Lesson 17 - Lymphoid Organs II (Notes) PDF

_____________ LESSON 17 _____________ LYMPHOID ORGANS (II) Secondary lymphoid organs Lymph nodes Lymph nodes (also incorrectly known in Spanish as “ganglio linfático”) are parenchymatous secondary lymphoid organs distributed along the lymphatic vessels. They are ovoid or kidney-shaped, with a depre...

_____________ LESSON 17 _____________ LYMPHOID ORGANS (II) Secondary lymphoid organs Lymph nodes Lymph nodes (also incorrectly known in Spanish as “ganglio linfático”) are parenchymatous secondary lymphoid organs distributed along the lymphatic vessels. They are ovoid or kidney-shaped, with a depression or hilum through which arteries and nerves enter and veins and lymphatic vessels exit. Like any parenchymatous organ, it is divided into a connective tissue stroma and a parenchyma that perform the specific functions of the organ. Thus, lymph nodes provide the microenvironment where lymphocytes are capable of responding to antigens carried by lymph and filtered by medullary macrophages through phagocytosis, as well as they phagocytose any other foreign substance. The stroma of the lymph node is made up of the capsule and trabeculae and a three-dimensional network of reticular fibres (type III collagen) and reticular cells, which are entrusted of synthesizing the former. The capsule is made up of fibrous connective tissue with abundant collagen and reticular fibres. Trabeculae depart from the capsule. Beneath the capsule and trabeculae there is a system of lymphatic sinuses that separate both from the parenchyma: subcapsular sinuses and trabecular sinuses, respectively. + Figure 1: Lymph node showing the capsule (arrow) and the parenchyma divided into cortex (+) and medulla (*). The parenchyma is divided into a peripheral zone or cortex and another central zone or medulla. The parenchyma is crossed by specialized lymphatic vessels or lymphatic sinuses. Afferent lymphatic vessels enter the lymph node through the capsule at various intervals on its convex surface and subsequently branch. Thus, the afferent lymphatic vessels flow into the subcapsular sinus that continues radially with the trabecular sinuses that, following the direction of the trabeculae, penetrate the cortical parenchyma. In the medulla, the trabecular sinuses continue with the medullary sinuses, which take on irregular shapes and have wide lumens. These branch and anastomose, fragmenting the parenchyma into medullary cords. The sinuses of the medulla meet in the subcapsular sinus in the area of the hilum and then flow into the efferent lymphatic vessel that leaves the parenchyma of the lymph node through the hilum. The afferent and efferent lymphatic vessels have valves that prevent the lymph from going back. The lymphatic sinuses are lined by a layer of endothelial cells that does not rest on a basement membrane. The endothelium is continuous in the subcapsular and trabecular sinuses but discontinuous in the medullary sinuses. The supportive framework of the sinus is made up of a layer of fibres and reticular cells. In the lumen of the medullary sinuses, extensions of dendritic cells and pseudopodia of macrophages located in the sinus wall are observed, as well as lymphocytes, macrophages and dendritic cells free in the lumen. These cellular elements slow down the flow of lymph, thus facilitating the interaction between antigens with macrophages and dendritic cells, responsible for initiating the immune response. In addition, they allow filtering into the parenchyma substances in solution and particles in suspension, as well as cells. The cortex is divided into an outer and an inner region. Lymphoid follicles and interfollicular areas are located in the outer region, known as outer or superficial cortex. As in other lymphoid organs, the interfollicular areas are made up of diffuse lymphoid tissue, composed mainly of T cells on a stroma of reticular cells and fibres. The inner region of the cortex, deep cortex or paracortex does not present lymphoid follicles, only diffuse lymphoid tissue. There is no clear boundary with the outer region. The deep cortex has specialized postcapillary venules called high endothelial venules (HEV) which are lined by cuboidal endothelial cells and lacking a muscular layer. These vessels are the port of entry into the lymph node for lymphocytes carried by the blood, coming either from the primary lymphoid organs or from their blood recirculation. These lymphocytes enter the parenchyma of the lymph node through the gaps between the endothelial cells. Figure 2: Cortex of a lymph node showing a secondary lymphoid follicle (*) and diffuse lymphoid tissue (arrow). Figure 3. Medulla of a lymph node showing the cords (*) and the medullary sinuses (arrow). The medulla is made up of cords of cells organized around small blood vessels. These cords occupy the space left by the medullary lymphatic sinuses and the trabeculae present in the medulla. The cords are made up of T cells, B cells, plasma cells, and macrophages. The blood supply to the lymph nodes is produced from the main arteries that penetrate the hilum, branching on one side towards the trabeculae and, on the other, towards the medullary cords where they become a network of capillaries, which irrigates lymphoid follicles and interfollicular tissue. Capillaries empty into high endothelial venules (HEV) which condense repeatedly in the interfollicular area and paracortex and then transition to medullary venules at the corticomedullary junction. Venules penetrate the medullary cords giving rise to small veins, which are tributaries of larger venous vessels inside the trabeculae and exit the lymph node through the hilum. DISTINCTIVE FEATURES OF THE PIG LYMPH NODES The structure of the pig lymph nodes, as in the elephant, rhinoceros, dolphin and hippopotamus, is different from the rest of the species. They are made up of several nodular units. The cortical parenchyma is located in the centre of each unit and around the afferent hilum. The medullary tissue is distributed around the periphery of the nodular units and around the efferent hilum, it does not present cords and the sinuses are very narrow and scarce. The medullary sinuses drain the efferent lymphatic vessels, which leave the node through the spaces between the nodular units. Lymphocytes circulate differently than they do in the lymph nodes of other mammals; thus, they do not leave the lymph nodes through the efferent lymphatic vessels, but rather through the postcapillary venules existing in the central portions of the nodular units. The blood vessels enter along with the afferent lymphatic vessels and exit with the efferent lymphatic vessels, thus no true hilum is seen.