L8 Microanatomy of Lymphoid Tissue PDF

The lymphatic system Dr. Evelyn 1  Learning outcomes At the end of the lecture the students will be able to understand: 1. Introduction to lymphatic system. 2. Describe the unique features of: lymph nodes, spleen, thymus and tonsil....

The lymphatic system Dr. Evelyn 1  Learning outcomes At the end of the lecture the students will be able to understand: 1. Introduction to lymphatic system. 2. Describe the unique features of: lymph nodes, spleen, thymus and tonsil. 2  Introduction ▪The lymphatic system consists of lymphatic vessels, lymphocytes and lymphatic organs. ▪The lymphatic capillaries are thin-walled tubes which form networks in most of the tissues of the body. ▪These collect tissue fluid called the lymph. ▪The lymphatic capillaries unite to form larger vessels, the largest of these open into veins. ▪Protects organism against invading pathogens or antigens by producing immune responses. 3  Lymphatic System consists of: A. Cells – Lymphocytes (B,T, natural killer) – Antigen-presenting cells (dendritic cells, Langerhans’ cells & macrophages) B. Lymphatic “tissue” – diffuse and nodular C. Lymphatic “organs” (lymph nodes, spleen, thymus) D. Lymphatic vessels that carry the cells and fluid.  Functions of Lymphatic System  Draining fluid into the bloodstream  Filtering lymph  Filtering the blood  Helps fight infection in many ways such as: o Helping to make special white blood cells (lymphocytes) that produce antibodies. o Having other blood cells called macrophages inside the lymph nodes which swallow up and kill any foreign particles 5  Lymphoid tissue Primary Lymphoid tissue/organ: Primary lymphoid organs provide a site for lymphocytes develop from a lymphoid stem cell. Eg. Thymus, Bone Marrow. Secondary Lymphoid tissue/Organ: It is the site where these formed lymphocytes are most active. Eg. Spleen, Lymph nodes Lymphatic vessels Body wide network of thin-walled vessels with abundant lymphatic valves. Walls made up of attenuated endothelium. Drains into lymph nodes.  Lymphatic vascular system Lymph moves only in one direction toward the junction of the lymph vessels with the large veins. Draining Fluid into Bloodstream Lymphatic organs  Lymph Nodes Lymph nodes are small bean-shaped structures, in the path of lymph vessels to serve as filters for the removal of bacteria and other foreign substances. Several lymph vessels enter (afferents) the node on its convex aspect and a single or a few lymph vessel leave (efferents) the node through its hilum with blood capillares.  Lymph node has outer fibrous capsule. This sends trabaculae inside. There is subcapsular sinus. Afferent lymphatics around the gland pierces the capsule and drain in to subcapsular space. Cortex contains lymphoid follicles with pale staining germinal center. Rim of the follicles have densely packed inactive B-lymphocytes. Germinal centers contain pale staining large, rapidly dividing B-lymphocytes.  In medulla, lymphocytes arranged like branching cords along the sinuses called as medullary cords. These are mainly macrophages and plasma cells. Lymph will drain from the afferent lymphatics into subcapsular sinus, then passes along the trabacular sinus into medullary sinuses. Finally, drains out from the hilum through the efferent lymphatics. As it passes through the sinuses it comes in contact with antigen presenting cells which present it to the lymphocytes.  Functions of lymph node Filtration of lymph Proliferation of lymphocytes & forms a part of immune system.  Spleen Spleen contains large amount of lymphatic tissue –which filter the blood instead of lymph. Spleen has thick capsule, sends septa into substance of spleen. White pulp is aggregation of lymphocytes around the central artery – made of T-lymphocytes. At some places it forms lymphatic nodules with germinal centers called as Malpighian bodies. They contain B- lymphocytes. Red pulp forms the major part of spleen. It is made of lymphocytes arranged like branching and anastomosing cords along the sinusoids.  Function Removes old RBC's. Production of lymphocytes. Removes blood borne antigens. In fetal life it also acts as a site of erythrocyte production.  Thymus Thymus is a primary lymphoid organ. It grows up to puberty and regress during adult hood. Capsule sends incomplete septa into the parenchyma, dividing it into lobules. Each lobule contains outer cortex and central medulla. Cortex is made of densely packed small lymphocytes. Also contains macrophages for phagocytosis  Medulla of adjacent lobules interconnected. It contains sparsely arranged larger lymphocytes and epitheliocytes. Degenerated epitheliocyte cell aggregations appear as concentric eosinophilic mass called Hassall's corpuscles in the medulla. Epithelioid cells form reticular system of thymus and forms blood thymus barrier. Also produce hormones for proliferation and maturation of T-lymphocytes. Thymus  Function of thymus Differentiation of T-lymphocytes to cells capable of reacting to antigen.  Palatine Tonsil Palatine tonsil is situated in tonsillar fossa, on lateral wall of oropharynx. It is active during early life. Mucosal lining is stratified squamous non-keratinized epithelium.  Mucosal lining invaginate to form crypt which are characteristic of palatine tonsil. Lymphocytes aggregated in the form of follicles with a central germinal center along the crypts. Follicles mainly contain B- lymphocytes. T- lymphocytes lie outside the follicle. Mucous acini are seen in the deeper plane.  Thank you https://histologyguide.com/slidebox/10-lymphoid-system.html