Lymphatic System Study Guide

Lymphatic System Lymphatic system The lymphatic system consists of lymphatic fluid, lymphatic vessels, lymphatic tissue, and lymphatic organs located throughout the tissues of the body. We have fluid exchange between blood and ISF Along the course of lymphatic vessels we have lymph nodes It functions to: 1- Drain excess interstitial fluid from the tissues and return to blood stream 2- Initiate an immune response against disease by producing and transporting lymphocytes 3- Transport dietary lipids absorbed by the gastrointestinal tract into the blood. Unite to form The lymphatic system starts as Lyphamtic cappillaries 20 liters of blood enters the capillary bed but only 17 liters gets out why? Because the hydrostatic force of the blood on the capillary forces 3 liters of fluids to leave to the interstitial fluid. And these three liters are drained back to the blood by the lymphatic system Tonsils Thymus Lymph is a colorless fluid that floats in the lymphatic vessels. It is similar in composition to blood plasma Spleen Peyer patch (small intestine) Lymphatic vessels Bone marrow Lymphatic vessels are thin vessels that accompany arteries and veins throughout the body and transport lymph. Lymphatic tissue is a specialized form of reticular connective tissue that is composed of masses of lymphocytes. These either occur alone as lymph nodules (follicles) or are organized into various lymphatic organs. Lymph nodes Lymphatic organs include the lymph nodes, spleen, thymus, and red bone marrow Fluid balance The tissues of the body are supplied by blood capillaries that bring oxygen-rich blood and remove carbon dioxide-rich blood. Fluid similar to blood plasma, called interstitial fluid, leaches from these vessels into the surrounding tissue. Around 20 liters of fluid leaves the arterial capillaries every day, but only 17 liters of fluid returns to the venous capillaries. Lymphatic vessels function to drain this excess fluid from the tissues as lymph and return this fluid to the blood. Arterial side Venous side Lined With simple squamous epithelium The arterial side is Rich in O2 and nutrients The venous side is Rich in CO2 and waste products the lymphatic vessels start as blindended lymphatic capillary Lymphatic vessels begin as “porous” blind-ended lymphatic capillaries in tissues of the body and converge to form a number of larger vessels, which ultimately connect with large veins in the root of the neck. Arterial side (blind-ended) Lymph returns back to the big veins (venous angle: the junction between subclavian and internal jugular veins) through the Thoracic duct and Right lymphatic duct. Arterial side Venous side When fluid accumulates in the tissue, interstitial pressure increases pushing the flaps inward, opening the gaps between cells, allowing fluid to flow in. As pressure inside the capillary increases, the endothelial cells are pressed outward, closing the gaps, thus preventing backflow. Unlike blood capillaries, the gaps in lymphatic capillaries are so large that they allow bacteria and immune cells (ex. Macrophages/ dendritic cells) to enter. This makes the lymphatic system a useful way for large particles to reach the bloodstream. Remember: lymphatic system is used, for example, for dietary fat absorption in the intestine. The cells in the lymphatic capillary are overlapped in a way that they act as a one-way valve Dr. Heba Kalbouneh Lymphatic capillaries are made of overlapping endothelial cells with large gaps. The overlapping flaps function as a one-way valve. Transport Some lipids are too large to pass through the capillary walls of the small intestine and therefore cannot be absorbed. The lymphatic capillaries within the small intestine, known as lacteals, can absorb these large lipid molecules and transport them into the venous circulation via the thoracic duct. Lymph containing these lipids becomes a creamy white color and is referred to as chyle. Lymphatic Organs and Tissues Lymphocytes can be found throughout the body, however, they aggregate in places where they are most likely to come into contact with pathogens. Tonsils Thymus Lymphocytes are produced within the red bone marrow and are transported via the blood vessels to lymphatic tissues and organs. Spleen Peyer patch (small intestine) Lymphatic organs are divided into: Primary lymphatic organs Bone marrow. Thymus gland. Are sites of Lymphocyte production, maturation, selection Bone marrow Self Non-Self Secondary lymphatic organs Diffuse lymphatic tissue (lymphatic nodules). Spleen. Lymph nodes. Are sites to encounter pathogens and become activated Lymph node Lymph nodes üAre kidney-shaped small encapsulated bodies located along the course of