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Overview of dense connective tissue.
Hey everyone! This is Joao from Kenhub, and on this histology tutorial, we will be discussing a type of connective tissue which is known as the dense connective tissue. Now, the first question in your mind might be, what is connective tissue? I think it's the most important thing to answer, and connective tissue is the most abundant tissue in your body. It provides form and structure to the rest of the body's tissues so without it, you'd be an unrecognizable mess. The term connective tissue is broad and covers a wide range of structures and those structures include tendons, bone, fat and even blood are all classed as connective tissue.
Generally, connective tissue is comprised of cells and then something else called an extracellular matrix. Here we see a micrograph showing connective tissue highlighted in green. Connective tissue functions to then surround other organs and tissues, provides structural and metabolic support. It also binds structures together and even assists in tissue repair.
Connective tissue can be classed into three different categories which are connective tissue proper, embryonic connective tissue and what is known as specialized connective tissue. In this tutorial, we'll only be looking at connective tissue proper.
There are two types of connective tissue proper. There is loose connective tissue and dense connective tissue. As I mentioned earlier, we will be focusing on the dense connective tissue in this tutorial. Aside from the cell types that comprise connective tissue, it can also be infiltrated by other cell types such as the ones we see here which are called plasmocytes. Plasmocytes are also known as plasma cells or plasma B cells. These antibody-secreting cells are also known as wandering cells because although they are found in connective tissue, they do not reside in or are not fixed within this tissue type. And this is relevant for us in this tutorial because plasmocytes can also be found in dense connective tissue.
During the rest of this tutorial, I'll be giving you an overview of the properties and functions of dense connective tissue and after this, we'll go into more detail about its two sub-types which are dense irregular collagenous tissue and dense regular collagenous tissue.
Now, let's dive in and take a closer look at dense connective tissue.
Here we see a micrograph of dense connective tissue taken from the apex of the lung. As we just saw, dense connective tissue is a type of connective tissue proper and as the name suggests, it is more dense than other types of connective tissue. This makes it less flexible than loose connective tissue but the tightly-packed fibers have the advantage of providing heavy duty strength, also support and stress resistance and also protection to various structures throughout the body.
As I mentioned earlier, there are two types of dense connective tissue – a dense irregular type and a dense regular type. But why is this? I can hear you ask. It's all to do with the arrangement of their collagen fibers which are the not-so-secret ingredient of the dense connective tissues. Some of these can be seen here highlighted in green and are what give the two different types of dense connective tissue – their differing physical properties depending on their arrangement. What makes collagen fiber so special is their high tensile strength combined with their flexibility making them well suited for the task of supporting the rest of the body's tissues.
Dense connective tissue also contains other cells types some of which we will see in this tutorial, but before we do so, let's take a closer look at dense irregular collagenous tissue. As we have just learned, both types of connective tissue contain a good amount of collagen. Dense irregular connective tissue is named as such because it contains collagen fibers that are arranged randomly in an irregular pattern with no defined orientation.
As you can see in this micrograph, it is pretty clear that fibers aren’t arranged in any sort of regular or ordered fashion. The three-dimensional network formed by the collagen fibers provides dense irregular connective tissue with properties of increased resistance and strength in all directions. Let's take a look at some examples of dense irregular connective tissue.
Here we have a micrograph showing the submucosa or tunica submucosa highlighted in green. This layer lies beneath the mucosa or also known as mucous membrane of hollow organs such as the organs of your digestive tract and it functions to support the mucous membrane and connects it to the underlying smooth muscle layer. The presence of this dense connective tissue in these hollow organs enables them to resist and withstand extensive stretching and distension.
Another example of dense connective tissue can be found in the skin and comprises of layer of the dermis known as the stratum reticulare, seen here highlighted in green. The reticular layer of the dermis, as it is also known, provides the skin with important properties such as strength and resistance to tearing. And as we age, the amount of collagen in the dermis naturally deteriorates which is unsurprisingly a big focus of the beauty industry. As well as dermis and submucosa, dense irregular connective tissue can also be found in joint capsules and muscle fascia.
