Lymphoid Organs

by Geoffrey Meyer, PhD

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    00:01 The lymphatic system contains lymphoid organs and tissues. And the job of these organs is to mount responses against any invading pathogens. So I’m going to talk about the lymphoid organs throughout this lecture. And hopefully at the end of this lecture, I’d like you to understand first of all a lymphatic nodule, what its structure is, and what it resembles.

    00:29 And we’re also going to look at the appendix and gut-associated lymphatic tissues.

    00:35 You should also recall how lymph is produced and how lymph flows through lymph nodes and the structure to the lymph node. Lymph nodes are designed for detecting antigens. So we’ll go through the structure of various components of the lymph node. We will then move on to look at the thymus. And at the end of this lecture, you should have a thorough understanding of the structure and function of the thymus. And lastly, we’re going to look not just to the lymph node, but we’re going to look at the spleen as well, a very similar organ.

    01:14 But you’ll appreciate that lymph node filters lymph and the spleen filters blood. You should know those differences, and also the structural differences to allow those two different roles.

    01:28 The function of the whole lymphatic system, as I mentioned at the start of this lecture, was to monitor all parts of the body, particularly mucosal surfaces, mucosal surfaces that are exposed to the exterior of the body, for instance, the respiratory system and the gut. And to monitor those surfaces and connective tissue spaces, and detect invading antigens.

    01:57 And once they're detected, they’d respond to them by then alerting the lymphoid organs and cells within each of those organs but its specialized to act against certain antigens. When we look at the cells that are involved in immune reactions, we can classify them into two different classes.

    02:17 First of all, there are the lymphocytes, and then there are the accessory cells or helper cells, antigen presenting cells they’re often referred to. The lymphocytes circulate around the body in the blood, and they leave the blood in various locations by means I’ll describe throughout this lecture. And then they can go through the tissues undergoing surveillance on sensory duty to detect an antigen. And these lymphocytes fall into two different classes. B cells which differentiate in bone-marrow and a bone-marrow derived, and the T cells which although they derive from bone-marrow, they need to go to the thymus gland where they’re educated to detect and respond to their antigens that they are trying to detect.

    03:11 If you remove the thymus from an experimental animal, they don’t develop T cells.

    03:19 And for that reason, the T cells are thymus-dependent on their education, T for thymus, T for T cells. The B cells were allegedly described in the bursa of Fabricius, which is a little gland they discovered in chicken embryos in the cloacal region. And they are found to contain lymphocytes and removal of this bursa of Fabricius experimentally result in no antibodies being produced in the animal. So they are called B for B cells, B for the bursa of Fabricius. So that’s where the terms B and T come from. And within the T cells, there are two categories, helper T cells, cytolytic T cells, and also other sorts of T cells.

    04:10 The accessory cells are macrophages, dendritic cells, and follicular dendritic cells, which I’ll refer to throughout this lecture. When we look at the B cells, the T cells, natural killer cells, they are the key players. But as I alerted to you in the previous slide, these other four groups of cells play a very important role. Usually, lymphoid tissues, organs such as the lymph node and spleen, they are dominated by a very, very elaborate spongy network that consist of reticular cells. These reticular cells secrete reticular fibres or collagen type III, and they wrap around those fibres and form this cobwebby spongy network right throughout the organ. And they express little flags on their surface to attract in all the four cell types that you see listed in this slide. The monocytes, macrophages, some of the blood cells, some of the white blood cells moving to these spaces, attracted by the reticular cells. Reticular cells also hone in dendritic cells. It’s important to understand that Langerhans cells are dendritic cells, but follicular dendritic cells are not.

    05:39 They’re not derived from the stem cell. They live in lymph nodes which we’ll see in a moment.

    05:46 And the epithelioreticular cell is the word I’ll describe later on that needs a little bit of elaboration because again, epithelioreticular cells are confined to the thymus. They are the supporting cell in the thymus. They’re not reticular cells. They don’t secrete reticular fibres. They’re epithelial cells. So perhaps, the name needs correction.

    06:13 One way in which we find the histology of lymphoids difficult in some respect is that, there are many many different sorts of lymphocytes, and they can be identified using certain markers by immunohistochemical staining using monoclonal antibodies. But if you don’t use these stains, it’s very hard to differentiate different sorts of cells. They express these CD molecules called cluster of differentiation markers. And various lymphocytes express certain sorts of these CD molecules. Some cells express particular CD molecules for their entire lifespan.

