Lymphatic System II - PDF

Lymphatic system II Dr. Silvia Boyajian 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) Spleen is the...

Lymphatic system II Dr. Silvia Boyajian 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) Spleen is the organ of odd Weighs 7ounces (200gm) numbers Lies under ribs 9 to 11 Functions I. Filtration of blood (defense against blood-borne antigens) II. The main site of old RBCs destruction. The spleen resembles a large lymph node III. Production site of antibodies and The spleen filters the blood while lymph nodes filter the lymph activated lymphocytes (which are delivered directly into 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 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. Penicillus The structure of the spleen is built around its blood supply 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: White pulp 1. Periarteriolar lymphatic sheaths (PALS): Penicillar arterioles tightly packed T cells arranged in cylindrical sheaths around central arterioles Lymphoid follicle Sheathed capillaries (terminal capillaries) 2. Lymphoid follicles: spherical aggregations PALS of B cells scattered throughout the PALS Primary (unstimulated) follicles contain resting (inactive) B cells Secondary (stimulated) follicles contain Central arteriole activated B cells in a central region (germinal center) When the lymphatic sheath expands to incorporate Splenic nodules (Malpighian corpuscles) the follicles, the central arteriole is displaced to one side and acquires an eccentric position in the follicle but is still called the central arteriole (Follicular Note: These follicles have the same structural arteriole). organization as those found in lymph nodes Function: The lymphocytes and APCs 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 sinusoids Closed circulation: the capillaries open directly into the splenic sinusoids (blood is enclosed 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  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 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) MALT is populated by: These aggregations are not encapsulated T cells MALT can be found in the following locations: B cells Plasma cells Palatine tonsils APCs Lingual tonsils Each of which is well situated to encounter antigens passing through the mucosal Pharyngeal tonsils epithelium Gut-associated lymphoid tissue (GALT) Bronchus-associated lymphatic tissue (BALT)  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 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) 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. 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 The cortex contains: 1.Immature T cells (T lymphoblasts, thymocytes) (in various stages of differentiation and maturation) 2.Macrophages 3.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  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 2-Line 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) 4-Act as APCs 6-Secrete hormones that promote the differentiation of T cells (endocrine thymus) Thymosin, Thymopoietin  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 Thank You