Dr. Samia Farrara. MBS University of Denver. USA Lymphatic system Dr. Samia Farrara. MBS University of Denver. USA

introduction The lymphatic system is a network of tissues and organs that primarily consists of lymph vessels, lymph nodes and lymph. The tonsils, adenoids, spleen and thymus are all part of the lymphatic system. The human circulatory system processes an average of 20 liters of blood per day through the capillary filtrations, which remove plasma while leaving the blood cells. Roughly 17 liters of filtered plasma get reabsorbed directly into blood vessels, while the remaining 3 liters are left behind in interstitial fluid. One of the main functions of the lymph system is to provide an accessory route for these excess 3 liters per day to get returned to the blood.

The other main function is that of defense in the Immune system The other main function is that of defense in the Immune system. It involved with lymphocyte production and immune response. Lymph is very similar to blood plasma but contain lymphocytes and other white blood cells. It also contain waste products and debris of cells together with bacteria and protein. Lymphoid organs are: 1.Primary or central: Thymus and bone marrow. 2.Secondary or peripheral: Lymph node, spleen, tonsils, solitary lymph nodules, appendix, Peyer ‘s patches of ileum.

Immunity Innate Immunity: non specific, Immediate, including physical barriers such as the skin, mucous membranes of GIT, Respiratory and urogenital tracts that prevent penetration of host body. Cell involved are neutrophil, natural killer cells. Adaptive immunity: acquired, specific, gradual, slower in response, involve B and T lymphocytes. Antigen presenting cells. T lymphocytes : it form 65-­‐75 % of circulating lymphocytes Originate in bone marrow. Migrate to Thymus where they mature and proliferate. and acquire surface markers CD4( helper T),CD8( cytotoxic), and T cell receptors. It recognize the antigen when it is presented as apart of MHC ( MHC restricted

B lymphocytes

Thymus Lymphoid organ present in superior mediastinum behind the upper part of sternum. Active at young people (puberty). And involutes in old age but continues to produce lymphocytes. Originate from mesoderm (lymphocyte ) and endoderm (epithelial reticular cells). Main function is central tolerance which prevent autoimmunity

Structure Capsule: vascularized connective tissue send septa to parenchyma divided the organ to incomplete lobules and forming short interlobular septa end at cortico-­‐medullary junction. These septa carry blood vessels branch at cortico-­‐ medulary junction. Parenchyma: composed of lobules consist of of dark stained zone Cortex and lightly-­‐stained medulla. Cortex: composed mostly of T-­‐lymphocytes( Thymocytes). Few macrophages and epithelial reticular cell

Types of TECs Squamous TECs: line the connective tissue capsule and septa form continues layer join by desmosomes and occluding junction. form blood barrier prevent unregulated exposure of thymocytes to antigens. Stellate TECs: deeper to first layer, has processes contain keratin tonofilaments join by desmosomes. These cells are antigen presenting cells express MHCI, MHCII. Other Squamous cortical TECs: express MHCII. From sheetlike structure contributing to cortico-­‐medullary barrier between the two regions of each lobule

Figure 14–9 Cortex of the thymus. (a) The cortical zone of an active thymus is packed with small, highly basophilic -lymphoblasts that proliferate as well as undergo positive and negative selection in that region. The lymphoblasts are -supported on a meshwork (or cytoreticulum) of unusual thymic epithelial cells (E). X400. PT. (b) The epithelial reticular cells throughout the cortex are APCs and extend long processes bound together by desmosomes to make the framework, the cytoreticulum, for the lymphocytes, having a cytoreticulum consisting of APC cellular processes. rather than the more common network of simple reticulin fibers, allows regulated specificity of lymphocyte binding via the changing antigens on MHC proteins. Some cortical epithelial cells also secrete cytokines that promote T-cell maturation.   Figure 14-9

T Cell: precursors with no surface receptor migrate from bone marrow and reside in cortex where they presented to self antigen bound to class I and class II MHC molecules on APCs surface. 95% of cortical T-­‐lymphocytes that react to self antigen are eliminated by process of apoptosis. The rest of T cells mature and attain T cell receptor and migrate to medulla and enter blood stream through the venules and go to secondary lymphoid organs.

medulla contain more epithelial reticular cells and few lymphocytes. It contain characteristic acidophilic staining Hassall’ s corpuscles: they consist of concentrically arranged epithelial reticular cells with keratin Filaments in the cytoplasm. Epithelial reticular cells degenerate and calcify. Function unknown. in young thymus corpuscles are small but in old thymus its large. ACTH and adrenal cortex hormone and male and female hormones accelerate thymus involution in experimental animals.

Arteries enter through the capsule: arterial branches follow the septa into the gland. Arteries leave the septa to enter parenchyma at cortico-­‐ medullary junction, break to capillaries. Capillaries which supply the cortex are impermeable to proteins and prevent the circulating antigens to reach the site of lymphocytes maturation, forming blood-­‐thymus barrier. BLOOD THYMUS BARRIER Thymus cortical capillaries have non-­‐fenestrated endothelial with occluding junction. very thick basal lamina. Reticular epithelial cells.

Post capillary venules in cortico-­‐medullary region with simple cuboidal cells allow lymphocytes in and out the thymus. Medulla supply by capillaries branch of arterioles at medulla-­ ‐cortical border. Medullary capillaries are fenestrated and highly permeable. Medulla drained by venules which joint to form veins Medullary vein s pass through the septa and leave the gland through the capsule

Unique feature it consist of reticular epithelial cells derived from endoderm while in other lymphoid organ is from mesoderm. Thymus has not lymphatic nodules instead it is divided into incomplete lobules . Thymus gland just has efferent lymphatic , no lymph sinuses. Thymus does not filter lymph. T thymoblast are thought to enter the thymic stroma through the large venules at the corticomedullary junction, and re-­‐ enter the circulation through the vascular lining of post-­‐ capilliary venules

Lymphatic nodules Found in all lymphatic organs except thymus. •  Found in lamina propria of digestive tract, upper respiratory and urinary passages. •  Spherical ( diameter 0.2 -1mm) without connective tissue capsule. •  Composed of dense aggregation of small B-lymphocytes. •  Cell at center are larger with more cytoplasm. Light stain central area is known as a Germinal center.

Germinal center is the site of proliferation and aggregation of B lymphocytes. It include B lymphoblast, plasma cells, follicular dendritic cells,and macrophages. Primary lymphatic nodules aggregation of B lymphocytes with help of T helper these B cells became activated and form much larger and more prominent Secondary lymphatic nodules which is characterized by light stain germinal center filled with lymphoblast. Non proliferating cells pushed aside produce dark zone Mantle