    07:02 Others only express these CD molecules at certain stage of their differentiation, or when they’re activated. And based on those three different criteria, immunologists and medical laboratory scientists can use histochemical staining, immunohistochemical staining using monoclonal antibodies that I mentioned a moment ago to actually identify and differentiate these different sorts of lymphocytes. There are two major regions that the immune cells are first of all derived, and also where they’re located in most concentrations.

    07:48 As I've mentioned earlier, the cells originally derive from the bone-marrow. That’s where they’re produced, or they can be produced in the gut-associated lymphoid tissues, or as I mentioned earlier, from the thymus. That’s where the T cells, remember, are educated.

    08:06 These cells can then go and locate in the lymphoid organs that I’ll describe in this lecture. The nodules, the nodes, tonsil, and also the spleen. They’re all antigen-dependent, and they are the effectors of T and B cells. Once they’re switched on by an antigen, they become effector lymphocytes. And as we’ll see also, they remember so that they can mount a response a lot quicker to a secondary invasion of a particular antigen. So let’s now have a look at some of these lymphatic tissues, and also the lymphoid organs. Here are two pictures, two images taken from a part of the gut. One happens to be the stomach on the left-hand side, the stomach mucosa. The one on the right is taken from the small intestine, the duodenum. And on the left-hand side, you see what we call diffuse lymphatic tissue.

    09:10 Lymphatic tissue is easily identified in mucosal surfaces because you see lots and lots of little tiny black dots. These represent lymphocytes. They don’t have much cytoplasm around them because they haven’t yet been activated. They’re waiting there on the surveillance duty. They are trying to recognize a particular antigen. And when they find that antigen that may have found its way across the surface of the epithelium, they recognize it, they bind to it, and then they can mount an immune response against it. On the right-hand side, you see evidence of that. When you see a nodule that you see on the right-hand side, a nodule is a pale staining germinal centre, and a darker staining corona, that represents a lymphatic nodule.

    10:04 That is the morphological or the histological evidence that there is an immune response going on, because what’s happening there is that an antigen is passed across the surface.

    10:18 It might have been ingested by an M cell or an enterocyte, a specialized enterocyte which is the M cell, and that might have ingested the antigen and then released it into the interstitial compartment underneath the epithelial surface to be recognized by a particular lymphocyte.

    10:39 And when that lymphocyte recognizes that antigen, it binds to it, and it could immediately go into a situation where it can differentiate into a lymphoblast and then massively proliferate.

    10:54 And that proliferation is evidence of that lighter staining germinal centre. And these cells can form memory cells or plasma cells. The plasma cells will secrete antibody. The memory cells are there to leave that area through part of the lymphatic system, a very small lymphatic channel, and then pass to a lymph node and beyond and get into the general circulation.

    11:26 When they go to the lymph node, they can be detected and they can spark up another immune response and produce more memory cells and more plasma cells, and they in turn can then move and populate other mucosal surfaces just in case these invading antigens is coming across another surface. They can go out and be located for a long time sitting in these mucosal surfaces waiting for that antigen to come through. The corona you see there is really just where the peripheral cells are being pushed aside. They actually are lymphocytes that have detected the specific antigen that is being wrecked against in this situation. So they’re just like lymphocytes, as I’ve mentioned, that have been in the area. But as all this activity, this germinal centre gets bigger, they just get pushed to this side. Here is a tonsil. On the left-hand side, the tonsil sitting in the oral cavity is a perfect spot to detect any antigens that come in with the food.

    12:34 They have tonsillar crypts. Tonsillar crypts are invaginations of the epithelial surface into the underlying connective tissue. And food debris gets caught in those crypts and it remains there for some time. And what happens is the lymphocytes, an antigen presenting cells, move across the epithelial surface into that debris, and then undergo surveillance to detect any antigens which they’re trying to detect. And if they do, again, they mount an immune response and you see a lymphatic nodule, as you see in this slide on the right-hand side. And again, these lymphocytes can always move away into lymph nodes nearby, create another lymphatic nodule, and again, proliferate more and more B cells, and more and more plasma cells, and again, more memory cells that can go and populate other surfaces as I described earlier. There is so much traffic across the tonsil surface, the epithelium of the tonsil that often, the epithelium is very, very difficult to recognize. It’s a stratified squamous epithelium, but as I said, in most areas where there’s so much traffic, it’s difficult to recognize that epithelial surface. You’ve seen on the left-hand side lymphatic nodules.

