Lymphatic System and Lymphoid Organs 2 PDF

Lymphatic System and Lymphoid Organs 2 Morphology of Lymph Nodes - Lymph nodes are glandular structures that vary in size. - They are encapsulated, spherical or kidney-shaped organs composed of lymphoid tissue distributed along lymphatic vessels. - They are a main source of lymphocytes in the bl...

Lymphatic System and Lymphoid Organs 2 Morphology of Lymph Nodes - Lymph nodes are glandular structures that vary in size. - They are encapsulated, spherical or kidney-shaped organs composed of lymphoid tissue distributed along lymphatic vessels. - They are a main source of lymphocytes in the blood (the other being bone marrow). - A lymph node consists of an outer cortex and an inner medulla. - Lymph nodes are found in the axilla and groin, and in large numbers in the thorax and abdomen. - Lymph nodules within the cortex contain lymphocytes. - Lymph nodes decrease in size with age unless enlarged by inflammation or tumors. - In the limbs, lymph nodes are largest and most numerous in the axilla and groin. - Lymph nodes are usually found in groups connected by lymph vessels. Structure of Lymph Nodes - Capsule: Dense connective tissue surrounds the node and sends trabeculae into the node. Reticular connective tissue forms the stroma for the interior of the node. - Outer Zone: Filled with lymphoid nodules composed of B lymphocytes. - Inner Zone (Paracortex): Filled with diffuse lymphoid tissue composed of T lymphocytes. - Sinuses in Cortex: Consist of a loose network of macrophages and reticular fibers through which lymph filters. - Subcapsular Sinus: Lies beneath the capsule and receives incoming lymph from afferent lymphatic vessels. - Intermediate Sinuses: Lie adjacent to the trabeculae, receiving lymph from the subcapsular sinus and continuing as medullary sinuses. Medulla of Lymph Nodes - Composed of: - Medullary Cords: B lymphocytes extend from the inner cortex into the medulla. - Medullary Sinuses: Continuations of the intermediate sinuses in the cortex. Lymph flows from medullary sinuses into the efferent lymph vessels that exit at the hilum of the node. - Blood Supply: Small arteries enter at the hilum to supply a capillary plexus in the outer cortex. The capillaries form high endothelial venules (HEVs) in the paracortex and small veins that exit at the hilum. Main Groups of Lymph Nodes - Axillary Lymph Nodes: Consist of groups that drain the upper limb and breast. They enter efferent lymphatic vessels forming the subclavian lymph trunk. - Inguinal Lymph Nodes: Receive lymph from the lower limbs and drain into external iliac and common iliac nodes. - Cervical Lymph Nodes: Receive lymph from the head and neck. Main Lymph Channels - Cisterna Chyli: Drains the abdomen. - Thoracic Duct*: Receives lymph from the cisterna chyli and the left upper limb and left side of head and neck. - Right Lymphatic Duct: Drains the right upper quadrant of the body. - Mesenteric Lymph Nodes: Superior nodes form a system where central nodes at the root of the superior mesenteric artery receive lymph from mesenteric, ileocolic, and right colic nodes. - Bronchomediastinal Lymph Nodes: Receive lymph from tracheobronchial and mediastinal lymph nodes. They terminate at the junction of the subclavian and internal jugular vein. Spleen Structure - The spleen is located in the upper left quadrant of the abdominal cavity. - Capsule: Surrounds the organ, sending trabeculae into the spleen. - Larger blood vessels traverse the trabeculae. Subdivisions of the Spleen - White Pulp: Appears white in fresh specimens, composed of periarterial lymphoid sheath (PALS) and surrounded by T lymphocytes around a central arteriole. - Lymphoid Nodules: Composed of B lymphocytes randomly located along and embedded in the PALS. Functions of the Spleen - The spleen contains the largest accumulation of lymphoid tissue and is the only lymphoid organ involved in blood filtration, important in defense against blood-borne antigens. - It is the main site of old erythrocyte destruction. - The spleen produces antibodies and activated lymphocytes, delivered into the blood. Histology of the Spleen - The spleen is surrounded by a capsule of dense connective tissue from which trabeculae penetrate the splenic pulp. - Large trabeculae originate at the hilum and carry branches of the splenic artery, vein, lymphatics, and nerves into the pulp. - The spleen is filled with reticular tissue containing reticular cells, fibers, lymphocytes, other blood cells, macrophages, and antigen-presenting cells (APCs). - The splenic pulp has two components: white pulp (20% of the spleen) and red pulp. - White pulp consists of lymphoid nodules and PALS; red pulp consists of blood-filled sinusoids and splenic cords. Functions of Splenic Red and White Pulp - The spleen monitors blood immunologically. The microvasculature contains unique regions. - Small trabecular arteries branch from the hilum, entering the parenchyma as arterioles primarily composed of T cells, with some macrophages and plasma cells in the white pulp. Morphology of the Spleen - The spleen