1. LYMPHOID TISSUE This resource is licensed under the Creative Commons Attribution Non-Commercial & No Derivative Works LicenseCreative Commons Attribution Non-Commercial & No Derivative Works License

2. Objectives 1.Appreciate that the lymphoreticular system is divided into primary and secondary lymphoid organs. 2.Recognise that the structure of the BONE MARROW and THYMUS provides an ideal environment for B cell and T cell differentiation. 3.Describe how the structure of the LYMPH NODE is well adapted for filtering antigens from the tissue fluid and initiating immune responses. 4.Describe the structure of the SPLEEN and appreciate that this is related to its role in initiating immune responses to antigens in the blood. 5.List the important components of the MUCOSAL ASSOCIATED LYMPHOID TISSUES (MALT) and describe how their structure relates to their function in protecting mucosal surfaces.

3. Bone Marrow See Blood and Haemopoiesis (year 1 of course 004)

4. SLIDE 147 Thymus (cat) Examine the thymus at very low magnification. Identify; a. Capsule. b. Thymic lobules with cortical and medullary zones. 1.0 mm

5. SLIDE 147 Thymus (cat) Examine the thymus at very low magnification. Identify; a. Capsule. b. Thymic lobules with cortical and medullary zones. 1.0 mm c : capsule cortex m : medulla septum m m m c thymic lobule

6. SLIDE 147 Thymus (cat) Examine the thymus at low magnification. Identify; a. Capsule. b. Thymic lobules with cortical and medullary zones. 0.5 mm

7. SLIDE 147 Thymus (cat) Examine the thymus at low magnification. Identify; a. Capsule. b. Thymic lobules with cortical and medullary zones. 0.5 mm capsule of connective tissue continues as septa between the lobules medulla cortex

8. SLIDE 147 Thymus (cat) What are the major differences in the histological structure of the thymus in the cortical and medullary zones? 100 µm

9. SLIDE 147 Thymus (cat) What are the major differences in the histological structure of the thymus in the cortical and medullary zones? 100 µm What are the major differences in the histological structure of the thymus in the cortical and medullary zones? Cortex: tightly packed with small lymphocytes, few stromal cells visible. cortex : packed with dark staining lymphocytes

10. SLIDE 147 Thymus (cat) What are the major differences in the histological structure of the thymus in the cortical and medullary zones? 100 µm What are the major differences in the histological structure of the thymus in the cortical and medullary zones? Cortex: tightly packed with small lymphocytes, few stromal cells visible. Medulla: fewer lymphocytes, more non-lymphoid cells visible (e.g. Epithelioreticular cells, macrophages) and more obvious blood vessels. medulla : fewer lymphocytes cortex : packed with dark staining lymphocytes

11. SLIDE 147 Thymus (cat) In which part of the thymic lobule would you expect to see mature lymphocytes and can immature and mature lymphocytes be distinguished morphologically? 50 µm

12. SLIDE 147 Thymus (cat) In which part of the thymic lobule would you expect to see mature lymphocytes and can immature and mature lymphocytes be distinguished morphologically? 50 µm In which part of the thymic lobule would you expect to see mature lymphocytes and can immature and mature lymphocytes be distinguished morphologically? Immature in cortex. Mature in medulla. medulla of thymus lymphocytes (dark staining) BV : blood vessel H : Hassall’s corpuscle pale staining non-lymphoid cells (epithelioreticular cells, macrophages) BV H

13. SLIDE 147 Thymus (cat) In which part of the thymic lobule would you expect to see mature lymphocytes and can immature and mature lymphocytes be distinguished morphologically? 50 µm In which part of the thymic lobule would you expect to see mature lymphocytes and can immature and mature lymphocytes be distinguished morphologically? Immature in cortex. Mature in medulla. It is difficult to distinguish between the two types of cell on morphology but you could look for expression of cell surface markers. e.g. CD4 & CD8 : double negative→double positive→single positive. medulla of thymus lymphocytes (dark staining) BV : blood vessel H : Hassall’s corpuscle pale staining non-lymphoid cells (epithelioreticular cells, macrophages) BV H

14. SLIDE 147 Thymus (cat) What is the function of epithelioreticular cells, how can they be identified in this section? cortexmedulla 25 µm

