Lymphoid Organs Introduction The slides for this lab are located in the “Lymphoid” folder on the Virtual Microscope. In this lab, you will learn about the lymphatic system and lymphoid organs that filter and clean the blood and contribute to immunity. Lymphoid organs can be classified based on whether or not they have a connective tissue capsule. So-called encapsulated organs include the thymus, spleen and lymph nodes. While un- encapsulated organs include the appendix, ileum and tonsils, many of which you have encountered during your studies of histology so far. Learning objectives and activities Using the Virtual Slidebox: A Examine the thymus and identify the developing T-cells and epithelial-reticular cells of the thymic parenchyma B Examine a lymph node and identify the cells that perform functions in generating immune responses. C Examine the spleen and identify the cells that are involved in filtering the blood and reacting to blood-borne antigens. D Differentiate between the MALT (Mucosa Associated Lymphoid Tissue) of the palatine, pharyngeal and lingual tonsils. E Review the GALT (Gut Associated Lymphoid Tissue) in the appendix and ileum. F Complete the self-quiz to test your understanding and master your learning Examine the thymus and identify the developing T-cells and epithelioreticular cells of the thymic parenchyma. Examine Slide 1 and approximate a thymic lobule in the thymus by identifying the following: The thymus is a bilobed lymphoepithelial organ located in the superior mediastinum. Multipotential lymphoid stem cells from the bone marrow are destined to develop into immunocompetent T-cells within the thymic parenchyma. The thymus is encapsulated by connective tissue which also divides the parenchyma into thymic lobules. i. General organization a. Connective tissue capsule Identify the thymic A thin outer layer of dense irregular connective tissue. Contains capsule in blood vessels, efferent lymphatic vessels and nerves. Slide 1a b. Trabeculae Extend into the parenchyma of the organ to establish domains Identify the within the thymus called thymic lobules. They also contain blood trabeculae in vessels, efferent lymphatic vessels and nerves. Slide 1b c. Thymic lobule Identify a thymic Defined by connective tissue trabeculae it is composed of the lobule in cellular parenchyma organized into a cortex and medulla. Slide 1c d. Thymic cortex The outer portion of parenchyma within a lobule. It is markedly Identify the thymic basophilic because it is packed full of T-lymphocytes and scattered cortex in epithelioreticular cells. Slide 1d e. Thymic medulla The inner portion of the parenchyma within a lobule. It is less Identify the thymic basophilic as the T-lymphocytes here are more loosely packed and medulla in are larger. It also contains prominent epithelioreticular cells called Slide 1e “Hassall’s corpuscles”. ii. Parenchyma of the thymus f. Thymocytes T-lymphocytes in the thymus. In the cortex they are small and Identify thymocytes tightly packed. In the medulla they are larger and more loose. in Slide 1f g. Epithelioreticular cells There are six types of epithelioreticular cell in the thymus Type I - separate connective tissue capsule/trabeculae/blood vessels Identify Type I from the parenchyma of the cortex. Epithelioreticular cells - adjacent reticular cells are connected by occluding junctions in Slide 1g - forms blood-thymus barrier to isolate T-cells from other tissues Type II - are stellate cells within the cortex Identify Type II - have long cytoplasmic processes joined by desmosomes Epithelioreticular cells - compartmentalize the cortex into isolated areas of T-cells in Slide 1g - express MHCI/MHCII molecules at their surface - involved in thymic education Type III and Type IV - are located at the cortico-medullary junction Identify Type III and IV - have long cytoplasmic processes joined by tight junctions Epithelioreticular cells - form a functional barrier between the cortex and medulla in Slide 1g - express MHCI /MHCII molecules at their surface - involved in thymic education Type V Identify Type V - located throughout the medulla Epithelioreticular cells - have long cytoplasmic processes joined by desmosomes in Slide 1g - compartmentalizes the medulla into groups of T-cells Type VI - form isolated masses of packed cells called Hassall’s corpuscles Identify Type VI - may contain keratin in the center Epithelioreticular cells - secrete IL-4/IL-7 that assists in differentiation/education of T- in Slide 1g cells h. Macrophages Located in the thymic cortex but difficult to see in standard H&E Macrophages cannot preps. They phagocytose T-cells that do not fulfill thymic be seen in Slide 1 education requirements. 