Lymphatic System

University:
Kentucky State University
Course:
BIO 301 | Human Anatomy
Academic year:

2021
Views:

117

Pages:

47
Author:

Campbell Wagner

Lecture title:lymphatic system 3/3/2022 • The outer cortex has lymphatic follicles that mostly contain B-cells. • The inner cortex (paracortex) contains mostly T-cells. • The medullary cordscontain mostly plasma cells.(and memory cells) •     Other cells in the lymph node: Macrophages Dendritic cells Follicular dendritic cells Reticular cells Both the macrophages, and the dendritic cells trap antigens and present them on their surfaces As B cells in lymphatic follicle are stimulated, they differentiate into plasma cells. Plasma cells move to medulla (medullary cords) Sheet Note Immunocompetent doesn’t necessarily mean an active lymphocyte Immunocompetent means it is able to differentiate between self and non-self antigen (weather it is active or not;we call it active when it find the antigen and distinguish it) Active lymphocyte is larger and lighter stained. So according to follicle's activity ,there are 2 types: primary and secondary The outer cortex houses lymphatic follicles (nodules) which are of two types: 1- Primary lymphoid nodules (follicles): • Lymphoid follicles without a germinal centre • Spherical aggregates of B lymphocytes (virgin B cells and memory cells) 2- Secondary lymphoid nodules (follicles): • • • • lymphoid follicles with a germinal centre (pale centre). Form only in response to exposure to antigens Sites for B memory cell and plasma cell generation Outer region: dense accumulation of small lymphocytes that are migrating away from their site of origin within the secondary nodule When activated by antigens (and T helper cells), B cells migrate to the centre of the follicle, forming a germinal centre. Germinal centres are the central regions of secondary follicles where activated B cells are proliferating (dividing by mitosis) and differentiating into plasma cells and memory B cells. When stimulated by antigens, lymph nodes enlarge due to the formation of germinal centres and B cell proliferation Macrophages and Dendritic cells capture antigen within tissues and transport antigen to secondary lymphoid tissue Sheet Note 2 Primary;contain only inactive form of lymphocyte-naïve Secondary;when B-lymphocyte find the antigen ,become activated, get larger and lighter .,it moves to the center and defferintiate to give plasma cells and memory cells The medulla is the deep, cavitated part of the lymph node it is composed of: large tortuous lymph sinuses surrounded by lymphoid cells that are organized in clusters known as medullary cords medullary cords: contain B lymphocytes and some plasma cells Functions of lymph nodes • Nodes filter the lymph, removing foreign material and microorganisms. All lymph is filtered by at least one lymph node beforeit returns to the blood. • Antibody-mediated and cell-mediated immune responses occur in the lymph nodes • Lymph nodes are production sites of antibodies and activated lymphocytes • Sites for antigen recognition Sheet Note 3 Lymph nodes contain b cells t cells and antigen presenting cells so its primary function is to initiate immune response,producing anti-bodies and activate lymphocytes Both types of immune :  Cell-mediateddue to T lymphocyte  Antibody-mediateddue to B lymphocyte Afferent vessels Many afferent lymphatic vessels enter the lymph node at different points over its convex surface Each containing valves to prevent backflow of lymph. • Subcapsular sinuses Each afferent vessel empties into the subcapsularsinus. • Trabecular sinuses The trabecular sinuses are a continuation of the subcapsular sinuses that follow the trabeculae •andMedullary sinuses drain into the medullary sinuses. Found separating the cords. The medullary sinuses converge at the hilum into the efferent lymphatic vessel. Efferent vessels The lymph is removed from the medullary sinus via one or two efferent lymphatic vessels that leave the lymph node at the hilum. Valves in the vessels prevent lymph from flowing in the wrong direction. Lymph slows down when it passes lymph nodes Lymphocyte recirculation Lymphocyte homing Serves critical functions in adaptive immune response: 1it enables the limited number of lymphocytes in an individual that are specific for a particular antigen to search for that antigen throughout the body 2it ensures that particular lymphocytes are delivered to particular tissue eg. Recirculation of naive lymphocyte differ from those of effector and memory lymphocyte  specifically naive lymphocytes recirculate through peripheral lymphoid organ and effector lymphocyte migrate to peripheral tissue at sites of infection and inflammation Sheet Note 4 Affernt vessels is a property exclusive to lymph nodes Afferant vessels enter bellow the capsules so we call it SUBCAPSULAR SINUSES When antigen presenting cells enter lymph nodes immune response activationB cells differentiate into plasma cells and memory cellsmove fromgerminal center to medulla and transported through effernt vessels. Most plasma cells go to bone marrow,so most antibodies are produced within bone marrow Memory cells will go to otherlymph