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Hyaline cartilage

A histological overview of the most common type of cartilage in the human body.

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Hello everyone! This is Joao from Kenhub, and in this tutorial, we will be discussing the most common type of cartilage found in the human body which is hyaline cartilage.

Cartilage is one of the types of connective tissue that is found in your body. It is an avascular flexible tissue containing cartilage cells known as chondrocytes that secrete an extracellular matrix in which they reside. It also contains protein sugar complexes known as proteoglycans and glycosaminoglycans. Fibers present in cartilage tissue include collagen fibers and elastic fibers which contribute to the flexibility properties of this tissue.

There are three types of cartilage which are defined according to the composition of the extracellular matrix and the cells which are then the chondrocytes. These types of cartilage are then hyaline cartilage, another one we can see here on the screen is elastic cartilage, and the other one is fibrocartilage. We will be then discussing the first type here on our list which is hyaline cartilage.

So in this tutorial, we will then discuss the parts of the body that hyaline cartilage can be found on. We will also look at many components of hyaline cartilage such as the perichondrium. We will also talk about cartilage cells found in this type of cartilage called chondroblasts and chondrocytes. In addition, we will also discuss the extracellular matrix, the capsular matrix, the territorial matrix, the interterritorial matrix, the tangential, radial and calcified layers, and finally, we will end this tutorial by discussing a clinical disorder associated with hyaline cartilage which is known as osteoarthritis.

So let's begin.

First, I just want to say that hyaline cartilage gets its name from the Greek word "hyalos" which loosely translated means glassy. In a way that is because this type of cartilage has a smooth translucent or glassy appearance. It is the most widespread type of cartilage found in the body and it can be found in the nose, also found in your trachea and larger bronchi, also the cartilages of the larynx. You can find it on the articular surfaces of movable joints, the epiphyseal plates of long bones, and the costal cartilages.

Here in this micrograph, we see an example of hyaline cartilage. This is a section taken from the cartilage rings found in the wall of the trachea. An interesting note to remember is that during fetal development, compact bone of the human skeleton is formed through a process known as endochondral ossification. Through this process which begins around the third month of fetal development, hyaline cartilage is gradually replaced by bone. Here we see a micrograph of the fetal elbow where endochondral ossification takes place.

I would like to show you guys another structure connected to hyaline cartilage. On this micrograph, we see the perichondrium which is highlighted in green. The perichondrium refers to a dense connective tissue layer that covers the surface of all hyaline cartilage except for the hyaline cartilage found on the articular surfaces of movable joints. This layer of dense connective tissue is comprised of type one collagen fibers and cells known as fibroblasts which are responsible for the production of the collagen fibers as well as the extracellular matrix.

The perichondrium is important in hyaline cartilage because it helps support the growth and maintenance of the cartilage. In addition, it also contains the vascular supply to the cartilage which provides nutrients to cartilage through diffusion as well as nerves and lymphatic vessels.

It should be noted that 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.

Moving on, we can now see a micrograph showing cartilage cells known as chondroblasts seen here highlighted in green. Chondroblasts are the progenitor cells of the chondrocytes and they are found in the inner layer of the perichondrium which we just saw. They have an oval shape as you can see here and they secrete extracellular matrix in which they are later embedded. They differentiate and mature to become then chondrocytes.

Chondrocytes are arranged in aligned isogenous aggregates as you can see here in this histology image. The term isogenous refers to a common progenitor cell. So when you say isogenous group of chondrocytes, we actually refer to a cluster of chondrocytes that originate from a division of a common progenitor cell or from the chondroblast that we have just mentioned. Although they are not as active as chondroblasts, chondrocytes still secrete extracellular matrix components such as sulfated glycosaminoglycans, proteoglycans, and collagen. They occupy space in the cartilage known as lacunae, filling this space completely, and their cytoplasm stains basophilic.

The nuclei of chondrocytes as seen here in this micrograph are also visible in preparations after hematoxylin and eosin staining. They always have a rounded shape in contrast with fibroblast nuclei which appear to have rather flattened and elongated nuclei.

As we have seen chondroblasts as well as chondrocytes produce and secrete the extracellular matrix of cartilage and are also responsible for maintaining it which is what we see now highlighted in green. This matrix secreted by the chondroblasts contains hyaluronic acid, collagen fibers, proteoglycans, and glycosaminoglycans. It also contains mainly type two collagen and stains basophilic in histological hematoxylin and eosin preparations. We can see the stain here on the image.

The negatively-charged sulfated GAGs or glycosaminoglycans in the extracellular matrix are bound to water molecules and in combination with the electrostatic bonds that occur between the elastic and collagen fibers also found in the extracellular matrix, this gives hyaline cartilage its properties which include stress and shock absorption. In addition, the extracellular matrix can be divided into three zones based on the distribution of some of its components. These three zones are termed by the capsular matrix, territorial matrix, and the interterritorial matrix.