15. SLIDE 147 Thymus (cat) What is the function of epithelioreticular cells, how can they be identified in this section? cortexmedulla 25 µm What is the function of epithelioreticular cells, how can they be identified in this section? They provide the structural framework of the thymus and secrete growth factors as well as essential cell-cell interactions with lymphocytes. These cells are bigger, lighter staining and more heterogeneous in shape compared to the lymphocytes. E : epithelioreticular cells Hassall’s corpuscle lymphocytes E E E E E

16. SLIDE 147 Thymus (cat) What is the distinguishing feature of Hassall’s corpuscles and are they found anywhere other than in thymic medulla? 25 µm

17. SLIDE 147 Thymus (cat) 25 µm What is the distinguishing feature of Hassall’s corpuscles and are they found anywhere other than in thymic medulla? They look like a section through an onion (degenerate epithelial cells forming squames). These are only found in the thymus. concentric whorls of degenerating epithelioreticular cells forming Hassall’s corpuscles (thymic corpuscles).

18. SLIDE 143 Lymph node (cat) Identify : a. cortex (outer layer). d. germinal centre. b. paracortex. e. fibrous capsule. c. medulla (deeper layer). 1.0 mm

19. SLIDE 143 Lymph node (cat) Identify : a. cortex (outer layer). d. germinal centre. b. paracortex. e. fibrous capsule. c. medulla (deeper layer). 1.0 mm fibrous capsule C : cortex PC : paracortex M : medulla G : germinal centre H : hilum PC C M H G

20. SLIDE 143 Lymph node (cat) 1.0 mm The sinuses characterise the structure of the lymph node. Try to visualise how the lymph carrying the antigen enters the subcapsular sinus and flows through the lymph node via cortical and medullary sinuses.

21. SLIDE 143 Lymph node (cat) 1.0 mm The sinuses characterise the structure of the lymph node. Try to visualise how the lymph carrying the antigen enters the subcapsular sinus and flows through the lymph node via cortical and medullary sinuses. lymph reaches subcapsular sinus from afferent lymphatics.

22. SLIDE 143 Lymph node (cat) 1.0 mm The sinuses characterise the structure of the lymph node. Try to visualise how the lymph carrying the antigen enters the subcapsular sinus and flows through the lymph node via cortical and medullary sinuses. lymph reaches subcapsular sinus from afferent lymphatics. flows through cortical sinuses in trabeculae.

23. SLIDE 143 Lymph node (cat) 1.0 mm The sinuses characterise the structure of the lymph node. Try to visualise how the lymph carrying the antigen enters the subcapsular sinus and flows through the lymph node via cortical and medullary sinuses. lymph reaches subcapsular sinus from afferent lymphatics. flows through cortical sinuses in trabeculae. flows through medullary lymph sinuses.

24. SLIDE 143 Lymph node (cat) 1.0 mm The sinuses characterise the structure of the lymph node. Try to visualise how the lymph carrying the antigen enters the subcapsular sinus and flows through the lymph node via cortical and medullary sinuses. lymph reaches subcapsular sinus from afferent lymphatics. flows through cortical sinuses in trabeculae. flows through medullary lymph sinuses. exits from hilus of lymph node via efferent lymphatics.

25. SLIDE 143 Lymph node (cat) 1.0 mm The sinuses characterise the structure of the lymph node. Try to visualise how the lymph carrying the antigen enters the subcapsular sinus and flows through the lymph node via cortical and medullary sinuses. lymph reaches subcapsular sinus from afferent lymphatics. flows through cortical sinuses in trabeculae. flows through medullary lymph sinuses. exits from hilus of lymph node via efferent lymphatics.

