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Overview of fibrocartilage.

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Hello everyone! This is Joao from Kenhub and on this tutorial – histology tutorial to be more specific – we will be discussing one of the three types of cartilage that we have in our body which is known as fibrocartilage. Now before we begin discussing fibrocartilage, let's first have a look at cartilage in general.

Here in this image, we can see the cartilage that cushions the spinal vertebrae which is known as an intervertebral disc. Cartilage is an avascular form of connective tissue that is tough as well as flexible. If we take a closer look at a diagrammatic view, I can show you that cartilage is characterized by its extracellular matrix which usually contains high amounts of glycosaminoglycans which are also called or abbreviated as GAGs, and the extracellular matrix also contains proteoglycans. Now, these extracellular matrix interacts with collagen and also elastic fibers and the variations in the composition of the cells and matrix of cartilage are what defined what type of cartilage it is.

The main cells of cartilage tissue are known as chondrocytes and are imbedded in the extracellular matrix that we mentioned before. There are three types of cartilage which are defined according to the composition of the extracellular matrix and chondrocytes. The first of which is shown in this micrograph here and it is known as hyaline cartilage. This is the most common type of cartilage found in your body and its extracellular matrix contains predominantly type two collagen fibers. This type of cartilage is mostly found on the joint surfaces, in the respiratory tract, and it is also widespread in the embryonic skeleton.

The second type is known as elastic cartilage and is histologically similar to hyaline cartilage but it contains a high amount of elastic fibers making it more pliable and distensible. This type of cartilage is mainly found in the outer ear as in this image that you see now as well as the epiglottis and the Eustachian tubes. And, finally, the third type is known as fibrocartilage which is the subject of our tutorial here. Fibrocartilage is essentially a combination of hyaline cartilage and dense regular connective tissue and one of its functions is to act as a sort of cushion for bones in your body.

In this tutorial specifically, we will begin by looking at some examples of where fibrocartilage tissue may be found in the body so that you can get a better idea of its application and function. We will then take a closer look at the histological structure of fibrocartilage and finally, we will examine the role of fibrocartilage in embryonic development which has important clinical applications that we will also be covering here on this tutorial.

So, without further ado, let's take a look at some examples of fibrocartilage that may be familiar to you.

First here on our list, we have the intervertebral discs. These fibrocartilage discs are extremely important for slide movements of the spine and they also act as shock absorbers in the spinal column preventing then damage to the underlying vertebrae during stress or movements. Fibrocartilaginous tissue can also be found in some places in your body where tendons can attach to bone such as here where we can see the calcaneal or Achilles tendon highlighted in green. Fibrocartilage can also be found at the pubic symphysis which is a cartilaginous joint formed between the superior rami of the left and right pubic bones right here. Here we see the pubic symphysis highlighted then in green.

In addition to the pubic symphysis that we just saw, fibrocartilage also comprises the articular disc of the temporomandibular joint, also known as TMJ. As we can see here in this illustration, the temporomandibular joint is a synovial joint formed between the squamous part of the temporal bone and the condylar part of your mandible which is also known as the lower jaw. Other areas where fibrocartilage can be found include the lateral and medial menisci of the knee joint seen here highlighted in green, and, finally, at the wrists, where fibrocartilage forms a triangular fibrocartilage complex.

So let's continue on and let's talk a little bit more about the structure and composition of fibrocartilage.

As we just saw, fibrocartilage is a combination of hyaline cartilage and dense regular connective tissue. The cartilage cells of fibrocartilage are also simply known as chondrocytes and we're going to be taking a closer look at these. Here we can see a micrograph with some chondrocytes highlighted in green. The chondrocytes in fibrocartilage are similar to those found in hyaline cartilage and are responsible for production of the extracellular matrix which consists mainly of collagen and proteoglycans and ground substance of fibrocartilage which they are embedded in. The cells are not only responsible for producing but also maintaining the extracellular matrix.

And let's go back briefly to our diagram where we can see that within the extracellular matrix, chondrocytes are contained within cavities which are known as lacunae. Here in this micrograph, you can see these lacunae highlighted in green with chondrocytes situated at their center. In fibrocartilage, the chondrocytes are arranged in aligned isogenous aggregates as you can see here in this micrograph and they can also occur singly. 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.