    14:00 This is gut-associated lymphoid tissue. It’s very common in the ileum, part of the small intestine, combating against any antigens that found their way, passed all the usual defenses we often have. There are defenses we have where our first line of defense can be mucous, can be lysozymes. It can be the secretions on our skin surface, that's antimicrobial.

    14:30 There's other ways in which we first try to prevent any antigen from getting into the body, the barriers. But if they do, then on the left-hand side, you can see a response against it. And on the right-hand side is the appendix, which is totally populated by lymphoid tissue. All those little dark dots are lymphocytes. And if you look very carefully in that slide, you can see nodules, again, reacting to antigens in the lumen of the appendix.

    15:03 A lymph node is one of the basic components of the lymphatic tissue. It filters lymph.

    15:11 On the left-hand side is a diagram illustrating the main structural features of a lymph node.

    15:18 And on the right-hand side is a section through part of a lymph node. Lymph is formed from interstitial fluid in the peripheral tissues of the body. That lymph or that fluid has accumulated there. It's passed out of capillaries into the interstitium. And sometimes, that fluid is not then passed back into the postcapillary venules and returned into the vascular system.

    15:47 There’s an accumulation of fluid in the interstitial space. Well, that excess of fluid is picked up by very fine little blind-ended lymphatic channels. These lymphatic channels communicate with each other. They join together to form a network and finally drain from tissues.

    16:11 And they pass through lymph nodes. They pass through chains of lymph nodes, several lymph nodes before that lymph is then finally returned into the venous system through a large lymphatic duct. For instance, the right lymphatic duct empties all the lymph into the vein at the junction between the right internal jugular vein and the right subclavian vein. Now, in that interstitial fluid, there may be microbes, antigens, pathogens, foreign cells. And because these blind-ended lymphatic channels are very permeable, much more permeable than the blood capillaries, then these toxins, microbes, etc, can get into the lymph.

    17:02 And therefore, the job of the lymph node is to actually clean that lymph to detect those antigens. On the left-hand diagram, let me just point out the main features of the lymph node. You can see a capsule on the far left-hand side. You can see lymphatic channels, these lymph vessels entering into the lymph node. They’re called efferent lymphatic channels. And those channels pass the lymph through the lymph node through sinuses. The lymph sort of percolates through the whole network of the lymph node with its antigens, with its toxins, and that lymph then finally passes out through the efferent lymphatic channel in the hilum of the lymph node. These lymph nodes are like little bean-shaped structures, little kidney-shaped structures.

    18:00 And then that efferent lymphatic channel will pass on and become maybe an efferent lymphatic channel on the next lymph node. So there is this series of lymph nodes or lymph passes through as I’ve explained earlier in this lecture. Now besides that, there are, and it’s not described here in the diagram, this massive cobwebby meshwork of reticular cells and reticular fibres forming the framework of the lymph node. And as I mentioned at the start of the lecture, those reticular cells can hold up little flags and attract in all the different accessory, cells and T and B lymphocytes. And because they attract these cells into the lymph node, those cells could detect any of these antigens that are passing through the lymph, percolating through this meshwork, and finally, getting into the efferent channel. So that’s a strategically good location to identify and mount a response against these antigens. Also, there’s the blood supply shown here. You have an artery, small vessel coming into the hilum of the lymph node, and then forming a capillary network.

    19:25 And then a postcapillary venule collects the blood from the capillary network and then passes out as a vein. That postcapillary network is very specialized. It’s specialized to allow lymphocytes to pass out of the blood into the lymph node. And this is how lymphocytes circulate. They go from the blood, pass these high endothelial cell venules, they are called, and they pass into the lymph, into the sinuses, into the spaces where the lymph is, and they circulate through the network, and they look for antigens that they are trying to detect.

