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Lymphatic Organs the Lymphatic Organs Are Classified in To: 1

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Lymphatic Organs the Lymphatic Organs Are Classified in To: 1 Lymphatic organs The lymphatic organs are classified in to: 1. Primary (central) lymphoid organs: responsible for development and maturation of lymphocytes. It consists of bone marrow and thymus gland. 2. Secondary (peripheral) lymphoid organs: Site where mature lymphocytes react with antigen. It consists of lymph node, spleen, lymphatic tonsil and diffuse lymphatic nodules. Thymus gland A primary lymphatic organ responsible for maturation of T lymphocytes to become immuno-competent (functional). Size of the thymus varies with age: - In infants, it is found in the inferior neck and extends into the mediastinum where it partially overlies the heart. It increases in size and is most active during childhood. - It stops growing during adolescence and then gradually atrophies. Structure: 1. Stroma: - Capsule: Thin CT capsule surrounding the gland - Septa: extend from the inner surface of the capsule into the gland tissue dividing it into lobules. - Epithelial-reticular cells (not reticular connective tissue): ñ don´t form reticular fibers. ñ joined together with desmosome forming the stromal background of the thymus. ñ Important for blood thymic barrier (will be mentioned later). Parenchyma: Thymic lobes contain an outer cortex and inner medulla Cortex: It is the outer dark part of the thymus lobule and contains • Lymphocytes: Most thymic cells are immature T-lymphocytes. They are rapidly dividing and densely packed. • Few macrophages. Medulla: It appears lighter than the cortex: • Few number of mature T lymphocytes • Thymic (Hassall’s) corpuscles: Consisting of concentric whorls of keratinized epithelial cells, which are thought to be degenerate epithelial cells. Recently it is evidenced that Hassall’s corpuscles are involved in the development of a class of T lymphocytes called regulatory T cells, which are important for preventing autoimmune responses. Blood supply of the thymus gland The arterial supply to the thymus gland is via the anterior intercostal arteries and small branches from the internal thoracic arteries. Venous blood drains into the left brachiocephalic and internal thoracic veins. - The thymic arteries follow the course of the CT septae and enter the organ substance at the cortico-medullary junction forming corticomedullary arterioles which ramify into capillaries that extend into the cortex and medulla. - In the cortex, non-fenestrated capillary network together with perivascular lymphocytes, macrophages and peripheral reticular epithelial cells form the blood-thymus barrier. - In the medulla, fenestrated capillaries are freely permeable to circulating antigens. - Blood drains into the post-capillary venules, returns to the corticomedullary junction in medullary veins that drain to the thymic vein. Blood- thymus barrier: It is a barrier between blood stream in the capillaries and immature T lymphocytes in the cortex of the gland. This prevents access of circulating antigens to the developing cortical lymphocytes. It is formed of:- 1- Non-fenestrated endothelium of blood capillaries. 2- Thick basement membrane of the capillaries. 3- A layer of perivascular CT containing macrophages. 4- Basement membrane of the epithelial reticular cells. 5- Continuous covering of epithelial reticular cells. Macr ophage The thymus differs from other lymphoid organs in important ways - It functions strictly in T lymphocyte maturation - It does not directly fight antigens - The stroma of the thymus consists of star-shaped epithelial cells (not reticular fibers) - These thymocytes secrete the hormones that stimulate lymphocytes to become immunocompetent. Lymph nodes Lymph node is a secondary lymphatic tissue where B-lymphocytes proliferate into antibody-secreting plasma cells. It is a bean-shaped organ, up to 1 inch long, located along lymphatic vessels throughout body. Function of lymph node: 1. Filtration of lymph from foreign bodies. 2. Lymphocytes storage and proliferation. 3. Antibody production by plasma cells. Lymph formation: 1. Filtration from plasma normally exceed reabsorption leading to formation of tissue fluid 2. Increase pressure in the interstitial spaces forcing fluid into lymphatic Circulation of lymph inside lymph node Afferent lymphatic vessel s cross the capsule at its convex surface and pour lymph into sub capsular sinus to the cortical and medullary sinus . It leaves the node through the efferent lymphatic vessel at the hilum. Enlarged regional lymph node is caused by acute infection, chronic infection, and neoplasia. capillaries forming lymph N.B: Lymph accumulation in interstitial spaces causes elephantiasis. Structure of lymph node: a) Stroma: the CT framework of lymph node includes: 1- CT capsule. 