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1. 20 The Lymphatic System and Lymphoid Organs and Tissues

2. Lymphatic System • Consists of three parts • A network of lymphatic vessels (lymphatics) • Lymph (fluid inside vessels) • Lymph nodes (cleanse the lymph)

3. Lymphatic System: Functions • Returns interstitial fluid and leaked plasma proteins back to the blood • Carry absorbed fat from intestine to the blood (through lacteals) • Once interstitial fluid enters lymphatics, it is called lymph • Together with lymphoid organs and tissues, provide the structural basis of the immune system

4. Lymphatic Vessels • One-way system, lymph flows toward the heart • Lymph vessels (lymphatics) include: • Lymphatic capillaries • Lymphatic collecting vessels • Lymphatic trunks and ducts

5. Venous system Arterial system Heart Lymphatic system: Lymph duct Lymph trunk Lymph node Lymphatic collecting vessels, with valves Tissue fluid Blood capillaries Lymphatic capillary Tissue cell Blood capillaries Lymphatic capillaries (a) Structural relationship between a capillary bed of the blood vascular system and lymphatic capillaries. Figure 20.1

6. Lymphatic Capillaries • Similar to blood capillaries, except • Very permeable (take up cell debris, pathogens, and cancer cells) • Endothelial cells overlap to form one-way minivalves, and are anchored by collagen filaments, preventing collapse of capillaries

7. Lymphatic Capillaries Filaments anchored to connective tissue Endothelial cell Flaplike minivalve Fibroblast in loose connective tissue (b) Lymphatic capillaries are blind-ended tubes in which adjacent endothelial cells overlap each other, forming flaplike minivalves.

8. Lymphatic Capillaries • Absent from bones, teeth, bone marrow and the CNS • Lacteals: specialized lymph capillaries present in intestinal mucosa • Absorb digested fat and deliver fatty lymph (chyle) to the blood

9. Lymphatic Collecting Vessels • Similar to veins, except • Have thinner walls, with more internal valves • Anastomose more frequently • Superficial collecting vessels • in the skin, travel with superficial veins • Deep collecting vessels travel with arteries • Nutrients are supplied from branching vasa vasorum

10. Lymphatic Ducts • Lymph is delivered into one of two large ducts • Right lymphatic duct drains the right upper arm and the right side of the head and thorax • Thoracic duct arises from the cisterna chyli and drains the rest of the body • Each empties lymph into venous circulation at the junction of the internal jugular and subclavian veins on its own side of the body

11. Cisterna Chyli • Located anterior to L1, L2 • Collects lymph from lower limbs and from the intestinal trunk that drains the digestive organs • In the intestines, specialized lymphatic capillaries called lacteals that transport absorbed fat from the small intestine to the blood stream • Fatty lymph is called chyle

12. Regional lymph nodes: Internal jugular vein Cervical nodes Entrance of right lymphatic duct into vein Entrance of thoracic duct into vein Axillary nodes Thoracic duct Cisterna chyli Aorta Inguinal nodes Lymphatic collecting vessels Drained by the right lymphatic duct Drained by the thoracic duct (a) General distribution of lymphatic collecting vessels and regional lymph nodes. Figure 20.2a

13. Right jugular trunk Internal jugular veins Esophagus Right lymphatic duct Trachea Right subclavian trunk Left subclavian trunk Right subclavian vein Left jugular trunk Right broncho- mediastinal trunk Left subclavian vein Brachiocephalic veins Entrance of thoracic duct into vein Superior vena cava Left bronchomediastinal trunk Azygos vein Ribs Thoracic duct Hemiazygos vein Cisterna chyli Right lumbar trunk Left lumbar trunk Inferior vena cava Intestinal trunk (b) Major lymphatic trunks and ducts in relation to veins and surrounding structures.Anterior view of thoracic and abdominal wall. Figure 20.2b

14. Lymph Transport • Lymph is propelled by • Pulsations of nearby arteries • Contractions of smooth muscle in the walls of the lymphatics • Skeletal muscle contraction • Breathing • Lymph movement is very slow and it depends on movement of adjacent tissues

15. Lymphoid Cells

16. Lymphoid Cells • Lymphocytes arise in red bone marrow and mature into 1 of 2 main varieties • T cells (T lymphocytes) • B cells (B lymphocytes)

17. Lymphocytes • T cells and B cells protect against antigens • Antigens are anything that the body perceives as foreign • These antigens will provoke an immune response • Bacteria and their toxins; viruses • Mismatched RBCs or cancer cells

18. Lymphocytes • T cells • Manage the immune response • Attack and destroy foreign cells • B cells • Produce plasma cells, which secrete antibodies • Antibodies bind to antigens and mark cells for destruction

19. Other Lymphoid Cells • Macrophages phagocytize foreign substances and help activate T cells • Dendritic cells capture antigens and deliver them to lymph nodes • Reticular cells produce stroma that supports other cells in lymphoid organs (structural cells)

20. Macrophage Reticular cells on reticular fibers Lymphocytes Medullary sinus Reticular fiber Figure 20.3

21. Types of Lymphoid Tissue • Diffuse lymphoid tissue • Lymphoid follicles (or nodes)

22. Diffuse Lymphoid Tissue • Loose arrangement of lymphoid cells and some reticular fibers • Found in virtually every body organ • Larger collections appear in mucous membrane areas, like the GI and respiratory tract