lymphatic vessels (Approximately 600 lymph nodes ) üReticular tissue forms the stroma of the lymph node üLymph nodes are up to 3 cm in length üImmunocompetent B cells and T cells are suspended throughout the lymph node üNodes filter the lymph, removing foreign material and microorganisms. üAll lymph is filtered by at least one lymph node before it returns to the blood. üAntibody- mediated and cell- mediated immune responses occur in the lymph nodes üLymph nodes congregate around blood vessels in clusters and are usually named according to the vessel or location that they are associated with. Lymph node enlargement can happen in cases of lymphoma (painless lymphadenopathy) or infection (painful). Lymph nodes are production sites of antibodies and activated lymphocytes The main groups of lymph nodes include: Name Location Associated vessel Axillary nodes Armpit Axillary vein Cubital nodes Elbow Basilic vein Popliteal nodes Posterior knee Popliteal vein Inguinal nodes (superficial and deep) Groin Great saphenous vein Femoral vein Cervical lymph nodes (superficial and deep) Neck Internal jugular vein External jugular vein Important table The lymph node consists of an outer cortex and an inner medulla Cortex Outer cortex Inner cortex Para cortex Medulla Cortex Medulla Contains lymphatic follicles No follicles Receives lymph from afferent vessels Forms sinuses that lead to efferent vessels at the hilum Afferent vessels Efferrent vessels Afferant vessels: carries lymph to the lymph node (usually multiple) Efferrent vessels: carries lymph from the lymph node (usually single) An Efferent vessel for a lymph node could be an Afferant for another lymph node The nodes are covered by a capsule of dense connective tissue, and have capsular extensions called the trabeculae, which provide support for blood vessels entering into the nodes. When lymph nodes become enlarged, the capsule is stretched and becomes painful The cortex is the outer, highly cellular part of the lymph node; it can be divided into an outer cortex and inner paracortex. Capsule B cells Outer Cortex Medulla Paracortex T cells Plasma cells The outer cortex has lymphatic follicles that mostly contain B-cells. The inner cortex (paracortex) contains mostly T-cells. The medullary cords contain mostly plasma cells. Other cells in the lymph node: Macrophages Dendritic cells Follicular dendritic cells Reticular cells Outer cortex 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) B-cells Paracortex T-cells Plasma cell Medulla Cartwheel Clock face appearance Plasma cells are located peripherally Plasma cells Small (6-9 µm) Inactive lymphocyte Darkly stained cell Appears basophilic Heterochromatic nucleus Large (9-18 µm) Active lymphocyte Lightly stained cell Appears lightly basophilic Euchromatic nucleus The outer cortex houses lymphatic follicles (nodules) which are of two types: Primary follicles: lymphoid follicles without a germinal center. (virgin B cells) Secondary follicles: lymphoid follicles with a germinal center. Sites for B memory cell and plasma cell generation When activated by antigens (and T helper cells), B cells migrate to the center of the follicle, forming a germinal center. Germinal centers 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 centers and B cell proliferation Lymphatic follicle → Aggregation of B lymphocyte virgin cells → not yet exposed to antigen secondary follicle (activated , stimulated follicle ) The enlarged secondary follicle are the cause of lymph node enlargement in bacterial infections - After activation of B cells, germinal centers will be formed in the center of the follicle forming secondary follicles and these activated B cells are going to defrenciate into plasma cells and memory cells. - The plasma cells move from the germinal center inorder to populate inside the medullary cords and then they secrete antibodies directly into the lymph, and this lymph leave the lymph node through efferent vessels and eventually will reach the blood and curculate everywhere inside the body. -At the infection site these antibodies form complexes with the antigens and these complexes are completely eliminated by the resident macrophages or the recruted macrophages (monocytes) Dendritic cell Reticular cell Macrophage Plasma cell Medullary cord Macrophage Dendritic cell Macrophages and Dendritic cells capture antigen within tissues and transport antigen to secondary lymphoid tissue Macrophage Dendritic cell Follicular dendritic cell Phagocytosis Most phagocytic Moderately phagocytic X Antigen presenting (via MHC-II) Moderate Agpresenter Very powerful Agpresenter X Location in lymph node Cortex and medulla Cortex and medulla Outer cortex Are antigen HOLDING cells Holds