    20:10 Should they find an antigen that they can detect, they start up an immune response and develop a germinal centre and produce B cells, plasma cells, and antibody. T cells do a similar thing except they attach and ingest the antigens. They don’t produce any antibodies. So that circulation is very important. And that’s why lymphocytes get into the lymph. They move back into the lymph, into the efferent vessel, and they can return back to the blood through the channels that I've described earlier. So when you see these efferent lymphatic channels coming into the lymph node, you’ll see them full of little lymphocytes on their way back to the blood system. On the right-hand side, you just see a low power magnification of the lymph node. You can see the cortex on the very outside. There’s a little nodule there if you look very carefully. You can see the hilum of the lymph node, the lighter stained area, and I’ll show you more details of that in the next slide. Here is a section of the lymph node taken at slight higher magnification. You can see the cortex labelled, the paracortex, the medulla, and the hilum. The medulla region contains lots of little tiny circular profiles, medullary rays of lymphocytes. And I’m going to mention those in a moment. The medulla is where the bulk of the lymph is going to drain through, on its way out from coming in from efferent lymphatics in the cortex. And that all drained into vessels and leave via the hilum shown there. B lymphocytes live in the cortical region. You can see a number of little nodules appearing there indicating these B cells there undergoing proliferation and differentiation into memory cells and plasma cells secreting antibodies. The paracortex is a region that is occupied primarily by T cells. So there is this difference in regional location of the B and the T cells. And we’ll also see that in the spleen, and I’ll point that out to you later on when we cover that organ. On this section, you can see a part of the cortex on the left-hand side. And there you can see, what I described before, little tiny lymphocytes in the vessel lumen. The efferent lymphatic vessels are coming in through the cortical region through the capsule of the cortex. And those little dark dots you see are lymphocytes packed in the lumen. On the right-hand side, you see a similar story.

    23:17 You see the large efferent lymphatic vessel, and that also contains lymphocytes.

    23:23 Those lymphocytes have probably come from other lymph nodes through the efferent one shown there, percolated through the lymph node. Others have come in through those venules I described earlier, circulate through the lymph node, and now they’re on their way out to finally go back into the vascular system, the blood. On the left-hand side here, you see part of the capsule. That capsule sends trabeculae of connective tissue into the lymph node. Just underneath the capsule there, you can see just a very clear space. And also near the trabeculum, that’s the beginnings of the sinus, the sinus spaces that contain the lymph. The lymph flows through those spaces, and then through all this network. The network you see in higher magnification on the right-hand side, you can see, if you look very carefully a reticular cell, you just see its nucleus wrapping around reticular fibres or collagen type III fibres which they produce. The collage type III fibres here are very well stained by the dark little lines you see indicating those fibres. Well, that’s that framework that I explained earlier. All these lymphocytes are percolating through that framework, and so to our antigens, and then they’re detected and dealt with. Now, the reticular sides are also very important because they attract, as I mentioned earlier, dendritic cells, macrophages, and other accessory cells. When they attract dendritic cells, they attract those dendritic cells specifically in the paracortex region. And there, those dendritic cells attract T cells. So that’s why there’s this localization of T cells in the paracortex mainly in the lymph node. On the left-hand side of this slide, you can see an image taken there at the cortex. You can see a lymphatic nodule. You can see the paracortex dominated by T cells. And then when the lymphatic nodule proliferates when the cells leave that nodule as B memory cells or plasma cells, they follow a little pathway down which is evident by the medullary cord you see on the right-hand side. The right-hand image is taken through the medullary region of the lymph node. And these long tiles of migrating lymphocytes go along these medullary cords. The plasma cells secrete antibodies. And finally, all these cells, all these lymphocytes coming from the germinal centre could leave the lymph node via the lymph, and then populate the rest of the body. This is a high magnification picture taken through the germinal centre of a lymph nodule. On the left-hand image, you can see a large lymphoblast. These are cells that have been reverted back from a stimulated lymphocyte and will undergo a series of mitosis or proliferation to give rise to cells that are going to form plasma cells and secrete antibodies, and also B memory cells which are going to populate other parts of the body so that they can recognize the antigen should've come across the surfaces at a later stage. On the right-hand section, there is an image showing you a follicular dendritic cell. These are dendritic cells. They extend long processes throughout the network. And again, this is the germinal centre. They also attract and bind antigen-antibody complexes and display them on the cell surface of these follicular dendritic cells. And then they interact with the developing B lymphocytes. And the B lymphocytes that can recognize these antigen immunoglobulin complexes on the cell surface and combine to that strongly survive. If you can’t bind to the surface of these antigen immunoglobulin complexes, as well as they should, then they go through a series of apoptosis because the follicular dendritic cell won’t support them. So these follicular dendritic cells are very important in training the B lymphocytes to be very specific for the immunoglobulin and antigen complexes on the surface.