23. Lymph Nodes • Principal lymphoid organs of the body • Embedded in connective tissue, in clusters along lymphatic vessels • Near the body surface in inguinal, axillary, and cervical regions of the body • These are clinically important, as they are palpable on PE • Firm, somewhat rubbery non-fixed mass the size of a pea or larger

24. Regional lymph nodes: Internal jugular vein Cervical nodes Entrance of right lymphatic duct into vein Entrance of thoracic duct into vein Axillary nodes Thoracic duct Cisterna chyli Aorta Inguinal nodes Lymphatic collecting vessels Drained by the right lymphatic duct Drained by the thoracic duct (a) General distribution of lymphatic collecting vessels and regional lymph nodes. Figure 20.2a

25. Lymph Nodes • Functions • Filter lymph—macrophages destroy microorganisms and debris • Immune system—lymphocytes are activated and mount an attack against antigens

26. Structure of a Lymph Node • Surrounded by a dense, fibrous capsule, from which connective tissue strands called trabeculae extend inward dividing the node into compartments (follicles) • 2 histologically distinct regions • Cortex • Superficial and deep • Medulla

27. Structure of a Lymph Node • Cortex • Superficial • Made up of follicles with germinal centers where B cells divide and proliferate • Deep • houses T cells • T cells circulate continuously between the blood, lymph nodes, and lymph, performing their surveillance role

28. Structure of a Lymph Node • Medulla • Medullary cords are thin inward extensions from the cortical lymphoid tissue and contain both types of lymphocytes • Lymph sinuses • Large lymph capillaries where macrophages reside and phagocytize foreign matter in the lymph as it flows by in the sinuses

29. Circulation in the Lymph Node • Lymph enters through many afferent vessels on convex side  large subcapsular sinus  smaller sinuses that cut through the cortex  medullary sinuses  exits node at hilum (indented region on concave side) with few efferent vessels

30. Circulation in the Lymph Node • Less efferent vessels, so the flow of lymph stagnates somewhat, allowing time for lymphocytes and macrophages to work • Lymph passes through several nodes before it is cleansed

31. Cortex • Lymphoid follicle Afferent lymphatic vessels • Germinal center • Subcapsular sinus Efferent lymphatic vessels Hilum Medulla: • Medullary cord • Medullary sinus Trabeculae Capsule (a) Longitudinal view of the internal structure of a lymph node and associated lymphatics Figure 20.4a

32. Follicles Trabecula Subcapsular sinus Capsule Medullary cords Medullary sinuses (b) Photomicrograph of part of a lymph node (72x) Figure 20.4b

33. Spleen • Largest lymphoid organ, size of a fist • Located in the left side of the abdominal cavity, just beneath the diaphragm, lateral to the stomach • Blood supply • Splenic artery and vein that enter and exit the hilum (concave anterior surface)

36. Diaphragm Spleen Adrenal gland Left kidney Splenic artery Pancreas (c) Photograph of the spleen in its normal position in the abdominal cavity, anterior view. Figure 20.6c

37. Spleen Function • Functions • Site of lymphocyte proliferation and immune surveillance and response • Cleanses the blood of aged cells and platelets and debris

38. Spleen • Stores breakdown products of RBCs (e.g., iron) for later reuse • Releases other breakdown products to the blood for processing in the liver • Stores blood platelets and monocytes to be released when needed • Contains lymphocytes, macrophages, and huge numbers of erythrocytes

39. Histological Structure of the Spleen • Has a fibrous capsule and trabeculae • Two distinct areas • White pulp • Red pulp

40. Histological Structure of Spleen • White Pulp • Where immune function takes place • Composed mostly of lymphocytes suspended on reticular fibers • White pulp forms clusters around central arteries (small branches of the splenic artery) and form what appear to be islands in a sea of red pulp

41. Histological Structure of the Spleen • Red Pulp • Where worn out RBCs and bloodborne pathogens are destroyed • Contains lots of RBCs and macrophages that engulf the RBCs • Consists of splenic tissue that is not white pulp (the rest of the spleen) • Splenic sinuses (venous sinuses) • Splenic cords (reticular CT that separates splenic sinuses)

42. Thymus

43. Thymus • Size with age • In infants, it is found in the inferior neck and extends into the mediastinum, where it partially overlies the heart • Increases in size and is most active during childhood • Stops growing during adolescence and then gradually atrophies

44. Thymus • T lymphocyte precursors mature into T lymphocytes • Even though the gland atrophies, it continues to produce immunocompetent cells as we age, though at a declining rate

45. MALT

46. MALT • Mucosa-associated lymphoid tissue (MALT) • Lymphoid tissues present in all mucous membranes of the body • Function: filter and remove potential pathogens • Largest groupings are tonsils, Peyer’s patches in small intestine, and appendix • Also present in the mucosa of the respiratory tract (bronchi), genitourinary (GU) organs and the rest of the GI tract

47. Tonsils

48. Tonsils • Simplest lymphoid organs • Form a ring of lymphatic tissue around the pharynx • Palatine tonsils • Located on either side at the posterior end of the oral cavity. Largest of the three types • Lingual tonsils • Located at the base of the tongue. Look lumpy. • Pharyngeal tonsil • Posterior wall of the nasopharynx • Aka adenoids

49. Tonsils • Function • Gather and remove pathogens entering the pharynx in food or inhaled air

50. Pharyngeal tonsil Palatine tonsil Lingual tonsil Tonsil Tonsillar crypt Germinal centers in lymphoid follicles Figure 20.8