meaning that the chondrocytes inside the cartilage divide and secrete new matrix which expands the cartilage just like in this image. And this sort of growth is important for the lengthening of long bones of the skeleton during childhood and adolescents. And, histologically, if we look at this image here on the right of the fetal epiphyseal plate, we can see the proliferation of the chondrocytes as they secrete new matrix.

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