Appearance and features of the reticular connective tissue.
Hey everyone! This is Nicole from Kenhub, and welcome to this Kenhub histology tutorial where we will be discussing the reticular connective tissue.
So as you're probably already aware, connective tissue is the collective term for the many types of tissue that served to maintain the body's form and structure. Without it, you just fall apart. Let's begin with an overview and a look at the images we'll be using to help us understand reticular connective tissue.
In this tutorial, we'll first take a look at what reticular connective tissue is, the various components it's made up of, the locations in the body where it may be found, and finally, its functions.
The image shown here is the histological section of an activated lymph node stained with hematoxylin and eosin or H&E. A lymph node is described as activated when it displays changes associated with inflammatory processes occurring somewhere in the body such as an infection. We're looking at a lymph node because it's a good example of an organ containing reticular connective tissue. Here we have some activated lymphoid follicles housing B lymphocytes getting ready for combat.
If we jump over to this part of the activated lymph node, we can see some sinuses – the subcapsular sinus – just here running beneath the capsule of the lymph node and the trabecular sinus which can be seen just here through which the lymphatic fluid flows. Increasing the magnification allows us to see a little bit more detail, and here again, we see the activated lymphoid follicles and here we have some sinuses. Throughout the lymph node including within the lumina of the sinuses runs the reticular connective tissue which is what we are particularly interested in right now. To get a closer look at it, we'll zoom in on the highlighted area. So if you remember those sinuses we pointed out, this is one of them just here.
So now that we've orientated ourselves a little, let's move on to the rest of the tutorial.
So the first question we want to ask is, what is reticular connective tissue? Reticular connective tissue is a mesh-like network of fibrous tissue. Unlike most tissues, the cells in connective tissue are not necessarily directly bound together and instead can be some distance apart separated by extracellular matrix. As with all connective tissue, reticular connective tissue is composed of a cellular component which consists of reticular cells and mesenchymal stem cells, fibres known as reticular fibres, and ground substance. It's worth noting that other cell types may be present depending on the organ in question. For example, lymphocytes attached to reticular fibres in lymph nodes.
We're now going to explore each component of reticular connective tissue individually. The first of these we'll look at are the reticular cells. In the image, we can see the general fibre's appearance of the reticular fibres. The multilobed nuclei of granulocytes can also be observed here. Reticular cells, on the other hand, have a single nucleus and are similar to fibroblasts in function and are interspersed throughout the reticular connective tissue. They can often be seen hanging around junctions of reticular fibres and produce collagen which forms the reticular fibres.
The reticular cell membrane extends and encapsulates the fibres. This is important as this allows other cell types to interact with the fibres via the reticular cell membrane.
Now for that other cell type we mentioned, the mesenchymal stem cell. Here we can see histological section of the root of a tooth lying within the tooth socket. This image shows mesenchymal stem cells highlighted in green. They're found within the layer of a tooth known as the dentin labeled here.
Mesenchymal stem cells are multipotent stem cells meaning that they can differentiate into multiple cell types including myocytes or muscle cells and osteocytes or bone cells. They're found in all types of connective tissue and are the source of all cells of the connective tissue type. This includes reticular cells in the reticular connective tissue.
Mesenchymal stem cells are the major component of the mesenchyme which is the embryonic precursor of connective tissue amongst other tissues. They exist to a lesser extent in the adult and are responsible for the replacement of damaged or aging connective tissue cells.
Now enough about cells, the real workhorses of reticular connective tissue are the reticular fibres. Reticular fibres are made of collagen – type three collagen to be precise – and these form the network that reticular connective tissue is famous for. So this network of fibres is called reticulin which is derived from the Latin word "reticulum" meaning network, and it's a major component of the stroma.
The stroma is a general term for the supporting structures of an organ and is composed of connective tissue, blood vessels, nerves, and lymphatics – in fact, pretty much everything that isn’t performing the main function of the organ. In the diagram, reticular fibres can be seen passing across a sinus within the red pulp of the spleen.
To be sure what you're looking at is a reticular fibre, you need to use special staining techniques. The most commonly used stains contain silver compounds. When we do this, we notice that reticular fibres have three components – collagen arranged in fibrils which is surrounded by an amorphous material and they're covered by extensions of the reticular cell membranes. This differentiates reticular fibres from other collagenous fibres which are naked by comparison.
So we said that cells and fibres in reticular connective tissue aren't packed together, so what fills the space? The space is filled by ground substance. This histology slide has the ground substance of this tissue highlighted in green and ground substance is an amorphous protein rich gel-like substance which serves to support the cells and fibres of the tissue. It's basically everything that isn't a cell or a fibre. The term extracellular matrix refers to the ground substance plus the fibres of a tissue. Like the reticular fibres, ground substance is a key component of the stroma.
So now we know what reticular connective tissue is made up, where can we find it? Let's take a look at some of the locations in the body.