2- Septa: descend from the capsule to divide the cortex into multiple compartments. 3- Reticular network: formed of reticular cells and reticular fibers. b) Parenchyma: arranged into cortex and medulla I. Cortex: the cortex is divided by septae into compartments. It contains: 1-Lymphatic or cortical nodules: round or oval masses formed of lymphocytes. -Types: of 2 types: 1. Primary lymphatic nodules: Dense packed small lymphocytes with no germinal center. 2. Secondary lymphatic nodules contain pale area in GC its center called germinal center. It contains lymphocytes, plasma cells and macrophage cells. 2 ry lymphatic nodule 2-Subcapscular and cortical sinuses: - Sub capsular sinuses: lymphatic sinuses between the capsule and the cortical parenchyma. - Cortical sinuses arise from the sub-capsular sinuses and penetrate the cortex between the cortical nodules and the septa. Thymus dependent zone (para-cortical zone) Present in the in the deep part of the cortex near the medulla (juxtamedullary region), rich in mature T lymphocytes migrated from the thymus that enter the lymph node through the postcapillary venules. It undergoes marked depletion following neonatal thymectomy. II. Medulla: A. Medullary cords: formed of lymphocytes, plasma cells and macrophages. B. Medullary sinuses: lymphatic sinuses between the medullary cords and trabeculae. Spleen It is the largest lymphatic organ in the body. Lies behind the stomach high up on the left side of the abdomen. Structure of the spleen 1. Stroma: 1- CT capsule: dense and contains smooth muscle cells. 2- Septa: short and irregular CT descend from the capsule. 3- Reticular network: formed of reticular cells and reticular fibers 2. Parenchyma: It is formed of white pulp and red pulp. - White pulp: • It is formed of lymphatic nodules contain B lymphocytes with pale germinal centers • They appear white in fresh sections. • The white pulp contains peri arterial sheath, which is formed of eccentric artery surrounded with sheath of T lymphocytes. - Red pulp: formed of: 1. Cords of Billroth or splenic cords: cords of cells between the venous sinuses, formed of lymphocytes, blood cells, macrophages and plasma cells embedded in reticular tissue. 2. Splenic sinuses - Are irregular branching and anastomosing channels. - They are lined by an unusual highly elongated, spindle-shaped endothelial cells, resting on discontinuous basement membrane, which lie parallel to the long axis of the sinuses. - Phagocytic cells are present in the wall of the sinus. Marginal zone • It lies between the white pulp and red pulp • It contains numerous macrophages, few lymphocytes and plasma cells. • Many arterioles that derived from the central artery are terminated in the sinuses of marginal zone. • Here the lymphocytes of the re-circulating pool leave the blood to enter the periarterial sheath of white pulp. Blood circulation in the spleen: W Splenic artery enters the spleen through the hilum. - It divides into trabecular arteries, which pass with the CT trabeculae. - It give rise to small arteries called central arteries that enter the white pulp where they are surrounded with lymphatic sheath. - Branches from the central arteries arise to supply the white pulps. - The central arteries leave the white pulps to enter the red pulps and divide into straight branches called penicillar or straight arterioles. - The penicillar arterioles end in many capillaries called sheathed capillaries or ellipsoids. Each ellipsoid is a thickened capillary surrounded with reticular cells and macrophages. - Sinuses are joined and drained into red pulp venules, then enter the trabecular veins to the splenic vein, which leaves the spleen at the hilum. W In red pulp the blood is delivered to the splenic sinuses by one of the following theories: Theories of splenic circulation: 1. Closed theory: Blood passes directly from the penicillar, the ellipsoid capillaries into the splenic sinuses. 2. Open theory: blood drains from the ellipsoid capillaries into cords of Billroth, then the blood passes into splenic sinuses through the gaps between the endothelial cells. 3. Closed and open theories: bot theories coexist at the same time. 4. Knisely theory (not in human): direct shunts between arterioles and venules. W Sinuses are joined and drained into red pulp venules, then enter trabecular veins to the splenic vein, which leaves the spleen at the hilum. Function of spleen: ñ Filtration of blood from any microorganism. ñ Formation of blood elements in early fetal life. ñ Storage of blood cells & platelets (blood reservoir) ñ Removal of deteriorating blood cells and platelets ñ Hemoglobin breakdown and iron storage ñ Immune defense mechanism In spite of its large size and numerous functions removal of the spleen has limited side effect on t he body. It seems that its function can be taken on by the liver or bone marrow. Lymphoid
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