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.