Histology of the lymphatic system + Anki flashcards

[00:00] In this session I am going to deal with the histology of the lymphatic tissue. Before dealing with the histology let's classify the lymphatic organs and tissues. There are primary lymphatic organs that provide environment for stem cells to develop.

[00:20] and mature and these divide and mature into B and T lymphocytes. The primary lymphatic organs are the red bone marrow. Actually the red bone marrow give rise to both T and B lymphocytes but the B lymphocytes they mature in the

[00:40] red bone marrow while the T lymphocytes they have to migrate to the thymus gland to become mature. So both the red bone marrow and the thymus are primary lymphatic organs. A secondary lymphatic organ or tissue is the site where immune responses occur.

[01:00] There is multiplication of lymphocytes but there is no maturation of lymphocytes and these can be subdivided into encapsulated and non-encapsulated. The encapsulated ones they have a connective tissue capsule of dense irregular connective tissue like the lymph nodes.

[01:20] like the spleen and the non-encapsulated ones are the lymphatic nodules that we find them in the small intestine forming the piers, patches in the terminal ileum in the appendix and also in the tonsils that

[01:40] are present in the pharynx. These are not covered by capsule of connective tissue but they are covered by mucus membrane epithelium as we will see shortly. Now let's start with the thymus. The thymus is a primary lymphatic organ.

[02:00] It is a lobulated organ invested by a connective tissue capsule, capsule of dense irregular connective tissue and from the capsule it sends interlobular receptor that divides the organ interlobules and it is through these interlobular receptors.

[02:20] that the blood vessels reach the substance of the thymus. The thymus is the place for the development and maturation of T lymphocytes from the lymphocytes that are derived from the bone marrow, red bone marrow. And here,

[02:40] in the thymus development of immunological self-tolerance takes place. The thymus is also considered as an endocrine gland because the erticular epithelial cells that are present in the parenchyma of the gland, they secrete hormones that are required.

[03:00] for the cell maturation of the T-cells in the thymus and in the fetal life it is a hemopoietic organ. If you look at the thymus in this section you can see again the interlobular septum. You can see also in each lobule there is a

[03:20] medulla and darkly staining cortex. Here again you can see the lighter staining medulla and darkly staining cortex. The cortex is highly packed with lymphocytes which are undergoing development while the inner pale staining medulla

[03:40] has fewer lymphocytes. In fact the medulla forms a continuous core that is covered by the cortex. An important thing to notice here is that in the cortex there are no nodules, no lymphatic nodules as we will see them in the lymph node. Here again

[04:00] You can see the darkly staining cortex and lighter staining medulla. These are blood vessels in the interlobular septum and in the cortex the immature T lymphocytes are located. These are most of the cells that are called thymocytes and they migrate from the red bone marrow.

[04:20] said the clones of these T-cells are produced by cell divisions and they are pushed as they mature, they are pushed toward the middle so that they leave through the blood vessel or through a lymphatic vessel and efferent lymphatic vessel. Also in the cortex there are

[04:40] epithelial reticular cells. You can see these cells, they have cell body and multiple processes as you can see them here in this diagram. These produce the thymic hormones hence the thymus is also on endoclinic gland. They aid in T cell maturation like nursing the

[05:00] The T-cells, these are the clones of the T-cells are located in between them and they form a stromal meshwork for support. In addition to that, they participate in the formation of the blood-thymus barrier. You can see here there is a capillary and some of the

[05:20] processes are wrapped around or they in sheaths, they form sheaths around the blood vessels creating the barrier to the entry of antigenic material into the cymic parenchyma. In addition to that, you can see in the cortex there are large parenchyma processes.

[05:40] cells here these are the macrophages. In the medulla you can see that the cells are separated from each other, it is less densely packed and so you can see in some places you can see these processes of the epithelial reticular cells which are also present in the medulla.

[06:00] not only present in the cortex. These fine cytoplasmic extension of the thymic epithelial cells. This is another section showing the cortex and medulla, but here the medulla clearly shows these structures which are very characteristic for the medulla.

[06:20] the love of the thymus. These are called the thymic corpuscles or the laminated corpuscles or the hassel corpuscles. They are clusters of concentric layers of dead cells. Of course these are located in addition to the mature T cells.

[06:40] which are going to leave via the blood. During their development, the lymphocytes that recognize self-antigens are eliminated before leaving the thymus and this results in what we call immunologic self-tolerance. Returning back to the Hassel's corpuscles, this is the particular

[07:00] feature of the thymus. You can see them here. Concentric arrangement is the concentric arrangement. But also you can see that there are debris of dead cells. These hassles, corpuscles, they appear in fetal life. They increase in number.