    28:14 And if they can recognize that complex very well, then that follicular dendritic cell saves that B cells from undergoing apoptosis and being digested by macrophages which also are located in the germinal centre to digest all the B cells that do not make the proper training and recognition of the antigens to which they are trained to recognize. Here is a section showing these high endothelial veins, high endothelial cell veins. They are cuboidal really. They’re not thin squamous like you see them in other capillaries. And they are the ones that put flags up and allow lymphocytes to attach to the endothelial cell, the lining of the capillary, and then move into the lymph node. So in a review, make sure you understand now the structure of the lymph node, how lymphocytes get into the node, the lymph nodule, and the region of the paracortex being specialized for T cells. Let’s now look at the thymus.

    29:18 Section here shows what the typical thymus looks like. It has got a capsule and thymic lobules. The thymic lobule has within it a dark staining cortex and a lighter staining medulla. And one feature of the thymus, one way in which you can identify the thymus from other organs is the fact that it contains thymic or Hassall’s corpuscles which I’ll show you in high magnification. The thymus is quite a complex organ, but quite easy to describe and identify histologically, as I just pointed out to you. On the left-hand side is a diagram illustrating the compartments of the thymus, the cortex and the medulla.

    30:06 What essentially happens is that the cortex contains T cells undergoing education.

    30:16 And they undergo education in various units or subdivisions of the cortex. And then they pass into the medulla where they undergo their final education before then moving out to the body. I sort of used the analogy that’s like a school. The cortex is like grade school or primary school where you’re receiving a certain education, and then you graduate and go to high school. Here in the cortex, T cells undergo a certain amount of training.

    30:47 They go to the medulla then and then graduate as fully mature immunocompetent T cells. But one difference here is that only 2% ever graduate and move out of the thymus medulla. The rest are apoptosed or digested by macrophages. They don’t complete their education. They recognize maybe self-antigens etc.

    31:11 So the body gets rid of them. On the diagram on the left, you can see rather a complex array of cells. There are six major cells that live in the thymus cortex and the thymus medulla. And they’re called epithelioreticular cells. They’re derived from the epithelium, and as I said before, they don’t secrete reticular fibres, they’re not reticular cells. The type I are the ones that are on the surface of the cortex. These are the ones that actually separate the trained or the educating thymus T cells from connective tissue.

    31:57 These cells form a barrier around all the connective tissue, the capsule, the trabeculae, and even the connective tissue around the blood. So they had jobs to separate these thymus cells or T cells undergoing education. Type II cells are in the network of the cortex.

    32:19 They compartmentalize the cortex into areas such as you can see shown on the diagram.

    32:26 And I’m not going to go into all the details of the physiology of the thymus here, I’m just pointing out some of the histological or structural features. The type II cells are also involved in training, educating the T cells. Type III cells are also involved in education of the T cells, and they form a barrier between the cortex and the medulla, a very strong barrier to stop cells from passing from the cortex into the medulla that haven’t gone through the proper training program. The type IV cells are in the medullary side.

    33:06 And what they do is they form a seal again on the medullary side so you have two cells, the type III and the type IV, making this barrier from the cortex into the medulla.

    33:17 Notice that the medullaries in most of that section you see on the right-hand side, lighter staining, lighter staining because there're far less cells in there because as I said, only 2% of cells ever make it out from the thymus medulla. The rest die. And then finally, get the type VI. These are called the Hassall’s corpuscles. They’re the ones that are probably very aged epithelioreticular cells, and they form these spiral coils very easy to identify in the medulla, and therefore, they're characteristic of the thymus, and as I said earlier, enables you to identify thymus compared to other organs. And finally, we need to say something about the blood-thymus barrier. We don’t want antigens and other components of the blood, self-antigens, interfering with the education of these T cells. So, these type II and type I epithelioreticular cells form a barrier, a wrap around these capillaries. And along with the endothelial cells and the basal lamina and macrophages, prevent the contamination of the educated T cells or T cells undergoing education from being exposed to those antigens.