node through its affernt vessel ,searching for antigen. antigen And this explains why we can find memory cells in the primary nodules Lymphocytes can enter lymphoid tissues in two ways: 1) Direct entry into lymph nodes via afferent lymphatics (10% of lymphocyte enter by this root**) 2)Entry from blood capillaries across specialized endothelial cells present in the postcapillary venules (High Endothelial Venules= HEV) within the paracortex of Howlymph is the process the node of lymphocyte recirculatioin regulated? Why naïve lymphocytes migrate preferentially to lymph node????? The structure of the post-capillary venule, in the paracortex is unusual in that it is not lined by simple squamous epithelium, but by a simple cuboidal epithelium. These are called high endothelial venules (HEVs) Lymphocytes recognize and adhere to these endothelial cells, and squeeze through them into the paracortex The process of lymphocyte recirculation is regulated by adhesion molecules on lymphocytes called Homing receptors and their ligands on vascular endothelial cells called Adressins This diagram of a lymph node shows the pathways that lymphocytes can take, in and out of the lymph node. Sheet Note 5 Remember :simple squamous epithelium is the tissue within capillaries and blood vessels While within High Endothelial Venule(paracortex), the cells are cuboidal in shape. The ligand presented on HEV attract lymphocytes to enter into lymph nodes (90% of lymphocytes enter through this root) T-cells remain in paracortex ,B-cells will migrate to the follicles within cortex Note: Most of the lymphocytes enter the lymph nodes via blood vessels, and about 10% enter through the lymph. Spleen • The spleen is an oval-shaped intraperitoneal organ • It is the largest of the lymphoid organs • Approximately 5 inches in height (12-13 cm) 3 inches in width (7-8 cm) 1 inch in thickness (2.5 cm) Weighs 7ounces (200gm) Lies under ribs 9 to 11 Spleen is the organ of odd numbers • Functions I. Filtration of blood (defense against blood-borne antigens) II. The main site of old RBCs destruction. III. Production site of antibodies and activated lymphocytes (which are delivered directly into the blood) The spleen resembles a large lymph node The spleen filters the blood while lymph nodes filter the lymph Sheet Note1 Remember: • lymph node filters the lymph • Spleen filters the blood Unlike lymph nodes the spleen: 1. has no afferent lymphatics 2. has no lymphatic sinus system 3. Its lymphatic tissue is not arranged into a cortex and medulla • The splenic artery is the largest branch of the celiac artery • enter the spleen at the hilum Abdominal Aorta sheet**no lymphatic sinuses,instead there are capillary sinuses -no cortex ,medulla ;instead there are pulps(white and red pulps) Celiac Trunk Splenic artery Structure of the spleen • The spleen is covered by a capsule of dense connective tissue, and have capsular extensions called the trabeculae • The spleen is composed of parenchyma and stroma 1. Parenchyma: Splenic pulps 2. Stroma: Reticular tissue (reticular fibres and reticular cells) The parenchyma of the spleen appears in fresh specimen as: • White pulp which appears white on gross examination (collection of both B and T lymphocytes) • Red pulp which appears red on gross examination (blood filled) Splenic artery Divides into trabecular arteries as it enters the hilum Trabecular arteries Follow the course of trabeculae Central arterioles Are branches of trabecular arteries entering the white pulp. They are surrounded by a sheath of lymphocytes. Penicillar arterioles Each central arteriole eventually leaves the white pulp and enters the red pulp, losing its sheath of lymphocytes and branching as several short straight penicillar arterioles that continue as terminal capillaries. The structure of the spleen is bui around its blood supply Penicillus Sheet**terminal capillaries are surrounded by macrophages terminal sheath capillaries White pulp (lymphoid tissue) • Constituting 25% of the spleen • the white pulp is responsible for the immunological (lymphatic) function of the spleen. • The white pulp contains: 1. Periarteriolar lymphatic sheaths (PALS): White pulp Penicillar arterioles tightly packed T cells arranged in Lymphoid Sheathed cylindrical sheaths around central follicle capillaries arterioles (terminal 2. Lymphoid follicles: spherical PAL capillaries) S aggregations of B cells scattered throughout the PALS Primary (unstimulated) follicles contain resting Central (inactive) B cells arteriole Secondary (stimulated) follicles contain activated B cells in a central region (germinal center) When the lymphatic sheath expands to incorporate the follicles, the central arteriole Splenic nodules (Malpighian is displaced to one side and acquires an corpuscles) eccentric position in the follicle but is still Note: These follicles have the same called the central arteriole (Follicular arteriole). structural organization as those found in lymph nodes **The PALS will surround both Function: The lymphocytes and APCs Follicles & Central arterioles monitor the blood for foreign antigens and respond in a similar way to those in the lymph nodes. Blood flow