98% of T-cells will be phagocytosed. Examine a lymph node and identify the cells that perform functions in generating immune responses. Examine the general organization of a lymph node in Slide 2 (H&E) and Slide 3 (Silver) The role of the lymph node is to filter the lymph (extracellular fluid derived from blood). Lymph enters the node via numerous afferent vessels and leaves via a single efferent lymphatic vessel at the hilum. It is composed of a cortex, medulla and intervening sinuses through which lymphatic fluid flows. i. General organization a. Connective tissue capsule Identify the capsule in A thin outer layer of dense irregular CT that surrounds the node Slide 2a and Slide 3a b. Trabeculae Identify trabeculae in CT extending into the parenchyma forming a gross framework Slide 2b and Slide 3b c. Subcapsular sinus A lymphatic channel interposed between the CT capsule and the Identify subcapsular outer cortex. Afferent lymphatic vessels bring lymph into this sinus in sinus. Slide 2c and Slide 3c d. Trabecular sinus Identify a trabecular Originates from the subcapsular sinus and carries lymph through sinus in the cortex into the medullary sinuses. Slide 2d and Slide 3d e. Reticular tissue You can only see Composed of reticular cells (fibroblast-like cells) and reticular reticular tissue in the fibers (Type II collagen) that forms a fine supporting meshwork silver stained throughout the organ. The fibers are best seen when stained with Slide 3e a silver stain. f. Cortex The outer portion of parenchyma within a lymph node. It is Identify the cortex in composed mainly of reticular tissue and lymphocytes. It can be Slide 2f and Slide 3f sub-divided into outer/superficial and para/deep cortex and is home to lymphoid nodules. g. Medulla The inner part of the lymph node. It contains medullary cords Identify the medulla in (composed of reticular tissue, lymphocytes (mostly B- Slide 2g and Slide 3g lymphocytes) and medullary sinuses through which lymph drains. ii. Cortex of the lymph node h. Outer/Superficial/Nodular cortex The outermost region of the cortex that contains primary and Identify the superficial secondary lymphoid follicle cortex in Slide 2h and Slide 3h i. Inner/Deep/Para/Thymus-dependent cortex The portion of the cortex that separates the outer cortex from the Identify the deep medulla. It does not contain lymphoid nodules and is mainly cortex in composed of T-cells (hence thymus-dependent, perinatal Slide 2i and Slide 3i thymectomy results in an underdeveloped deep cortex). It is home to multiple high endothelial venules (HEV). j. Primary nodule/follicle Located in the superficial cortex, nodules are considered ‘primary’ Identify a primary if they consist of small lymphocytes that have not been exposed nodule in to antigen and have not commenced differentiation. As a result Slide 3j and Slide 3j primary nodules appear as basophilic aggregates of lymphocytes. k. Secondary nodule/follicle Located in the superficial cortex, nodules are considered Identify a secondary ‘secondary’ if the lymphocytes have been exposed to antigen and nodule in have commenced differentiation to form memory B-cells and Slide 2k and Slide 3k plasma cells. These cells predominate in a germinal center. Most nodules are secondary. l. Germinal center The central pale region of a secondary nodule. Recall that a germinal center develops when a B-lymphocyte has recognized an Identify a germinal antigen and has interacted with a complimentary T-lymphocyte. center in This interaction between B-cell and T-cell triggers proliferation Slide 2l and Slide 3l and differentiation of the B-lymphocyte to form memory B-cells and plasma cells within the germinal center. m. High endothelial venules (HEVs) Some lymphocytes enter lymph nodes through afferent lymphatic vessels (as a component of lymph) but most enter the node through the walls of HEVs located in the deep cortex. They are so Identify an HEV in named because they are lined with cuboidal endothelial cells (not Slide 2m and Slide 3m squamous). These special endothelial cells have receptors for antigen primed lymphocytes triggering them to migrate across the endothelium into the lymph node. ii. Medulla of the lymph node n. Medullary cords Cords of reticular fibers synthesized by surrounding reticular cells Identify medullary (fibroblasts) that serve as a framework for lymphocytes (mainly B- cords in cells), plasma cells from the cortex and macrophages that clean up Slide 2n and Slide 3n the lymph. o. Medullary sinuses Identify the medullary A pathway for the flow of lymph that runs between medullary sinuses in cords. Sinuses converge at the hilum where they drain into Slide 2o and Slide 3o efferent lymphatic vessels. trabecular sinus trabecula capsule afferent lymphatics efferent lymphatic vessel inner (para) cortex medullary cord outer cortex medullary sinus germinal center secondary lymphatic subcapsular sinus nodules Examine the spleen and identify the cells that are involved in filtering the blood and reacting to blood-borne antigens. Examine the general organization of the spleen in Slide 4 The spleen has a number of important roles. It acts as a store for platelets; reacts to blood borne antigens by producing antibodies (and is in fact the main source of circulating antibodies); it also removes defective red blood cells and platelets from the circulation. i. General organization a.