    34:51 Macrophages live within the thymus, the cortex, and the medulla. And they are busy wrapping up or phagocytosing cells that don’t pass the grade. So again, in a review, make sure you’re aware of the structure of the cortex and the thymus medulla, and the role or function of these special epithelioreticular cells. Finally, let’s look at the spleen. A lymph node filters lymph and the spleen filters blood. They’re very similar organs really. It’s just that they just clean different components of the body, blood or lymph. When you look at the spleen which is about the size of a fist, clenched fist, it has got essentially two major components to it, when you look at the components in a histological section.

    35:42 You have white pulp which here stained as tiny circular blue-stained components. And you have the red pulp. This name is from what you see when you cut spleen freshly. The white pulp is where lymphocytes are. The red pulp is where all the vascular components are.

    36:02 The red pulp is the component that cleans the blood. You need to know a little bit about the blood supply to the spleen. On the left-hand diagram, trabecular arteries come into the spleen, and they form central arteries. These central arteries, shown on the right-hand side, move into the body or the substance of the spleen, and they get surrounded by lymphoid tissue. They’re called central arteries because they’re actually in the centre of the splenic lobule, although you don’t really appreciate the lobular structure of the spleen when you look at histological sections. And those central arteries then pass out to little radial arteries, which then open into sinuses. And those sinuses eventually are surrounded by macrophages that can phagocytose old red blood cells. Those sinuses also open to the free space where the circulation then goes into an open circulation. And the blood meanders through the reticular network just like lymph meanders through the lymph node.

    37:14 And then the blood can be checked for contents of antigens, foreign cells etc, and cleaned. That blood can then flow back into the venule system through sinusoids, and then back into the circulation. On the right-hand side, you can see the central artery, and it’s surrounded by a high load of lymphoid tissue. That lymphoid tissue I’ve abbreviated here as being PALS. It stands for peri, around, arterial, the central artery, periarterial lymphatic sheath. And it’s essentially T cells. Down below that, you can see a nodule, a lymph nodule. And what happens when blood passes out of these central arteries through the radial arteries into the blood spaces of the spleen, it flows along the length of these arteries. And along the length of all these lymphoid tissue, that lymphoid tissue has got reticular cells around it that attract all the lymphocytes into it. And then, if an antigen is detected, then an immune response is created, as you see here. The germinal centre starts, plasma cells are formed, memory B cells are formed, antibodies are formed, and also the T-cells that work together with the B cells, but the T cells can also ingest cells etc, that happens to have percolated entered via the blood stream. The arteries in the spleen, the trabecular arteries are shown here with a bit of smooth muscle around them, and they’re supported by connective tissue of the trabeculae. Sometimes, these trabeculae have smooth muscle in them that helps to contract the spleen in certain animals, and therefore, return blood cells quickly into the circulation. The vein, on the other hand, is just an endothelial lining around some of these trabecular collagenous sort of units, separating components of the spleen, trabeculae coming in from the capsule. And you also see the pulp vein, very thin-walled vein coming in, and eventually, joining out and being part of one of these large trabecular veins. The blood passing out of these blood vessels filter through the reticular network, as I explained. It filters through splenic cords lined by reticular cells and macrophages.

    40:00 And again, these reticular cells can hone in all the sorts of accessory cells I’ve mentioned before. Macrophages can break down red blood cells that happen to have passed through that network, they are aged. They can’t whine their way through the very fine meshwork of the reticular network, and therefore, they haven’t got the elasticity to do that.

    40:23 Therefore, the macrophages detect that and then destroy them. On the left-hand side, you can see some little bright red components that represent trabeculae, connective tissue. You can see the red pulp or the blood cells passing through that area, the splenic cords. And then you see the whitish spaces. They are the sinusoids that eventually, those cells will pass into and return to the rest of the vascular system. Higher power, you can see on the left-hand side the splenic sinusoids. And if you look very, very closely, you can see little gaps in the cell wall. Large spaces in the endothelial wall that allow these cells to return back into the vascular system, and then leave again in the same area, but mostly to return into the vascular system, and as I said before, rejoin the circulation. And those endothelial linings are wrapped up by incomplete reticular fibres, very fine little fibres you see just underneath the endothelial cells. And the diagram on the left-hand side shows you one of these capillaries, one of these sinusoids. The endothelial cells are elongated along the length of the sinusoid. And there are gaps between them as I pointed out, an incomplete basement membranes, basal lamina. And you can see the cells moving across the wall of these very leaky sinusoids.

