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Elastic cartilage

An overview of histological features of elastic cartilage.

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Hello everyone! This is Joao from Kenhub and welcome to another histology tutorial where this time we're going to be talking about elastic cartilage. Now before we begin talking about elastic cartilage, let's first quickly define what cartilage is.

Cartilage is a type of connective tissue found in the human body that is characterized by cartilage cells known as chondrocytes which are embedded in the extracellular matrix. Cartilage is a flexible avascular type of connective tissue found in many different parts of the body and can be subdivided into three types according to its properties and components.

The three types of cartilage are hyaline cartilage which contains large amounts of type two collagen in its extracellular matrix and is the most common type of cartilage found in your body, then we have fibrocartilage whose matrix mainly contains type one collagen and can be found in areas of the body that are subject to pulling forces, and the other type is the star of this tutorial, the elastic cartilage, which as the name suggests contains numerous elastic fibers and as we will see later on this tutorial is quite a pliable type of cartilage. Cartilage essentially doesn't possess vasculature on its own so it derives its nutrition through diffusion from capillaries in neighboring tissue.

So that's it, an introduction of what cartilage is and the different types, and we're going to move on to talk about then elastic cartilage.

Elastic cartilage, as we just saw, is one of the three types of cartilage found in the human body. Here we see a micrograph of the external ear which is a typical structure in our body comprised of elastic cartilage. In addition to the external ear, elastic cartilage can also be found in the walls of the Eustachian tube seen here which is also known as the pharyngotympanic tube or auditory canal and the epiglottis seen here from the dorsal aspect is also made up of elastic cartilage.

Like the other two types of cartilage, the cartilage cells of elastic cartilage are known as chondrocytes and chondrocytes comes from the Greek word "chondros" which means cartilage and "cytos" that means cell. Here in this micrograph, you can see these cartilage cells highlighted in green. These cartilage cells are responsible for production or secretion of extracellular matrix, also known as ECM of elastic cartilage, and these cells remain embedded in the ECM that they produce.

A typical image of a chondrocyte nucleus is seen in this image highlighted in green in which we can easily distinguish the round shape of the cells in contrast with the flattened shape of the nuclei of the fibroblasts found in the same tissue. The chondrocytes of elastic cartilage and also those that we find in the other two types of cartilage, these chondrocytes are found inside cavities or spaces within the extracellular matrix known as lacunae. Here we see these cavities or lacunae highlighted in green. Remember that the singular form of lacunae is lacuna.

Within the cartilage matrix or ECM, chondrocytes can either be found as single cells or they can be found clustered together in aligned aggregates where they are collectively known as isogenous groups of chondrocytes. Here in this micrograph, you can see an example of isogenous groups of these cartilage cells. It is also important to pinpoint that we can see the chondroblasts in the periphery. This is relevant if we want to understand the origin of the chondroblasts and, therefore, the origin of the isogenous group of chondrocytes.

The term isogenous refers to a common progenitor cell, so when we say isogenous group of chondrocytes, we actually refer to a cluster of chondrocytes that originate from a division of a common progenitor cell which is also known as a chondroblast.

As we have already seen, chondrocytes are responsible for the production of the extracellular matrix of cartilage which is also simply known as the cartilage matrix. This matrix contains high amounts of heavily glycosylated proteins known as proteoglycans and long chain polysaccharides that contain an amino sugar known as glycosaminoglycans.

Now, in this slide, we can see the cartilage matrix highlighted in green. As we were saying, the presence of these glycosylated proteins in the matrix gives cartilage tissue the capability to withstand compressional forces. The cartilage matrix of elastic cartilage also contains elastic fibers and collagen making this tissue tough and flexible at the same time. These elastic fibers found in the extracellular matrix are responsible for the elastic properties of this type of cartilage. Here in this micrograph, you can see the elastic fibers highlighted in green. These elastic fibers are arranged in dense, branching networks in the cartilage matrix. All the other components of the extracellular matrix allow the cartilage to bear mechanical stresses.

Elastic cartilage found in the body or all elastic cartilage found in the body is surrounded by a dense connective tissue sheath known as the perichondrium. This connective tissue sheath helps support the cartilage and also contains vessels that supply the cartilage as well as lymphatic vessels and nerves. It should be noted that the perichondrium is comprised of two layers – an outer fibrous layer which contains elastic and collagen fibers and an inner chondrogenic layer from which new cartilage cells arise.

Chondroblasts seen here in this micrograph right now are found in the chondrogenic layer of the perichondrium that we just mentioned and they are immature chondrocytes which are derived from the perichondrial cells. Remember that these cells also produce extracellular matrix.

We have covered all the relevant information connected to elastic cartilage - yes, this is a very short tutorial. It is time for us to now talk about a few clinical notes connected to this type of tissue.

An interesting clinical note to bear in mind is that cartilage tissue has an inability to regenerate or repair fully. This feature can be attributed to the fact that chondrocytes or cartilage cells are immobile as they are situated within the lacunae in the cartilage matrix. Taking into the account the fact that chondrocytes have low mitotic and metabolic rates, their inability to regenerate or repair fully can be then explained.

In addition to that, the lack of vessels in cartilage tissue provides another explanation for this inability of the tissue to regenerate. However, a limited amount of repair can take place in injured cartilage with the help of chondroblasts and fibroblasts if the injury that occurs involves the perichondrium in which case these cells are mobilized to repair the injury. It should be noted that in most such cases, the new tissue that is formed during the repairing process is dense connective tissue and as such, the cartilage does not necessarily retain its normal function. That is to say then that the function is very often impaired.

And this brings us to the end of this tutorial, but before we go, I would like to quickly recap what we've learned.

So far, we have learned that elastic cartilage is one of the three types of cartilage found in the human body. By now, you should be able to identify that elastic cartilage can be found in the pinna of the external ear, the epiglottis of the larynx, and Eustachian or auditory tube. We also saw that like the other two types of cartilage, elastic cartilage is comprised of cartilage cells known as chondrocytes and extracellular matrix which contains elastic fibers and type two collagen. In addition to that, we also saw that because of the composition of the extracellular matrix, elastic cartilage has somewhat pliable and resilient properties.

And that's it for this tutorial. Thank you for watching and I will see on the next one.

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