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10.03.2015 Lymphoid Tissue Dr. Archana Rani Associate Professor Department of Anatomy KGMU UP, Lucknow What is lymphoid tissue? • Specialized form of connective tissue • Supporting framework: reticular cells & reticular fibres • Large number of lymphocytes • Other cells: Plasma cells & macrophages Consists of……. • Lymphatic vessels • Specific lymphoid organs (lymph node, spleen, thymus) • Lymphatic tissue found within the tissues of other organs (in bone marrow, GI tract, urinary tract, respiratory tract) Functions • Defense of body • Phagocytosis of foreign cells • Involved in production of lymphocytes and plasma cells Lymphatic Vessels – Originate as lymph capillaries – Capillaries unite to form larger lymph vessels • Resemble veins in structure • Connect to lymph nodes at various intervals Lymphatic Capillaries Lymphatic capillary & Vessel Lymphatic Vessels Channels of Lymphatics – Lymphatics ultimately deliver lymph into 2 main channels • Right lymphatic duct –Drains right side of head & neck, right arm, right thorax –Empties into the right subclavian vein • Thoracic duct –Drains the rest of the body –Empties into the left subclavian vein Channels of Lymphatics Major Lymphatic Vessel of the Trunk Lymphatic Tissue – 3 types • Diffuse lymphatic tissue – No capsule present – Found in connective tissue of almost all organs • Lymphatic nodules – No capsule present – Oval-shaped masses – Found singly or in clusters • Lymphatic organs – Capsule present – Lymph nodes, spleen, thymus Diffuse lymphatic tissue • Called as mucosa associated lymphatic tissue (MALT). • Accumulation of lymphatic tissue in the mucous membrane of gastrointestinal, respiratory, urinary and reproductive tracts. • Located where they come in direct contact with antigens. Lymphatic Nodule • Circumscribed concentration of lymphatic tissue (lymphocytes and related cells). • Not surrounded by capsule. Lymphatic Nodule Lymphatic Organs Lymph Node – Consists of connective tissue framework & numerous lymphocytes. – Bean shaped structures placed in pathway of lymphatic vessels. – Enclosed by a fibrous capsule. – Cortex = outer portion • Germinal centers produce lymphocytes – Medulla = inner portion • Medullary cords Lymph Node • Lymph enters nodes through afferent lymph vessels, flows through sinuses, exits through efferent lymph vessel. Lymph Node 1.Capsule 2. lymphoid nodule with germinal center 3. subcapsular sinus 4. intermediate sinus 5. medullary cords 6. medullary sinuses 7. trabecula Cortex of Lymph Node Medulla of Lymph Node Medullary sinuses Medullary cords Medullary sinus of a lymph node containing reticular cells with long processes and elongated nuclei, macrophages, and many lymphocytes. (1) Macrophage; (2) reticular cell; (3) trabecula. H&E stain. High magnification. (Courtesy of PA Abrahamsohn.) Cells of Lymph Node • Lymphocytes • Plasma cells • Reticular cells • Macrophages and other phagocytic antigen processing cells • Lymphatic and vascular endothelial cells Functions of Lymph Node • Filtration of particles and microorganisms to keep them out of general circulation. • Interaction of circulating antigens in lymph with lymphocytes to initiate immune response. • Activation, proliferation of B lymphocytes and antibody production. • Activation, proliferation of T lymphocytes. Spleen • Largest lymphoid organ • Encapsulated • Structure is similar to a node • Capsule present • But no afferent vessels or sinuses Spleen • Substance is arranged in form of: White pulp (basophilic) Red pulp (reddish) • Supporting Elements: Capsule Trabeculae Trabecular network Lymphocytes, macrophages, blood cells Red pulp: splenic cords Spleen White pulp: Lymphoid Nodule (Malpighian corpuscle) 1. Germinal center 2. Central artery Red Pulp of Spleen (Splenic cords: Cords of Billroth) Functions of Spleen • Filtration of blood. • Immune response against antigens circulating in blood. • Site for production of B & T lymphocytes. • Formation of blood cells during fetal life. • Storage of blood. • Site of destruction of aged erythrocytes. Thymus – Location – behind the sternum in the mediastinum – Development: • Infant – conspicuous • Puberty – maximum size • Maturity – decreases in size – Function • Differentiation and maturation of T cells Thymus • Consists of 2 lobes (rt. & lt.) covered by connective tissue capsule. • Septa passing inwards from the capsule subdivide the lobe into a large number of lobules. • Lobule: outer cortex, inner medulla • Supporting stroma: epithelioreticular cells Thymic Lobule • Cortex: densely packed small lymphocytes. • Medulla: Lymphocytes are less densely packed. Presence of Hassall’s corpuscles. Functions of thymus • Provides the environment for stem cells where they can divide and mature into T lymphocytes. • Thymopoietin induces T cell production & maturation. • Thymosin supports T cell activities. Tonsils – Multiple groups of large lymphatic nodules – Location – mucous membrane of the oral and pharyngeal cavities. – Palatine tonsils • Posterior-lateral wall of the oropharynx – Pharyngeal tonsil • Posterior wall of nasopharynx – Lingual tonsils • Base of tongue Palatine Tonsil • Aggregation of lymphatic nodules within diffuse lymphoid tissue. • Covered by stratified squamous epithelium. • Tonsillar crypts (opening of numerous mucous glands) Functions of tonsil • Production of lymphocytes. • Immunological response against antigens & organisms coming in contact with epithelium. References 1. diFiore’s Atlas of Histology with functional Correlations, 12th Edition. 2. Textbook of Human Histology. Inderbir Singh, 1st Edition. 3. Textbook of Histology. GP Pal, 3rd Edition. MCQ • The supporting framework of lymphatic tissue is formed by all except: 1. Reticular cells 2. Plasma cells 3. Macrophages 4. Fibrocyte MCQ • Afferent lymphatics in lymph node pour their lymph into: 1. Trabeculae 2. Subcapsular sinus 3. Medullary sinus 4. Lymph nodule MCQ • Cords of Billroth is a feature of: 1. White pulp of spleen 2. Red pulp of spleen 3. Thymus 4. Tonsil MCQ • The tonsil is covered by: 1. Simple squamous epithelium 2. Stratified squamous epithelium 3. Stratified cuboidal epithelium 4. Stratified columnar epithelium MCQ • Hassall’s corpuscles is a feature of: 1. Lymph node 2. Spleen 3. Thymus 4. Tonsil .
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