through the splenic red pulp can take either of two routes: Open circulation: the capillaries open into the spaces of the red pulp (splenic cords) and then the blood returns to the venous system through the wall of the splenic 90% Of blood sinusoids Closed circulation: the capillaries open directly into the splenic sinusoids (blood is enclosed 10% of blood by endothelium) Note the wide gaps between endothelial cells which allow for movement of entire cells from cords to sinuses Red pulp (blood filled) • Constituting 75% of the spleen • The red pulp contains: 1. Splenic cords (Billroth’s cords): consist of all cells between the sinusoids in the red pulp (reticular cells, macrophages, plasma cells, lymphocytes, RBCs, platelets, other leukocytes) 1. Splenic sinusoids: are blood-filled spaces located throughout the red pulp. They have large, dilated, irregular lumens and large pores (spaces between the endothelial cells) 1. The endothelial cells (stave cells) are elongated, fusiform cells that lie parallel to the long axis of the vessel 2. The cells lie side by side around the vessel but not joined by any type of intercellular junctions 3. The endothelial cells are supported by highly discontinuous basal lamina (forms bars and encircles the sinusoid) Function: Destruction of worn-out RBCs and platelets **lots of spaces beween cells in the vessel because we need to return all the blood into the closed circulation • In this route plasma and all the formed elements of blood must re-enter the vasculature by passing through narrow slits between the stave cells into the sinusoids. • These small openings present no obstacle to platelets, to the motile leukocytes, or to thin flexible erythrocytes. • However stiff or swollen RBCs at their normal life span of 120 days are blocked from passing between the stave cells and undergo selective removal by macrophages • Deformed or less pliable RBCs cannot squeeze effectively from the cord into the sinus and upon their mechanical fragmentation are removed by resident macrophages (lie just next to the sinusoids) Macrophages monitor erythrocytes as they migrate from splenic cords between the endothelial cells into the splenic sinusoids Old erythrocytes lose their flexibility They cannot penetrate the spaces between the endothelial cells and are phagocytosed by macrophages Old erythrocytes lose sialic acid from their cell membranes  Galactose exposed Induce phagocytosis of RBCs After surgical removal of the spleen (splenectomy), the number of abnormal erythrocytes in the circulation increases although most such cells are then removed by macrophages in sinusoids of the bone marrow and liver **Macrophages The whole structure is called terminal sheath Marginal zone •sinuses Located between the white and the red pulp • The spaces between these sinuses are wide (2-3 µm) • It is here the blood-borne antigens and particulate matter have their first free access to the parenchyma of the spleen The following events occur at the marginal zone: 1-APCs sample the material travelling in blood searching for antigens 2-Macrophages attack microorganisms present in the blood 3-The circulating B and T cells leave the blood stream to enter the preferred location within the white pulp T cells: PALS B cells: lymphatic follicles Lymphocytes come into contact with APCs, if they recognize their antigen, the lymphocytes initiate immune response within the white pulp Functions of the spleen: 1. Blood cell production: During the fetal life, blood cells are produced in the spleen 2. Blood storage: A small quantity of blood is stored in the sinusoids of the red pulp 3. RBC destruction: Most worn-out or damaged red blood cells are destroyed in the spleen (some in the liver and bone marrow). They are phagocytozed by macrophages 4. Defense mechanism: Macrophages phagocytoze microbes that have penetrated the blood. Antigens in the blood activate B and T cells residing in the spleen, triggering immune response Production of antibodies and activated lymphocytes (which are delivered directly into the blood) Summary: Diffuse lymphatic tissue (lymphatic nodules) • Is formed by aggregations of lymphatic tissue • Is found in various mucosal sites of the body The mucosa or inner lining of the digestive, respiratory, and genitourinary tracts is a common site of invasion by pathogens because their lumens open to the external environment. It can therefore be referred to as: Mucosa-Associated Lymphatic Tissue (MALT) • These aggregations are not encapsulated • MALT can be found in the following locations: Palatine tonsils Lingual tonsils Pharyngeal tonsils Gut-associated lymphoid tissue (GALT) Bronchus-associated lymphatic tissue (BALT) MALT is populated by: T cells B cells Plasma cells APCs Each of which is well situated to encounter antigens passing through the mucosal epithelium sheet**MALT is found mostly in GI ,Urinary,respiratory system • Tonsils are large, irregular masses of lymphoid tissue embedded in the mucous membranes of the throat • Function of tonsils: Protect the body from inhaled and ingested pathogens. Palatine tonsils • Are located at the lateral wall of oropharynx**2 in number • Are covered by