Now it is time for us to take a look at dense regular collagenous connective tissue – that is certainly a tongue twister. And here we see a micrograph showing a close-up view of the dense regular connective tissue. As you've probably guess, if we take a closer look at this micrograph, we can see that this tissue type differs from its regular sibling we just looked at. In that, its structure is arranged in a regular defined pattern. It's important to note that these aren’t actually the collagen fibers themselves we're looking at here. Those are much too small to see at this magnification. So what are we looking at?
The cells here are known as fibroblasts. We can see some more of these in this micrograph. These cells function to synthesize the extracellular matrix and collagen fibers so they are a vital component of the dense connective tissue. A parallel configuration of the cells provide strong resistance to stresses exerted over time in the same direction. Fibrocytes which are considered to be a less active form of fibroblasts are also found in dense regular connective tissue and tend to have elongated nuclei and also a very little cytoplasm.
Here in this image we have an excellent example of dense connective tissue in the body which is the Achilles tendon. Let's take a closer look.
Tendons attach muscle to bone and so they need to be really strong. Here we see a micrograph of a tendon where we can clearly see the parallel arrangement of the fibers that is characteristic of dense regular connective tissue. The tightly-bound strands ensure that tendons are structurally strong and also flexible. See important to note here that the fibrocytes found in tendons are then called tendinocytes, some of which we can see here in green. Their cytoplasm isn't usually visible when stained with hematoxylin and eosin.
Although tendons are comprised of dense regular connective tissue, they are additionally enveloped by a connective tissue sheath that is comprised of dense irregular connective tissue. This dense irregular connective tissue that surrounds the tendon is, in some cases, covered by cells known as synovial cells. These cells produce a lubricating fluid which reduces friction at synovial joints. These actually offers us a glimpse of how the different types of connective tissue work together to help your body function properly.
Like tendons, ligaments and aponeurosis are also comprised of dense regular connective tissue. Here we see the patellar ligament which helps to stabilize the patella or kneecap. If we take a closer look at a ligament at its cellular level, we can get a better idea of its structure. A typical ligament contains fibroblasts and collagen fibers that are arranged in parallel rows as well as elastic fibers which are highlighted in green here. This is especially true for ligaments associated with the vertebral column. Unlike tendons, the fibers in ligaments are less regularly arranged and may be found slightly crisscrossed or overlapping.
That’s just about it for this tutorial but before we wrap up, let's take a look at dense connective tissue from a clinical standpoint.
As we have seen, tendons are comprised of dense regular connective tissue. These cordlike structures that attach muscles to bone can become inflamed as a result of overuse or injury to the tendon and this leads to a condition known as tendinitis.
Tendinitis can affect any tendon and is usually associated with sports injuries. Common areas of injury that result in tendinitis include the elbow, the shoulder rotator cuff and the Achilles tendon. Tendinitis is characterized by joint stiffness, pain, swelling in the area where the injury occurs and even a burning sensation at the site of the injury. Tendinitis can be treated with the use of nonsteroidal anti-inflammatory drugs. In some cases, other anti-inflammatory drugs such as cortisone can also be injected at the site of injury to relieve the symptoms. Rest and physical therapy can also be prescribed depending on the severity of the injury.
And this brings us to the end of our tutorial here so let's just do a quick recap.
So we began our tutorial by looking at connective tissue in general which can be defined as being either specialized, embryonic or connective tissue proper. Connective tissue proper was where our focus began today and this can be further divided in being dense connective tissue or loose connective tissue. Of course, our main focus was dense connective tissue. This type of connective tissue is particularly suited for providing strength, support, stress resistance and protection throughout the body due to its particularly high concentration of collagen fibers.
From dense connective tissue, we saw once again that this type of tissue can be subdivided into dense irregular connective tissue which contains collagen fibers that are arranged randomly and these provide this type of tissue with increased resistance and strength in all directions. Examples of dense irregular connective tissue include tendons, ligaments and aponeurosis. Finally, we saw that the second type of dense connective tissue called dense regular connective tissue contains collagen fibers that are arranged in a regular defined pattern.
And that brings us to the end of this tutorial. Thanks for stopping by and I will see you on the next one.