the Ag for long time These cells are not phagocitic cells, instead these cells have special receptors for Ag-Ab complexes, so that after the infection is eleminated they hold these complexes to keep our immune system active and in a semi-stimulated state. The medulla is the deep, cavitated part of the lymph node; it is composed of medullary cords The cords are separated by spaces known as medullary sinuses The medullary sinuses converge at the hilum. The hilum is a slight indentation on one side of the node. Here, an artery, vein, and an efferent lymphatic vessel enter and leave the node. Medullary cords Hilum The paracortex can also be called, thymus dependent zone so if there is a defect in the thymus this zone will be underdeveloped --> can’t defend intracellular pathogens Medullary sinuses 1 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. How lymph moves in a lymph node Afferent vessel Efferent vessel Subcapsular sinuses 2 Each afferent vessel empties into the subcapsular sinus. Trabecular sinuses 3 The trabecular sinuses are a continuation of the subcapsular sinuses that follow the trabeculae and drain into the medullary sinuses. Medullary sinuses 4 Found separating the cords. The medullary sinuses converge at the hilum into the efferent lymphatic vessel. Subcapsular sinus Medullary sinuses Efferent vessels 5 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. Trabecular sinuses Sinuses are irregular spaces through which the lymph percolates Lymph flow Lymph nodes are linked together by lymphatic vessels. Lymph flows through a lymph node via a series of sinuses and lymphatic tissue Lymph, containing micro-organisms, soluble antigens and antigen presenting cells, enters the lymph node via afferent lymphatic vessels (1) which enter the subcapsular sinus (2). It then runs through trabecular (cortical) sinuses (3) then into medullary sinuses (4) and leaves through the efferent lymphatic vessels (5), at the Hilum as efferent lymph. Efferent lymph contains lots of activated Tlymphocytes, activated B-lymphocytes, plasma cells and antibodies. 1 2 3 5 4 Medullary cords All the lymphatic sinuses are lined by a discontinuous layer of simple squamous endothelium Afferent lymphatic vessel Trabecula Capsule Subcapsular sinus Dendritic cell Reticular cell Trabecular sinus Lymphatic follicle Paracortex (Thymus dependent zone) Macrophage Hilum Plasma cell Medullary cord Efferent lymphatic vessel Vein Artery Medullary sinus Subcapsular sinus Afferent lymphatic vessel Capsule Trabecular sinus Trabecula Lymphatic follicle (B cells) Artery Medullary sinus Vein Medullary cord (plasma cells) Efferent lymphatic T cells (thymus dependent zone) Lymphocytes can enter lymphoid tissues in two ways: 1) Direct entry into lymph nodes via afferent lymphatics 2)Entry from blood capillaries across specialized endothelial cells present in the postcapillary venules (High Endothelial Venules= HEV) within the paracortex of the lymph node 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. Note: Most of the lymphocytes enter the lymph nodes via blood vessels, and about 10% enter through the lymph. The venule directly after the cappillary bed Post capillary Venule (High Endothelial venule (HEV)) The lining epithelium is simple cuboidal epithelium instead of simple squamous epithelium so we call them high endothelial venules T cells enter through the HEV and stay there because it is in the paracortex, but B cells migrate to the primary follicle Arteriole paracortex Adressins(ligand on the apicl surface) that attach to the homing receptors on lymphocyest Lymphatic trunks and ducts All lymphatic vessels coalesce to form larger trunks which eventually converge to form the right lymphatic duct and the thoracic duct Right lymphatic duct ü Is formed by right jugular and right subclavian trunks üDrains lymph from the upper right quadrant of the body (the right side of the head and neck, the right side of the thorax and the right upper limb) üEmpties into the junction where right internal jugular vein joins the right subclavian vein (Rt venous angle) Thoracic duct (Left lymphatic duct) üIs larger and drains lymph from the rest of the body. üOriginates in the abdomen as cisterna chyli Cisterna chyli is a dilated sac at the lower end of the thoracic duct (anterior to the bodies of L1 and L2) formed by confluence of the right and left lumbar trunks and the intestinal trunk üPasses through the diaphragm at the aortic aperture üEmpties into the junction where left internal jugular vein joins the left subclavian vein (Lt venous angle) Cisterna chyli Try to describe these histological sections and the clinical picture a patient would have in each case Answered in the next lecture