is the largest accumulation of lymphoid tissues, located in the abdomen. - It is the site of destruction of aged red blood cells. - It has white pulp composed of lymphoid tissue and red pulp containing sinusoids. - The spleen lies beneath the left diaphragm, close to the 9th, 10th, and 11th ribs. - Anteriorly: Stomach, tail of the pancreas, left colic flexure. - Posteriorly: Diaphragm, left pleura, left lung. Blood Supply - The splenic artery is the largest branch of the celiac artery, running along the upper border of the pancreas. - The splenic artery divides into about six branches entering the spleen at the hilum. Relations and Blood Supply of the Spleen - The splenic vein leaves the hilum, running behind the tail and body of the pancreas. - Behind the neck of the pancreas, the splenic vein joins the superior mesenteric vein to form the portal vein. - Lymph Drainage: Lymph vessels emerge from the hilum, passing through lymph nodes along the splenic artery and draining into celiac nodes. - Nerve Supply: Nerves accompany the splenic artery, derived from the celiac plexus.  Lymphatic system and Lymphoid Organs 1 Classificauon of lymphoid organs Primary lymphoid organs and structures : - Bone marrow. Site of origin of T and B lymphocytes. B lympho-cytes directly seed secondary lymphoid rstructures ii and organs. - Thymus. T lymphocytes from bone marrow undergo further maturation in the thymus before seeding secondary lymphoid structures and organs. Secondary lymphoid organs and structures: www Diffuse lymphoid tissue in Lymphoid nodules. Both solitary and in aggregates. we Tonsils, lymph nodes and spleen Mucosa-Associated Lymphoid Tissue (MALT) - Located in the lamina propria of the respiratory and digestive systems. is - Composed of clusters of lymphocytes and other cells capable of responding to antigens. - Filters and provides immune surveillance for tissue fluid in the lamina propria. Lymph Vessels - Lymph vessels are fine tubes that contain lymph. - They assist the cardiovascular system in removing tissue fluid from body spaces and returning it to the blood. - Found in all tissues and organs except the CNS, eyeball, internal ear, epidermis, cartilage, and bone marrow. Functions of Lymphoid Tissue The lymphoid organs: - Provide immune surveillance and defense against foreign substances and microorganisms. - Provide immune tolerance, distinguishing between "self" and "non-self." I - Absorb lipids into small lymphoid vessels (lacteals) in intestinal villi for distribution to the bloodstream. - Help maintain fluid balance by accumulating tissue fluid and white blood cells in lymph vessels and returning them to the blood. Circulation of Lymph - Lymph capillaries are a network of fine vessels that drain lymph from tissues. - They have a similar structure to blood capillaries but are wider and less regular. - Small lymph vessels drain this capillary plexus and unite to form larger lymph vessels, which converge on primary lymph nodes. - The lymph passes through these nodes and leaves in vessels that usually converge on secondary and tertiary lymph nodes. sina.fm Lymphatic Vessels - Lymph drains through a number of lymph nodes and is gathered into larger lymph vessels before returning to the bloodstream via great veins at the root of the neck. 5 - Lymph vessels have numerous valves. The vessels that carry lymph an to a node are called afferent vessels, and those that carry it away are efferent lymph vessels. i Tonsils fi - Pharyngeal, lingual, and palatine tonsils are located at the junction of the oral cavity with the pharynx. - Composed of aggregations of lymphoid nodules and diffuse lymphoid tissue.  Thymus d - A primary lymphoid organ that receives immature lymphocytes (thymocytes) from the bone marrow. - These cells mature in the thymus and are carried to secondary lymphoid structures byvia the blood vascular system. - A connective tissue capsule surrounds the thymus and extends into it, dividing it into lobules. bies www.iw Thymus Structure - Each lobule contains: - An outer cortex densely packed with thymocytes (developing T lymphocytes). - An inner medulla with fewer thymocytes. - A blood-thymic barrier around capillaries in the cortex to protect developing lymphocytes from circulating antigens. Diffuse Lymphatic Tissue I - Located in the lamina propria of any organ opening to the exterior of the body. - Composed of clusters of lymphocytes and other cells capable of responding to antigens. - Diffuse lymphatic tissue is also located in tonsils, lymph, nodes, and spleen. Lymph Nodules - Found in the lamina propria of any organ opening to the exterior. - May occur singly (solitary) or in clusters (aggregates) such e as in tonsils and Peyer's patches in the small intestine. - Primary nodules are present before antigen stimulation. consists primarily of densely packed B lymphocytes. - secondary nodules appear after stimulation. central pale core, the germinal center, appears i This center is composed of immunoblasts that divide to form lymphocytes that accumulate inthe densely packed, peripheral zone of the nodule.