Lymphatic system- Lymph nodes and Spleen

About the histology of lymphatic system. Detailed discussion about Lymph nodes and Spleen.

1. A discussion about the Microanatomy

2. Lymphoid Organs

3. Reticuloendothelial System Mononuclear Phagocytic system

4. Lymphoid Cells and other immune cells B cells and T lymphocytes These two subclasses of lymphocytes are distinguished on the basis of where they differentiate and mature into immunocompetent cells, and on the types of surface receptors present on their cell membranes. Macrophages natural killer cells Antigen presenting cells

5. Endothelial Cell The endothelium is a monolayer of flattened polygonal cells which extends continuously over the luminal surface of the entire vascular tree. T hin but extend over a relatively large surface area. They usually adhere firmly to each other at their edges, so that the lining of the lumen presents no discontinuity, other than in sinusoids . Transcytotic (pinocytotic) vesicles Wiebel – Palade body (p-selectin and vWF ) Junctional complex Apart from vimentin intermediary filaments , the endothelium also contains a contractile filament system ( actin-myosin system )

6. Endothelial Cells in Lymphatic vessels A basal lamina is incomplete or absent lack associated pericytes endothelial cells have numerous transcytotic vesicles within their cytoplasm Unlike capillaries , their endothelium is generally quite permeable to much larger molecules Permeability is facilitated by gaps between the endothelial cells , which lack tight junctions, and by pinocytosis.

7. L, a lymph capillary; Arrow head, an arterial capillary, Arrows, endothelial cells of arterioles

8. Lymphatic capillaries They begin as dilated, blind-ended tubes larger diameters basal lamina is incomplete or absent lack associated pericytes Permeability is facilitated by gaps between the endothelial cells , which lack tight junctions, and by pinocytosis.

9. Lymphatic vessels T ake up residual fluid (about one tenth ) by passive diffusion and the transient negative pressures in their lumina which are generated intrinsically by contractile activity of smooth muscle in the largest lymphatic vessel walls, and extrinsically, by movements of other tissues around it. The unidirectional flow of lymph is maintained by the presence of valves in the larger vessels.

10. LYMPH NODES encapsulated centres of antigen presentation and lymphocyte activation. They generate mature, antigen-primed, B and T cells, and filter particles, including microbes, from the lymph by the action of numerous phagocytic macrophages small, oval or kidney-shaped bodies, 0.1–2.5 cm long lying along the course of the lymphatic vessels

11. Lymph Node: Panoramic View

12. Capsule of the Lymph Node Lymph nodes are surrounded by a fibrous connective tissue capsule that enters the organ as trabeculae that define a cortex and medulla. The capsule and trabeculae are the source of reticulin fibers that are found throughout the node and form the main supporting network of the organ. Beneath the capsule is a subcapsular sinus into which lymph flows from the afferent lymphatic vessels.

13. Trabeculae Connective tissue divisions from the capsule, the trabeculae, extend into the interior node The larger trabeculae springing from the capsule break up into finer bands, and these interlace to form a mesh-work in the central or medullary portion of the node . Trabeculae form the basic skeleton of the organ . The web contains considerable numbers of lymphocytes and macrophages ( lymphoreticular organ ).

14. Reticulin Framework The reticular fibers are the rather delicate, black-staining strands supporting the parenchyma and contributing to the perivascular connective tissue . reticular fibers (Type III collagen) Such reticular tissue is found primarily in the blood forming organs and in glands where it forms a supportive framework around the gland cells and blood vessels. Silver impregnations are commonly used to demonstrate the fine reticular ( argyrophilic i.e. "silver-loving") fibers . They are not visible in H&E preparations.

15. Lymph Node: Capsule, Cortex and Medulla

16. Lymphatic channels inside the node Lymph nodes are permeated by channels through which lymph percolates after its entry from the afferent vessels .