26. SLIDE 143 Lymph node (cat) Identify : a. cortex (outer layer). e. medullary sinuses. b. paracortex. f. fibrous capsule. c. medulla (deeper layer). g. connective tissue trabeculae. d. medullary cords. 0.5 mm

27. SLIDE 143 Lymph node (cat) Identify : a. cortex (outer layer). e. medullary sinuses. b. paracortex. f. fibrous capsule. c. medulla (deeper layer). g. connective tissue trabeculae. d. medullary cords. 0.5 mm cortex paracortex medulla Mc : medullary cords Ms : medullary sinuses capsule trabeculae Mc Ms

28. SLIDE 143 Lymph node (cat) Identify : a. cortex. d. sub-capsular sinus. b. fibrous capsule. c. connective tissue trabeculae. 100 µm

29. SLIDE 143 Lymph node (cat) Identify : a. cortex. d. sub-capsular sinus. b. fibrous capsule. c. connective tissue trabeculae. 100 µm capsule trabecula cortex sub-capsular sinus

30. SLIDE 143 Lymph node (cat) Part of the trabecula of a lymph node. The sub-capsular sinus continues as cortical sinuses in the trabeculae. Smooth muscle fibres can be seen in the trabeculae. 50 µm

31. SLIDE 143 Lymph node (cat) Part of the trabecula of a lymph node. The sub-capsular sinus continues as cortical sinuses in the trabeculae. Smooth muscle fibres can be seen in the trabeculae. 50 µm cortical sinus trabeculae with smooth muscle fibres cortex

32. SLIDE 143 Lymph node (cat) Under higher magnification distinguish different cell types in the germinal centre and peripheral corona of follicles and diffuse paracortex. 100 µm

33. SLIDE 143 Lymph node (cat) Under higher magnification distinguish different cell types in the germinal centre and peripheral corona of follicles and diffuse paracortex. 100 µm germinal centre peripheral corona capsule trabecula paracortex

34. SLIDE 143 Lymph node (cat) Under higher magnification distinguish different cell types in the germinal centre and peripheral corona of follicles and diffuse paracortex. 50 µm germinal centre peripheral coronaparacortex

35. SLIDE 143 Lymph node (cat) What are the major cell types found in the paracortex and how does this differ from the cells present in the follicles or medulla? 50 µm

36. SLIDE 143 Lymph node (cat) 50 µm What are the major cell types found in the paracortex and how does this differ from the cells present in the follicles or medulla? T cells, interdigitating DCs and macrophages in paracortex. B cells and follicular DCs in follicles. Macrophages and plasma cells in medulla.

37. SLIDE 143 Lymph node (cat) Examine the medulla with medullary cords and sinuses. 250 µm

38. SLIDE 143 Lymph node (cat) Examine the medulla with medullary cords and sinuses. 250 µm capsule Medulla : MC : medullary cords blood vessels MS : medullary sinuses cortex paracortex MC MS

39. SLIDE 143 Lymph node (cat) The medulla with medullary cords and sinuses lined with an endothelium containing lymphocytes, plasma cells and macrophages. 100 µm

40. SLIDE 143 Lymph node (cat) The medulla with medullary cords and sinuses lined with an endothelium containing lymphocytes, plasma cells and macrophages. 100 µm medullary cords medullary sinuses

41. SLIDE 125 Lymph node (dog) View the larger of the two sections. This has been stained to demonstrate reticular fibres. 2.0 mm

42. SLIDE 125 Lymph node (dog) Identify the main regions of this lymph node stained for reticular fibres. 1.0 mm

43. SLIDE 125 Lymph node (dog) Identify the main regions of this lymph node stained for reticular fibres. 1.0 mm capsule cortex paracortex medulla trabeculae

44. SLIDE 125 Lymph node (dog) At high magnification the network of reticular fibres can be seen in this area of medulla. What is this protein fibre composed of? 25 µm

45. SLIDE 125 Lymph node (dog) At high magnification the network of reticular fibres can be seen in this area of medulla. What is this protein fibre composed of? Collagen type III. 25 µm medullary sinus medullary cords R : reticular fibres R R

46. SLIDE 145 Spleen (rat) Whole section seen under low magnification. Identify : a). capsule. d). central artery. b). white pulp. c). red pulp. 1.0 mm

47. SLIDE 145 Spleen (rat) Whole section seen under low magnification. Identify : a). capsule. d). central artery. b). white pulp. c). red pulp. 1.0 mm W : white pulp R : red pulp W W R R R W capsule central artery

48. SLIDE 145 Spleen (rat) 1.0 mm W : white pulp R : red pulp W W R R R W capsule central artery What features distinguish the spleen from lymph nodes and other lymphoid tissues?