[07:20] thereafter initially they begin as a medillary epithelial cells which enlarge and then degenerate, it form a vaculated eosinophilic mass into which the lymphocytes with debris dying cells which were sensitized

[07:40] against the self, that's to say that they did not develop immunologic tolerance, they are dumped here. Actually the function of these thymic corpuscles is not clear but this is probably one of the functions is that it is a dumping place for the

[08:00] faulty lymphocytes as they mature. During puberty, the thymus undergoes involution and is largely replaced by fatty tissue. So this is an adult thymus. You can see that although it undergoes involution but there is still evidence of thymic tissue and the thymus is

[08:20] still functional even in adult life. Now let's deal with another lymphatic organ. It's a lymph node and this is a secondary lymphatic organ where immune responses are initiated. It is encapsulated as you can see here the capsule of dense regularization.

[08:40] connective tissue and also surrounded by fat in many places. From the capsule, trabeculae enter into the lymph nodes and divide the nodes into compartments as you can see here in this diagram. In addition to that there are particular fibrin and fibrin.

[09:00] and these they form a supporting network. These are reticular fibers, not reticular epithelial cells like the ones that are present in the science. These are reticular fibers kind of connective tissue fibers like the collagen, elastic fibers and the reticular fibers.

[09:20] Now the parenchyma of the lymph node which is the functional tissue has a region here which is called the outer cortex. It is darkly staining like the one in the thymus but it is characterized in the lymph node by the presence of these lymphoid follicles and these are aggregates of B cells.

[09:40] Many of them, they have a center that is lighter stained and it's called the Gerbinal Center and the follicle as a whole is called the Secondary Follicle. They contain macrophages as well as proliferating B-cells. These activated B-cells in the Gerbinal Center, they have more cytoplasm.

[10:00] and so the terminal center is less densely populated with the cells and hence it appears as a lighter stain. So we have a cortex that is divided into an outer cortex and then there is a region here which is called the inner cortex. In the inner cortex there are

[10:20] no lymphatic nodules but there are T lymphocytes, the T lymphocytes they proliferate, they multiply, they do not mature, they multiply as well as there are some dendritic cells which are antigen-presenting cells that are present in multiple locations.

[10:40] These dendritic cells in fact are present in the outer cortex and in the inner cortex. And then we have the central region here which is the medulla and the medulla is in two parts. It consists of medulla cords and these are extensions of the cortex.

[11:00] into the medulla and again they contain B cells, plasma cells and macrophages as well as medullary sinuses. You can see here these are medullary sinuses which are actually a network or a plexus of interconnected lymphatic channels that

[11:20] are going to collect and form an efferent lymphatic vessel. Here again this is another section of a lymph node. You can see the surrounding fatty tissue like for example probably this is from a lymph node in the axilla where there is a lot of fat in the axilla and you can see.

[11:40] clearly here in this section how the medullary cords are extending into the medulla and these are the medullary sinuses, the light are staining medullary sinuses. The function of the lymph node is that it acts as a filter for lymph. So how does this take place?

[12:00] It takes place because it has multiple afferent lymphatic vessels. You can see here this is a vessel, this is another vessel. The vessel here is showing even it's showing a valve which prevents backflow of lymph. So multiple lymphatic vessels are coming from the periphery and there is here

[12:20] at the hilum there are multiple profiles of vessels. Some of them are blood vessels, the ones with thicker wall and one or two are with thinner walls and they form efferent lymphatic vessels. So multiple efferent lymphatic vessels and one or two efferent

[12:40] lymphatic vessels. Here this is a higher magnification of an afferent lymphatic vessel showing the presence of the valve and as the lymph is flowing through the lymph node, the lymphocytes are exposed to antigens in the lymph as it flows through the nodes.

[13:00] will go through a subcapsular sinus and then a trabecular sinus until it reaches the medullary sinus and then leaves through the efferent lymphatic vessel. Here again a subcapsular sinus below the capsule, trabecular sinus, a medullary sinus.

[13:20] and then it leaves it through an efferent lymphatic vessel. Now let's deal with another lymphatic organ which is an encapsulated secondary lymphatic organ. This is the spleen. The spleen again has a stroma of connective tissue capsule with multiple trabeculae.

[13:40] And within the substance of the spleen, there is also a meshwork of reticular fibers, connective tissue fibers. In this slide which is stained with a special stain to show the reticular fibers as a black meshwork as you can see it here within the

[14:00] the parenchyma of the spleen, supporting the cells that are present in the spleen. The parenchyma of the spleen is in two parts. There are these rounded profiles which constitute what we call the white spout because it is mainly lymphoid aggregates, white blood cells.

[14:20] and the in-between, the red pulp which is highly vascular. As you can see here that there is no segregation into cortex and medulla. And so this differentiates the spleen from the lymph nodes which also had lymphatic aggregates in the form of.