The reticular connective tissue network provides structure within solid organs and soft tissues so it is found in locations such as the spleen, the lymphatic system, the liver, and bone marrow which in adults is found primarily in the axial skeleton. Additionally, reticular fibres can be found and wrapped around the surface of myocytes or muscle cells, adipocytes or fat cells, and Schwann cells which are insulating cells found wrapped around nerve cells. Reticular fibres are also distributed through the mesenchyme of the embryo.
Now let's talk about the functions of the reticular connective tissue which will help explain why it exists in all these different places. The main function of reticular connective tissue is to provide a structure or framework for soft tissues similar to how the skeleton supports the human body. It does this through the reticular fibre network – the reticulin – which I mentioned earlier. This allows other cells to bind to it and even allows themselves to walk along it. In this way, reticular connective tissue facilitates organogenesis during embryonic development.
B and T lymphocytes are two cell types which can use the reticular system for transport. Reticular connective tissue guides B and T lymphocytes and antigen-presenting cells to a desired location as part of an immune response. Let's take a look at some examples where the reticular connective tissue is useful.
The red pulp of the spleen is a good example of a location where reticular connective tissue acts as a supporting structure for a solid organ. Macrophages are attached to the reticular fibres within the sinuses of the spleen. We can see some sinuses on the red pulp highlighted in green here. Attachment to the reticular fibres holds macrophages stationary within the sinus allowing them to sample red blood cells as they flow past. The macrophages subsequently remove any older faulty red blood cells from circulation and can then begin the process of breaking them down into their component parts.
Reticular connective tissue in lymph nodes highlights its other functions. Within lymph nodes, there are two types of reticular cell – fibroblastic reticular cells located in the T-cell cortex of the lymph node and marginal reticular cells seen on this image situated just deep to the subcapsular sinus. We'll discuss each one in turn.
Fibroblastic reticular cells have three functions in the adult. One, to support the immune cells of the lymph node specifically the antigen-presenting cells like the macrophages seen here and the lymphocytes observed here, two, to guide cells by providing them with a walkway to travel along as shown by this slide, and three, to manufacture and maintain the reticular network which acts as molecular sieve to help the lymph nodes filter out any unwanted particles or cells. This is being performed by the reticulin shown here.
Marginal reticular cells, on the other hand, appear to have just a single function in the adult and also a function in embryonic development. When an organ is developing in the embryo during a process called organogenesis, it requires a framework around which the functional tissues can grow. The marginal reticular cells produce the reticular network which is the name for that framework.
In the adult, the reticular fibres created by marginal reticular cells extend from the subcapsular sinus to the B cell follicles in the lymph node. They guide antigen-presenting cells such as dendritic cells from the subcapsular sinus where they collects the antigens to the B cell follicles where they present those antigens for assessment by B lymphocytes. The B lymphocytes will then produce antibodies to the antigens that have been presented.
So that's all well and good but why should you bother to learn about reticular connective tissue?
There are three reasons. Firstly, cancers commonly spread via the lymphatic system. Cancer cells invade the surrounding tissue and enter the lymphatic vessels, draining the area where the primary tumor has risen. The cells are carried to the first lymph node in the lymphatic chain. This lymph node is known as the sentinel lymph node. Within the node, the cancer cells are caught and trapped by reticular fibres where the immune system will attempt to destroy many of them, however, with continued assault, eventually the cancer cells start to collect, survive and grow in the lymph node. The cancer can continue to climb the lymphatic chain and entre the bloodstream within lymphatic fluid. Being able to recognize cancer cells in lymph nodes allows pathologists to say how far the patient's cancer has progressed. This piece of information is known as cancer staging.
Secondly, reticulum cell carcinomas arise directly from reticular cells. This is a rare type of tumor usually originating within bone marrow or lymph nodes. A sarcoma is a cancer arising from a cell of mesenchymal origin. Knowledge of what is normal and what is abnormal on histological section which contains reticular connective tissue is vital in identification of such tumors.
Thirdly, a type of mesenchymal cell called follicular dendritic cells provide supporting germinal centres of lymph nodes. Presence or absence of follicular dendritic cells is used to differentiate between activated lymph nodes and lymph nodes containing lymphoma. The cells specifically expressed two types of antibody – CD21 and CD23 – which can be stained for in order to identify their presence.
So, hopefully, by now, you've learned a lot more about reticular connective tissue. Before we finish the tutorial, let's quickly recap.
One of the key points that we covered was that reticular connective tissue is a mesh-like network of connective tissue. Also, no matter where it's located, reticular connective tissue is made up of three basic parts – a cellular component which consists of reticular cells as shown in the slide and mesenchymal stem cells, a fibrous component a.k.a. the reticular fibres highlighted here, and ground substance, the amorphous gel in between.
Reticular connective tissue is found mainly in soft tissues including the spleen, liver, bone marrow, and the lymphatic system. There it supports and provides structure to allow the functional cells to do their jobs. In some cases, for example, in the lymph nodes and during organogenesis, the reticular network is utilized to guide movement of the cells.
And that's it! Thanks for watching this tutorial on reticular connective tissue. Happy studying and see you next time!