Sounds like we're going to be talking about a movie but we are certainly not. Let's start by talking about this type of matrix. This one is the capsular matrix which you see now on the screen. This one is closely surrounding chondrocytes within hyaline cartilage. This type of matrix stains rather densely and contains a very high, if not the highest, concentration of sulfated proteoglycans and other multiadhesive glycoproteins. This part of the cartilage matrix is called the capsular matrix because it is the part of the matrix that immediately surrounds each lacuna. It also contains collagen fibers – type six to be more specific – that form a tight network enclosure around individual chondrocytes and through the integrin receptors on cell surfaces that they bind to helping then anchoring cartilage cells to the extracellular matrix.

We are now highlighting another set of structures on this image, this is what we call the territorial matrix, which is a part of the extracellular matrix that surrounds isogenous aggregates of chondrocytes which as we mentioned earlier are clusters of chondrocytes originating from the same progenitor cell. This territorial matrix contains a network of type two collagen fibers that are randomly arranged within the matrix. It is also rich in glycosaminoglycans and is part of the extracellular matrix that immediately surrounds the capsular matrix.

We are now going to change the highlight here on this micrograph to show you that the extracellular matrix that surrounds the territorial matrix, the structure that is known as interterritorial matrix. It is part of the extracellular matrix found between the cluster of chondrocytes described above. As we saw earlier, the perichondrium surrounds all hyaline cartilage except for the hyaline cartilage found on the articular surface of movable joints. This articular cartilage possesses four layers or zones.

The most superficial layer is known as the tangential layer or the superficial zone which you can now see highlighted in green and its function is to protect the three deeper layers from shear stress. This layer of hyaline cartilage is pressure-resistant and contains the highest concentration of collagen with the lowest concentration of proteoglycans. It also contains flat, elongated chondrocytes which are surrounded by a heavily calcified area known as a tidemark and is in direct contact with the synovial fluid of the joint capsule. Chondrocyte proliferation occurs about the tidemark providing new cells for interstitial growth. Keep this in mind before we go into the clinical notes that this superficial zone is the first to show changes in osteoarthritis.

The next layer of hyaline cartilage located just beneath the tangential zone is the transitional layer seen now highlighted in green. This layer amounts to about forty to sixty percent of the total amount of articular hyaline cartilage. It contains low density of spherical chondrocytes as well as collagen fibrils and a very high concentration of proteoglycans.

The layer located right beneath the transitional layer is known as the radial layer. This layer of hyaline cartilage also known as the deep radial layer provides the highest amount of resistance to compressive forces. This is because this layer contains the largest diameter of collagen fibers which are arranged perpendicular to the articular surface. It also contains a high number of proteoglycans and the lowest concentration of water of the four layers. The chondrocytes in this layer of articular hyaline cartilage are arranged perpendicular to the collagen fibers.

The fourth and innermost layer of the articular hyaline cartilage is known as the calcified layer. Now, this layer is important because it secures the articular cartilage to the bone and also anchors the collagen fibers from the radial layer to the subchondral bone. The calcified layer also known as a zone of calcified cartilage forms an important interface between cartilage and bone and it contains a few chondrocytes and the chondrocytes present in this layer are abnormally large.

We have covered everything we need to know for now for hyaline cartilage. We're ready to move on and talk a bit about clinical notes connected to this topic. We have just looked at the hyaline cartilage that covers the articular surface of movable joints. There is a condition known as osteoarthritis which arises when the articular cartilage lining these movable joints is gradually lost, degraded or changed over time. This condition usually occurs as a result of aging.

As a result of the degeneration of articular cartilage due to wear and tear, there'll be fragments from the cartilage which are released and trigger the activation of macrophages and the release of several factors as well including matrix metalloproteinases from adjacent tissues. The release of these factors causes then pain and also triggers the inflammation process within the joints.

Here you can see a patient with osteoarthritis showing inflammation at the interphalangeal joints with the typical articular distortion. Pain medication and non-steroidal antiinflammatory drugs can be used to treat the symptoms of osteoarthritis.

And this brings us to the end of this tutorial where I'm going to be then doing a summary or recapping the material that we covered. So what have we learned so far?

We have then seen that cartilage is a type of connective tissue and that hyaline cartilage is one of the three types of cartilage. We have also seen that it contains cells known as chondroblasts and chondrocytes which secrete extracellular matrix and collagen fibers. We also have seen that most hyaline cartilage is surrounded by a dense connective tissue layer known as the perichondrium except for the hyaline cartilage that covers the articular surface of movable joints.

In addition, we learned that the hyaline cartilage found on the articular surface of movable joints can be subdivided into four layers – the tangential layer, the transitional layer, the radial layer, and the calcified layer. Finally, we looked at a condition that comes about as a result of degeneration of hyaline cartilage of articular surfaces of movable joints and that was then osteoarthritis.

I hope you have enjoyed this tutorial. Don’t forget that here at Kenhub, we still have a quiz connected to the hyaline cartilage topic that you can use to test your knowledge on this video, not to mention the related articles and atlas section that we have so you can keep learning more stuff about the hyaline cartilage.

Thank you for watching and I will see you on the next tutorial.

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