17. Afferent lymphatic vessels dense intracapsular plexus , subcapsular sinus radial cortical sinuses medullary sinuses . efferent vessel

18. C ellular zones of lymph nodes In the cortex, cells are densely packed and in the outer cortical area they form lymphoid follicles or nodules, which are populated mainly by B cells and specialized follicular dendritic cells (FDC ) A primary follicle is uniformly populated by small, quiescent lymphocytes S econdary follicle has a germinal centre

19. Zones of Germinal Centre In the ‘dark zone’ the B cells ( centroblasts ) undergo rapid proliferation. Light zone: B cell occur as Centrocytes The mantle zone is produced as surrounding cells are marginalized by the rapidly growing germinal centre.

20. Medulla The medulla consists of anastomosing cords of lymphatic tissue, the medullary cords interspersed with medullary sinuses that drain the lymph from the node into the efferent lymphatic vessels that are located at the hilus .

21. Hilum In the hilum are found numerous arteries and veins . The lymph leaves the lymph node via the efferent lymphatic vessels with valves at the hilum .

22. Spleen

23. Spleen Microscopically, the parenchymal tissue of the spleen consists of two major components, white pulp and red pulp The white pulp is composed of lymphoid tissue in which B and T lymphocytes mature and proliferate under antigenic stimulation The red pulp is a unique filtration device that enables the spleen to clear particulate material from the blood as it perfuses the spleen. It is composed of a complex system of interconnected spaces populated by large numbers of phagocytic macrophages that remove effete red blood cells, microorganisms , cellular debris and other particulate matter from the circulation .

24. Capsule The capsule has an outer and an inner lamina in which the directions of the collagen fibres differ Numerous trabeculae extend from the capsule into the substance of the spleen, where they branch to form a supportive framework. The largest trabeculae enter at the hilum and provide a conduit for the splenic vessels and nerves, dividing into branches in the splenic pulp. Within the spleen, these branches become continuous with a delicate network of type III (reticular ) collagen fibres that pervades both red and white pulps and is secreted by fibroblasts within its meshes.

25. White Pulp In the spleen parenchyma, branches of the splenic artery radiate out from the hilum within trabeculae, ramifying and narrowing to arterioles . In their terminal few millimetres, their connective tissue adventitia is replaced by a sheath of T-lymphocytes, the periarteriolar lymphatic sheath (PALS ). Follicles are usually situated near the terminal branches of arterioles and typically protrude to one side of a vessel, which consequently appears eccentrically placed within the follicle. Arterioles branch laterally within follicles to form a series of parallel terminal arterioles , called penicilli .

26. Red Pulp The red pulp constitutes the majority (75%) of the total splenic volume . It contains large numbers of venous sinusoids that ultimately drain into tributaries of the major splenic veins . The sinusoids are separated from each other by a fibrocellular network consisting of small bundles of delicate collagen type III fibres , the reticulum, and numerous reticular fibroblasts and splenic macrophages . Seen in two-dimensional sections, these intersinusoidal regions appear as strips of tissue, the splenic cords

27. Venous Sinusoids

28. Reticular tissue of the splenic cords

29. Marginal Zone

30. Splenic Microcirculation The segmental splenic arteries enter the hilum and ramify in the trabeculae throughout the organ. The splenic vein forms in the splenorenal ligament from tributaries emerging from the hilum; they are similar in number to the arterial branches. Small arteries tapering to arterioles pass through the white pulp then turn abruptly to form penicillar branches which, after a course of approximately 0.5 mm, pass out of the white pulp into the marginal zone and red pulp. intermediate circulation of the spleen.

31. Open and Closed Circulation Views on the intermediate circulation of the spleen differ on whether blood passes from the arterioles (or their terminal capillaries) directly into the venous sinuses ( a closed circulation ), or is instead discharged into a network of spaces in the splenic cords before entering the sinuses through the minute slits in their walls ( an open circulation ). In humans, evidence favours the presence of an anatomically and physiologically open circulation, in which blood percolates slowly through the reticular tissue of the splenic cords and filters through slits in the sinus walls before joining the majority of the blood flow There is thought to be an additional closed vascular route, but this is likely to provide only a minor contribution to splenic circulation.