stratified squamous epithelium. • The surface area of each is enlarged with 10-20 tonsillar crypts (deep invaginations ) • Many lymphoid nodules around the crypts • Has an underlying • capsule (partial capsule) **when inflammation occurs ,pus will be collected within crypts As strawberry Pharyngeal tonsils • Are located in the posterior wall of the nasopharynx. • It is most prominent in children, but begins to atrophy from the age of seven. • Epithelium: respiratory epithelium (ciliated pseudostratified columnar epithelium) • Contain diffuse lymphoid tissue and lymphatic nodules • No crypts • Thin capsule • Hypertrophied regions of pharyngeal tonsils resulting from chronic inflammation are called adenoids. Lingual tonsils • Are located on the posterior 1/3 of the tongue. • Smaller and more numerous than palatine and pharyngeal tonsils • Covered by stratified squamous epithelium Waldeyer's tonsillar ring (Waldeyer's lymphatic ring) is a ringed arrangement of lymphoid tissue In the nasopharynx, oropharynx, and base of the tongue. **found as a ring in the entry of respiratory and digestive system So ,any pathogen we inhale or digest pass through tonsils first! Gut-associated lymphoid tissue (GALT) Is located in the mucosa of the intestine. Examples: 1 Peyer'spatches of ileum 2 Lymphatic nodules of appendix Function: Protects the body from ingested pathogens. Bronchus-associated lymphatic tissue (BALT) Is located in the mucosa of the bronchioles. Function: Protects the body from inhaled pathogens. Thymus • The thymus is a bi-lobed gland located in the anterior mediastinum, posterior to the sternum and anterior to the trachea • Within the thymus, immature T-cells develop, differentiate, and multiply, as well as gaining their antigen specificity and immune tolerance to the body’s own tissues. • It is large in the newborn and young child • From puberty onwards, it gradually becomes replaced by fat. • The thymus has a connective tissue capsule that extends septa, dividing the organ into many incomplete lobules. • Each lobule has an outer darkly basophilic cortex surrounding a more lightly stained medulla. • The staining differences reflect the much greater density of lymphocytes in the cortex than the medulla **90%of T-lymphocyte get destructed within the cortex (selecting the immunocompetent cells and destroying the rest) cortex appear darker than medulla The cortex contains: • .Immature T cells (T lymphoblasts, thymocytes) (in various stages of differentiation and maturation) • .Macrophages • Unique thymic epithelial cells (TECs) The medulla contains: 1.Fewer and more mature lymphocytes. 2.Macrophages 3. Dendritic cells (APCs) 4. Unique thymic epithelial cells (TECs) 5. Large aggregates of TECs called Hassall corpuscles As T cells mature, they migrate to the medulla Hassall corpuscles are unique to the thymic medulla • 20-100 μm in diameter • Are concentric aggregates of squamous cells with central keratinization(acidophilic) • Consists of flattened epithelial reticular cells wrapped about one another in concentric lamellation • Tend to grow larger with age • Function  unknown *Ex;lymphocytes that identify self antigen as a pathogen ;will be destroyed to prevent autoimmune disease Blood-thymic barrier The developing T cells of the cortex are protected from contracting blood-born macromolecules. However self-molecules are permitted to cross blood thymic barrier (probably controlled by epithelial reticular cells)  possibly to eliminate those T cells that are programmed against self-antigens Thymic Epithelial Cells (TECs) (Epithelial reticular cells) 1 Form a stroma to which macrophages and developing lymphocytes attach instead of reticular fibres 2Line the capsule and septa and surround all blood vessels in the cortex Form a blood-thymus barrier preventing antigens in the blood from making contact with the developing T cells (in cortex) 3 Envelop groups of T cells that are multiplying and maturing (in cortex) 4Act as APCs, expressing MHC class II and MHC class I molecules (in cortex) 5Express many specialized proteins specific to cells of other organs, tissue specific antigens (in medulla) 6 Secrete hormones that promote the differentiation of T cells (endocrine thymus) Thymosin, Thymopoietin **Has role in  Programming  Stroma  Secrete hormones Thymic epithelial cells are also called Nursing cells They form sheets for cells deep to the capsule, around septa and blood vessels, to isolate developing T lymphocytes in the cortex from contact with antigens (present in the blood), while they are programmed Lymph Drainage of Breast Sheet Note 3 2 pathways Medially ,Lateral  Mediallydrainage in the internal thoracic lymph node(also called intermamillary lymph node),25% of lymph  Lateral75% There is connection between right and left side ,so if cancer develop in the right breast it can metastasize to left breast through lymph nodes– Enlarged ,tender,soft lymph nodeimmune response Hard,no pain usually cause by metastisis. Inguinal Lymph Nodes **superfecial lymph node Beside Great saphenous vein then inguinal lymph node join with the deep ,,,complete the circulation

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