In addition to chondrocytes, fibroblasts can also be found in fibrocartilage. These fibroblasts can be distinguished from chondrocytes because their nuclei appear elongated and flattened while the nuclei of chondrocytes have a rounded appearance. The matrix then is produced by the chondrocytes found in fibrocartilage contains both type one and type two collagen and is called the cartilage matrix.

Here in this micrograph, you can see the cartilage matrix or extracellular matrix highlighted in green. As we get older, the ratio of type one to type two collagen in fibrocartilage changes and more type two collagen is produced by chondrocytes in older individuals. And this is due to the metabolic activity of chondrocytes.

Here in this micrograph, we can see some collagen bundles that form part of the cartilage tissue. The proportions of type one and type two collagen in the extracellular matrix varies depending on the location of the fibrocartilage. For example, if we take another look at the menisci of the knee, the fibrocartilage found here contains more type one collagen and only a small amount of type two collagen.

We have seen that many examples of fibrocartilage are located at the terminal end of several bones and so it is worth looking at fibrocartilage within the context of bone formation. Here in this micrograph, we see a longitudinal section of a fetal elbow joint. You can quite clearly see the humerus right here and the ulna around here. As I mentioned at the start of this tutorial, fibrocartilage is made up of hyaline cartilage and dense regular connective tissue. During fetal development, bone formation occurs through two processes which are known then as intramembranous ossification, the other one is endochondral ossification. In this tutorial, we are most interested in one of them which is endochondral ossification.

Here on this micrograph, we can actually see two areas at which endochondral ossification is taking place. Now let's take a closer look. The endochondral ossification here is occurring in this direction and let's zoom in even further where we can see highlighted in green the area where ossification has taken place. Endochondral ossification is the process by which cartilage is converted to bone during embryological development. Most bones, especially the long bones, are formed through endochondral ossification. In this case, hyaline cartilage is used as a template for bone formation. Secondary ossification centers are then found in the epiphyseal plate shown here highlighted in green. These plates that are also known as growth plates are located at each end of all long bones.

At this stage, you can see quite a large number of chondrocytes present at the epiphyseal plate. From the epiphyseal plate later in childhood and adolescence, the bones lengthened through the production of more cartilage cells and essentially the formation of more bone tissue through the process of endochondral ossification.

After the ossification process, bone matrix formation takes place. As you can tell just by looking at this micrograph, the bone matrix appears significantly different from the cartilage matrix we saw earlier. For one, the cartilage matrix is completely avascularized which means that it's devoid of blood vessels whereas the bone matrix is vascularized or is infiltrated by blood vessels. Thus, the bone matrix is the intercellular substance of bone tissue consisting of collagen fibers, ground substance, and inorganic bone salts produced by osteoblasts which are the progenitor cells of bones.

We are done talking about fibrocartilage and the histology of fibrocartilage. We're ready to talk about a few clinical notes that are connected to this topic. So we have just seen that all long bones of your body are formed through a process known as endochondral ossification. When this process does not function or work properly during fetal development, this will then result in a disorder known as achondroplasia which is more commonly known as dwarfism.

It is an autosomal dominant disorder and comes about as a result of a mutation in the fibroblast growth factor receptor three gene. It results in abnormal endochondral ossification such as in the epiphyseal growth plate where the zone of proliferation is disorganized and narrowed which then results in abnormal bone growth and, therefore, severely shortened bones. This is more visible in the long bones as individuals with achondroplasia typically have a normal or average sized trunk, short limbs and midface hypoplasia.

Now this brings us to the end of this tutorial but before we go, let's briefly recap what we looked at. First of all, we introduced cartilage in general and saw that there are three types of cartilage which are then hyaline cartilage. We also looked at elastic cartilage and the topic of this tutorial, fibrocartilage. We saw that fibrocartilage is comprised of hyaline cartilage and dense regular connective tissue.

Looking at the histological composition of fibrocartilage, we then looked at chondrocytes which are cartilage cells found in fibrocartilage that produce the extracellular matrix. The extracellular matrix of fibrocartilage comprises type one and type two collagen fibers, and then we went on to discuss endochondral ossification, the epiphyseal plates found at both ends of all long bones, and finally looked at a clinical disorder of abnormal endochondral ossification named achondroplasia.

I hope you have learned a lot from this tutorial. Feel free to take the quiz to test your knowledge. Thank you for watching and I will see you on the next one.

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