Lymphatic System PDF

The Structure of the Lymphatic System Ms. Sharmili Ravindrarajan BSN (Hons) – UoP Lecturer (Probationary) Introduction When circulating blood reaches the capillaries part of its fluid content passes into the surrounding tissues as tissue fluid Most of this fluid re enters the capillar...

The Structure of the Lymphatic System Ms. Sharmili Ravindrarajan BSN (Hons) – UoP Lecturer (Probationary) Introduction When circulating blood reaches the capillaries part of its fluid content passes into the surrounding tissues as tissue fluid Most of this fluid re enters the capillaries at their venous ends. However, sometimes returned to the circulation through a separate system – Lymphatic vessels The fluid passing through the vessels is lymph The smallest lymphatic vessels are lymphatic capillaries that join together to form larger lymphatic vessels The largest lymphatic vessel in human body – Thoracic duct Introduction Scattered along the course of lymphatic vessels there are numerous small bean shaped structures are present – Lymph Nodes Lymph nodes are masses of lymphoid tissue Lymph from any parts of the body passes through one or more lymph nodes before entering the blood stream Lymph nodes act as filters removing bacteria and other particulate matter from lymph Lymphocytes are added to lymph in these nodes Thymus and spleen are almost entirely made up of lymphoid tissues Introduction Aggregation of lymphoid tissue in the regions of pharynx – Tonsils Isolated nodules of lymphoid tissue and larger aggregations called Peyer’s patches are present in the mucosa and submucosa of the small intestine The mucosa of the vermiform appendix contains abundant lymphoid tissue Collections of lymphoid tissues are also seen in the walls of the trachea, large bronchi and urinary tract Lymph It is a transudate from the blood contains the same proteins as in plasma Suspended in lymph there are cells that are chiefly lymphocytes Large molecules of fat (Chylomicrons) that are absorbed from the intestine enter lymph vessels After a fatty meal, these fat globules may be numerous that lymph becomes milky – Chyle Lymphatic Vessels Lymphatic capillaries begin blindly in tissue where they form a network The structure of lymph capillary is similar to blood capillary but high permeability is present The structure of a thoracic duct and of other larger lymph vessels is similar to that of veins Main Groups of Lymph Nodes in the Body Palpable lymph nodes (When enlarged during infections) ✓Superficial and deep lymph nodes – Neck ✓Axillary lymph nodes – Axilla ✓Inguinal lymph nodes – Front of the thigh just below the inguinal ligament Important groups of deep lying nodes are present in the thorax and in the abdomen Structure of Lymph Nodes Each lymph node consists of a connective tissue framework and of numerous lymphocytes, and other cells that fill the interstices of the network The entire node is bean shaped Contains two zones: ✓Cortex (Dark color) – Densely packed lymphocytes; Outer layer ✓Medulla (Light color) – Few lymphocytes; Inner layer Within the cortex there are several rounded areas – Lymphatic follicles or nodules Structure of Lymph Node Structure of Lymph Nodes Each nodule has a paler staining Germinal center surrounded by a zone of densely packed lymphocytes Within the medulla, the lymphocytes are arranged in the form of branching and anastomosing cords Each lymph node is covered by as capsule A number of septa or trabeculae extend into the node from the capsule and divide the node into lobules The remaining spaces within the node are filled by a delicate network of reticular fiber Lymphocytes Both B lymphocytes and T lymphocytes are present in lymph nodes and these are enter from blood The cortex part of the lymphatic nodules consists of B lymphocytes The cells in the paler germinal centers of the nodules – Lymphoblast B lymphocytes have been stimulated, by antigens to enlarge and undergo multiplication The lymphocytes divide repeatedly and give rise to more B lymphocytes. These mature into plasma cells. Plasma cells produce antibodies Section through a Lymph Node 1. Cortex 2. Germinal center 3. Outer zone of lymphatic follicle 4. Medulla 5. Blood vessel Lymphocytes The diffuse lymphoid tissue intervening between nodules is made up mainly of T lymphocytes These are also present in medulla T cells enter lymph nodes from the blood. After few hours they leave the node via efferent lymph vessels When activated by antigen they multiply to form a large number of activated T cells that are sensitive to specific antigen Apart from lymphocytes and plasma calls, fibroblasts and macrophages are also present  Scheme to show the circulation of B Lymphocytes and T Lymphocytes through a lymph node The Spleen – Introduction The spleen is in the upper part of the abdomen, on the