    42:04 On the right-hand side, you can see the reticular network stained, and there're little gaps in that network, again, creating space for the cells to move in and out of the lining of the vessel. So in summary then, make sure that you understand diffuse lymphatic tissue in the mucosal areas. It’s representing spaces where cells can undergo surveillance and then mount a response against an invading antigen. Make sure you understand that lymph nodes filter lymph and they have the capacity to mount responses against antigens because the circulation of lymphocytes and the presence of certain lymphocytes. The thymus is where T cells are educated. It’s a complex structure, but histologically, it’s quite simple to identify. And the spleen filters lymph, and also, it can respond to antigens because of the presence of the central artery being housed or wrapped up by the periarterial lymphatic sheath, which actually replaces the tunica adventitia that you see in other vessels of the body.

    About the Lecture

    The lecture Lymphoid Organs by Geoffrey Meyer, PhD is from the course Lymphoid Histology. It contains the following chapters:

    • Lymphoid Organs
    • Cells of the lymphatic system
    • Lymphatic tissues and organs
    • Lymph nodes
    • Lymph node in detail
    • Thymus
    • Spleen
    • Summary of the lymphatic system

    Included Quiz Questions

    1. T cells differentiate from an activated lymphoblast in the lymphatic nodule
    2. Reticular cells and reticular fibres produced by these cells form elaborate networks in lymphatic tissues including a lymph node and the spleen
    3. In the thymus, epithelioreticular cells form the structural framework but they do not produce reticular fibres and are not reticular cells
    4. Cluster of differentiation (CD) molecules are unique cell surface molecules and can be visualized by immunohistochemical staining methods using monoclonal antibodies
    5. B cells differentiate and give rise to cells that secrete antibodies
    1. Follicular dendritic cells are derived from activated lymphoblasts in the lymphatic nodule
    2. The germinal centre in a lymph nodule is a histological sign that lymphatic tissues is responding to an antigen
    3. B cell predominate in the cortex and T cells predominate in the paracortex of a lymph node
    4. The coronal zone is a population of lymphocytes that surround a germinal centre
    5. A medullary ray is a population of B lymphocytes and plasma cells migrating into the medulla of the lymph node
    1. Lymphocytes can enter a lymph node via efferent lymphatic channels and postcapillary high endothelial venules (HEV’s)
    2. Lymph nodes filter lymph
    3. Follicular dendritic cells play an important role in supporting B lymphocytes during their differentiation
    4. Macrophages are present in the lymph nodules and phagocytose B lymphocytes deemed to be incompetent
    5. Lymph flowing through lymph nodes can expose any antigen in the lymph to immune cells and initiate an immune response
    1. Type II epithelioreticular cells compartmentalize developing T cells in the thymus medulla
    2. Type I epithelioreticular cells are located adjacent to the thymus capsule and separate T cells undergoing education
    3. Type I epithelioreticular cells are located between the cortical zone and the connective tissue trabeculae/septa
    4. Type IV epithelioreticular cells create a barrier at the junction of the thymus cortex and medulla
    5. Type VI epithelioreticular cells form thymic/Hassall’s corpuscles
    1. The lymphocytes in the periarterial lymphatic sheath (PALS) are mostly B lymphocytes
    2. The spleen filters blood and can initiate an immune responses to antigen carried in the blood
    3. The white pulp of the spleen consists of lymphatic tissue
    4. Red pulp is characterized by the presence of splenic sinusoids and splenic cords
    5. The periarterial lymphatic sheath (PALS) surrounds the central artery in the spleen

    Author of lecture Lymphoid Organs

     Geoffrey Meyer, PhD

    Geoffrey Meyer, PhD

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    very well detailed!
    By Niamh D. on 30. April 2017 for Lymphoid Organs

    I like how the questions are integrated at the appropriate part of this lecture.