Lymphatic System PDF

JABIR IBN HAYYAN MEDICAL Lecture By : Hussein bahaa UNIVERSITY COLLEGE OF MEDICINE Lec :9 DEPARTMENT OF HUMAN ANATOMY Section of Histology Histology of lymph...

JABIR IBN HAYYAN MEDICAL Lecture By : Hussein bahaa UNIVERSITY COLLEGE OF MEDICINE Lec :9 DEPARTMENT OF HUMAN ANATOMY Section of Histology Histology of lymphatic system All lymphocytes (T & B) types originate in the red bone marrow, but become mature & functional in two different central or primary lymphoid organs. Cells destined to become B lymphocytes (Bone marrow dependent) remain and differentiate further in the bone marrow. Cells destined to become T lymphocytes (Thymus dependent) move from bone marrow via the circulation into the thymus. After maturation in these primary structures, B and T cells circulate to the peripheral or secondary lymphoid organs, which include the MALT, the lymph nodes, and the spleen. 1 THYMUS The primary or central lymphoid organ in which T cells are produced is the thymus, a bilobed structure in the mediastinum. Fully formed and functional at birth, the thymus remains large and very active in T-cell production until puberty, during which it undergoes involution, decreasing greatly in size and activity and becoming largely filled with adipose tissue. The thymus has a vascularized connective tissue capsule that extends septa into the parenchyma, dividing the organ into many incompletely separated lobules. Each lobule has an outer darkly basophilic cortex surrounding a more lightly stained medulla. The staining differences reflect the much greater density of lymphocytes in the cortex than the medulla. The thymic cortex contains an extensive population of T lymphoblasts (or thymocytes), some newly arrived via venules, located among numerous macrophages and associated with the unique thymic epithelial cells (TECs) that have certain features of both epithelial and reticular cells. 2 Thymic epithelial cells (TECs) are stellate cells with pale nuclei and long cytoplasmic processes that join together and completely isolate the cortex from the medulla The more lightly stained thymic medulla contains fewer and larger, more mature lymphocytes, Large aggregates of TECs, sometimes concentrically arranged, called Hassall corpuscles or (thymic corpuscles) are unique to the medulla. TECs are responsible for the secretion of factors which promote the maturation of the T cells. MUCOSA-ASSOCIATED LYMPHOID TISSUE (MALT) The mucosa or inner lining of the digestive, respiratory and genitourinary tracts is a common site of invasion by pathogens because their lumens open to the external environment. To protect against such invaders mucosal connective tissue of these tracts contains large and diffuse collections of lymphocytes, plasma cells and lymphoid nodules, all of which comprise the MALT. Most of the immune cells in MALT are dispersed diffusely in the connective tissue; others are found in aggregates that form large, conspicuous structures such as the tonsils, the Peyer´s patches in the ileum, and the appendix. Collectively the MALT is one of the largest lymphoid organs, containing up to 70% of all the body’s immune cells. Tonsils are large, irregular masses of lymphoid tissue in the mucosa of the posterior oral cavity and nasopharynx where their cells encounter antigens entering the mouth and nose. Named by their location these masses are the palatine, lingual, and pharyngeal tonsils. In all tonsils the lymphoid tissue is closely associated with the surface epithelium. 3  Palatine tonsils, located posteriorly on the soft palate, are covered by stratified squamous epithelium. The lymphoid tissue is filled diffusely with lymphocytes, with many secondary lymphoid nodules. Lingual tonsils are situated along the base of the tongue, are also covered by stratified squamous epithelium. The single pharyngeal tonsilis situated in the posterior wall of the nasopharynx, is covered by pseudostratified ciliated columnar epithelium. Diffuse MALT extends from the pharynx along the entire gastrointestinal tract but becomes very well-developed again in the mucosa and submucosa of the ileum. Here large aggregates of lymphoid nodules comprise the Peyers patches, each containing nodules with no underlying connective tissue capsule. Another significant collection of MALT occurs in the mucosa of the appendix, a short, small- diameter projection from the cecum. Typically the mucosa of the appendix is almost completely filled with lymphoid tissue LYMPH NODES 4 Lymph nodes are bean-shaped, encapsulated structures, generally only 10 mm by 2.5 cm in size, distributed throughout the body along the lymphatic vessels, most abundantly in the axillae (armpits) and groin, along the major vessels of the neck, and in the thorax and abdomen, especially in mesenteries. The nodes constitute a series of in-line filters of lymph that defend against the spread of microorganisms and tumor cells and provide enclosed environments that facilitate production of plasma cells secreting non-IgA antibodies. Before merging with the blood stream, all lymph is filtered and has antibodies added by at least one lymph node. The lymph node has a convex surface where afferent lymphatics enter and a concave depression, the hilum, where an efferent lymphatic leaves and where an artery, vein, and nerve penetrate the organ. A dense connective tissue capsule surrounds the lymph node, extending trabeculae internally through which the blood vessels branch. Valves in the lymphatics ensure that lymph flow is unidirectional. The most abundant cells of lymph nodes are: 1. Lymphocytes 2. Plasma cells 3. Macrophages 5 All of these cells are arranged in a stroma of reticulin fibers and reticular cells to form three major regions: an outer cortex, a central medulla, and a smaller area between these two called the paracortex.These regions are not physically compartmentalized like those of the thymus. The cortex includes the following components: Subcapsular sinus, immediately inside the capsule, receives lymph from the afferent lymphatics. From this space cortical sinuses(or trabecular sinuses) branch internally among the lymphoid nodules along trabeculae. Lymphoid nodules, with or without germinal centers, fill most cortical areas. The region between the cortex and medulla, the paracortex does not have precise boundaries but can be distinguished from the outer cortex by its lack of lymphoid nodules. The Medulla of a lymph node has two major components: Medullary cords are branched cord like masses of lymphoid tissue extending from the paracortex. Medullary sinuses are dilated spaces lined by discontinuous endothelium that separate the medullary cords, They are continuous with the cortical sinuses and converge at the hilum as the efferent lymphatic vessel. SPLEEN The spleen contains the largest single accumulation of lymphoid tissue in the body and is the only lymphoid organ involved in filtration of blood, making it an important organ in defense against blood-borne antigens. It is also the main site of old erythrocyte destruction. The Spleen is surrounded by a capsule of dense connective tissue from which emerge trabeculae to penetrate the parenchyma or splenic pulp. Large trabeculae originate at the hilum, on the 6 medial surface of the spleen, and carry branches of the splenic artery, vein, lymphatics, and nerves into the splenic pulp. The splenic pulp has two components: (A)- White pulp (20% of the spleen). The small masses of white pulp consist of: Lymphoid nodules and the periarteriolar lymphoid sheaths (PALS) Splenic artery penetrates the hilum, branched to give trabecular arteries that leave the trabeculae and enter parenchyma of spleen. The arteriole then surrounded by sheath of lymphocytes called peri-arterial lymphatic sheath (PALS). The sheath contains mainly T-lymphocytes (thymus dependent zone) whereas the lymphoid follicles contain B-lymphocyts. (B)-Red pulp. The red pulp consists of: Blood-filled sinusoids and splenic cords. The red pulp is composed almost entirely of splenic cords (of Billroth) and splenic sinusoids and is the site where effete RBCs in blood are removed. The splenic cords contain a network of reticular cells and fibers filled withT and B lymphocytes, macrophages, other leukocytes, and red blood cells. The splenic cords are separated by the sinusoids. 7