left side in contact with the diaphragm, the stomach, and the left kidney Its size is approximately to a clenched fist The arterial supply to the spleen is through the splenic artery The splenic vein joins the portal vein, that all blood has to pass through the liver Connective Tissue Basis The spleen is the largest lymphoid organ of the body The surface is covered by peritoneum (Serous coat) and deep to the serous layer the organ has a capsule Trabeculae arising from the capsule extend into the substance of the spleen The spaces between trabecula are filled by a network of reticular fibers Fibroblasts (Reticular cells) and macrophages are also present in relation to the reticulum The interstices of the reticulum are pervaded by lymphocytes, blood vessels, macrophages and blood cells Section through Spleen 1. Capsule 2. Septum 3. Red pulp 4. Cords of densely packed lymphocytes around arterioles Circulation Through the Spleen On reaching the hilum of the spleen, the splenic artery divides into about five branches Each branch divides and subdivides as it travel through the trabecular network Arterioles arising from the network leave the trabeculae to pass into the inter trabecular spaces In some places the arteriole is surrounded by a dense sheaths of lymphocytes – White pulp of the spleen Circulation Through the Spleen The arterioles divide into small vessel that open into the reticular framework, which leads blood flows into spaces lined by reticular cells coming into direct contact with lymphocytes – Red pulp The circulation in the red pulp of the spleen is thus an ‘open’ one in contrasts the ‘closed’ circulation in other organs Blood from spaces of the red pulp is collected by wide sinusoids. These drain into veins in the trabeculae The RBC trapped in the spleen and destroyed by macrophages The cords of white pulp are thicker than elsewhere and contain lymphatic nodules – Malpighian bodies Scheme to Show Some Features of Splenic Circulation Circulation Through the Spleen Each nodule has a germinal center and a surrounding cuff of densely packed lymphocytes The nodules are easily distinguished from other lymph nodes due to the presence of arteriole in each of them T lymphocytes are abundantly present in white pulp and the lymphatic nodules of the white pulp are aggregations of B lymphocytes The red pulp is like a sponge and is permeated by spaces lined by reticular cells The spaces are filled by B and T lymphocytes, macrophages and blood cells. These arranged in a cords to form splenic cord The Thymus It is lies in the thorax, behind the manubrium sterni, and in front of the heart. Consists of two lobes, right and left At birth thymus weighs 10 – 15g. The weight increases to 30 – 40g at puberty T lymphocyte production is present throughout the life The right and left lobes are joined together by fibrous tissue Each lobe has a connective tissue capsule Connective tissue septa passing inwards from the capsule incompletely subdivide the lobe into a large number of lobules Each lobule has outer cortex and inner medulla  The Thymus Gland The Thymus Both cortex and medulla contain cells of two distinct kinds of cells: ✓Epithelial cells (Epitheliocytes) oFlattened cells with branches and form a network ✓Lymphocytes / Thymocytes oFill the spaces in the reticulum oStem cells formed in bone marrow travel to the thymus. There they come to lie in the superficial part of the cortex, and divide repeatedly to form small lymphocytes oLymphatic nodules are not present in the normal thymus The Thymus As thymocytes divide they pass deeper into the cortex and medulla, ultimately they leave the thymus by passing into blood vessels and lymphatics Apart from epithelial cells and lymphocytes the thymus contains a fair number of macrophages (Mononuclear phagocyte system) Corpuscles of Hassall These are small rounded structures present in the medulla of the thymus Each corpuscle represents a mass of epithelial cells that have undergone degeneration and forms a pink staining hyaline mass Around this mass there is a wall formed by concentrically arranged epithelial cells Mucosa Associated Lymphoid Tissue in the GI Tract Near the junction of the oral cavity with the pharynx – Tonsils ✓Left palatine tonsils are largest and present on either side of the communication between the mouth and the pharynx ✓Another midline collection of lymphoid tissue, the pharyngeal tonsil is present on the posterior wall of the pharynx ✓In children, it get hypertrophy and then referred as adenoids ✓Smaller collection of lymphoid tissue are present on the dorsum of the posterior part of the tongue which is Lingual tonsils and around the pharyngeal openings of the auditory tubes is Tubal tonsils Scheme of various tonsils present near the junction of the oral cavity and the pharynx Section through a Palatine Tonsil