[14:40] follicles but the follicles are located in the outer cortex, they are not dispersed here in the entire thickness of the spleen. Let's go into a higher magnification of the white pulp. You can see in the white pulp here that there are aggregates of lymphocytes, macrophages,

[15:00] pages and the most important thing here that there are central arteries in the white pulp. These are called central arteries and surrounded by the lymphatic sheaths which contain T-cells organized in the form of nodules. Some of them are even with a

[15:20] germinal center as you can see here, proliferating and larger cytoplasm so they are lighter stained and the peripheral ones are B lymphocytes. In the red pulp which is highly vascular this is the region of the red pulp between the white pulp. It is rich with macrophageal disease.

[15:40] plasmodium cells, blood cells and lymphocytes and in addition to that there are a lot of sinuses, large venous sinuses. These are not lymphatic sinuses, they are venous sinuses. So here there is a kind of an open circulation, the blood reaches.

[16:00] the spleen and the artery is divided into central arteries and then from these central arteries capillaries will blood will leave the capillaries into the red pulp before it reaches the venous sinuses. You can see

[16:20] terminal capillaries and these are the venous sinuses. So in the spleen, immunological reaction takes place to antigens in the blood. In other words, the spleen acts like a filter of blood. It's not like the lymph node which acts

[16:40] as a filter of lymph. Here it acts as a filter of blood not only because there is an immunological reaction taking place to antigens in the blood but because the worn-out RBCs are removed here from the spleen. These RBCs and

[17:00] pylchis, they cannot easily fold to move into the venous sinus and so they will not be able to return back to the blood and they will be engulfed by the macrophages that are present in the breath.

[17:20] It also acts as a blood reservoir, that's why the capsule of the spleen also contains smooth muscle fibers that contract in order to push blood to the circulation or squeeze the blood to the circulation when it is required. We should keep in mind that the spleen is not a blood reservoir.

[17:40] is not a vital organ and when removed, most of its functions can be performed by other organs. So far about the encapsulated lymphatic organs. Now I will deal with secondary lymphatic organs that are non-encapsulated and these are located in the mucosa.

[18:00] and submucosa of GI tract, respiratory and urogenital tract and they are called mucosa-associated lymphoid tissue or MALTS. The ones that are present in the submucosa and the mucosa of the gut is called GOUT, gut-associated lymphoid tissue and they

[18:20] provide protection against invasion by pathogens via the exposed absorptive surfaces. These lymphoid organs, these non-incorpsulated lymphoid organs, they can be either aggregated into nodules or they can be diffuse. So let's have a

[18:40] an example here, an aggregated lymphoid tissue. This is located in the submucosa of the small intestine. This is a small intestine. You can see the multiple villi. And to be specific, this is the ileum. And these lymphatic nodules, they might aggregate

[19:00] into larger and larger masses that can be seen by the naked eye, especially in the terminal ileum and are called the piers patches. Another example of the grout is located in the appendix. This is a section of the appendix. You can see the mucosa, the submucosa, and the ileum.

[19:20] and the muscular slayer and this is a higher magnification in the mucosa here and this is the submucosa. You can see a lymphoid nodule even with a germinal center in the submucosa and this is large intestine because I can see crypts but there are no villi. This is another

[19:40] place where there is an aggregated non-encapsulated lymphoid tissue. There's no connective tissue capsule surrounding the lymphoid aggregates. The other place is in the peratime tonsil. You can see here that this tonsil is covered by epithelium.

[20:00] at it in higher magnification it is stratified squamous epithelium and it is non-teratinized, stratified squamous epithelium like the one lining the mouth, the pharynx, the palate and you can see that there are multiple crypts. These are the nodules with germinal centers. So this is

[20:20] is a palatine ponsel, which is characterized by these crypts. Another place where they are present in the oropharynx is these lymphoid nodules that are present on the posterior third of the tongue. You can see here the tongue is covered.

[20:40] by stratified squamous non-carotidized epithelium and we can say that this is the tongue because there are multiple bundles here of muscle fibers not very clear but also I can see here that beneath the epithelium there are collecting

[21:00] infection of mucus acini of accessory-celevery glands. So this is a lingual tonsil that is present in the posterior third of the tongue. The lingual tonsil together with the palatime tonsil together with the adenoid or the pharyngeal tonsil together with the pneumatic tonsil.

[21:20] tubal tonsils, they constitute a ring at the beginning of the upper respiratory and digestive passages which is called the Waldeir's ring. This is an example of diffuse lymphoid tissue that is present in the mucosa. Here this is a section in the

[21:40] you can see the stratified squamous epithelium known keratinized and these cells they are lymphocytes and plasma cells they are diffused in the mucosa and another example here of diffuse lymphoid tissue as the alveolar macula

[22:00] macrophages that are present in the alveoli of the lung and the alveolar septum, you can see they are darkly staining because these macrophages they have engulfed the particulate matter which is like dust cells, carbon particles or the some

[22:20] they phagocytize red blood cells that may enter the alveoli and heart failure as well as their main function which is to phagocytize infectious organisms. Thank you.

[22:40] Thanks for watching.