49. SLIDE 145 Spleen (rat) 1.0 mm W : white pulp R : red pulp W W R R R W capsule central artery What features distinguish the spleen from lymph nodes and other lymphoid tissues? Central arteries in the white pulp. No cortex and medulla.

50. Spleen – Dog ▪ Cadaver in right lateral recumbency. Abdominal wall has been removed. Ribcage & diaphragm intact. Left ribcage is retracted laterally with forceps. ▪ Identify : tendinous & muscular portions of diaphragm left, lateral & left medial lobes of liver right medial lobe of liver fundus & body of stomach greater curvature of stomach & gastrosplenic ligament spleen

51. Spleen & Thymus – Sheep ▪ Diaphragm partially removed (cut edge apparent). Pars caudalis of cranial lobe of left lung reflected dorsally. Pericardium partially removed to expose heart. ▪ Identify : abomasum reticulum diaphragm spleen left ventricle right ventricle left interventricular paraconal sulcus apex of heart thymus

52. SLIDE 145 Spleen (rat) Note the random scattering of lymphatic tissue, white pulp, centred around the branches of the central artery in the blood filled parenchyma called red pulp. Note the red pulp is composed of elongated structures called splenic cords that lie between the sinusoids. 250 µm

53. SLIDE 145 Spleen (rat) Note the random scattering of lymphatic tissue, white pulp, centred around the branches of the central artery in the blood filled parenchyma called red pulp. Note the red pulp is composed of elongated structures called splenic cords that lie between the sinusoids. 250 µm red pulp : white pulp : marginal zone PALS (periarterial lymphatic sheath) splenic cords central artery blood filled sinusoids

54. SLIDE 145 Spleen (rat) The lymphocytes around a central arteriole are called a periarterial lymphatic sheath or PALS. The lymphatic follicles are also present at the edges of the PALS. 50 µm

55. SLIDE 145 Spleen (rat) The lymphocytes around a central arteriole are called a periarterial lymphatic sheath or PALS. The lymphatic follicles are also present at the edges of the PALS. 50 µm C : central artery PALS periarterial lymphatic sheath C

56. SLIDE 145 Spleen (rat) How does the antigen arrive in the spleen and how does this differ from a lymph node? 25 µm

57. How does the antigen arrive in the spleen and how does this differ from a lymph node? Antigen arrives via blood to the spleen, compared to afferent lymphatic vessels for the lymph node. Splenic artery → trabecular arteries → central arteries (surrounded by white pulp). SLIDE 145 Spleen (rat) 25 µm central artery marginal zone surrounding lymphocytes of the periarteriolar lymphatic sheath (PALS).

58. SLIDE 145 Spleen (rat) Identify : connective tissue support represented by a). capsule. b). trabeculae. Reticular connective tissue is extensive in the spleen; it supports the PALS and lymphatic follicles of the white pulp and supports the pulp spaces in the blood filled red pulp. 50 µm

59. SLIDE 145 Spleen (rat) Identify : connective tissue support represented by a). capsule. b). trabeculae. Reticular connective tissue is extensive in the spleen; it supports the PALS and lymphatic follicles of the white pulp and supports the pulp spaces in the blood filled red pulp. 50 µm capsule covered by mesothelium trabecula with smooth muscle blood vessel

60. SLIDE 145 Spleen (rat) Identify : connective tissue support represented by a). capsule. b). trabeculae. Reticular connective tissue is extensive in the spleen; it supports the PALS and lymphatic follicles of the white pulp and supports the pulp spaces in the blood filled red pulp. 50 µm

61. SLIDE 145 Spleen (rat) The capsule is covered by a layer of mesothelium and contains bundles of smooth muscle fibres. In this higher magnification of red pulp close to the capsule, the tissue can be seen to be packed with erythrocytes in sinuses and blood vessels. 25 µm

62. SLIDE 145 Spleen (rat) The capsule is covered by a layer of mesothelium and contains bundles of smooth muscle fibres. In this higher magnification of red pulp close to the capsule, the tissue can be seen to be packed with erythrocytes in sinuses and blood vessels. 25 µm mesothelium of capsule capsule blood vessel erythrocytes

63. SLIDE 145 Spleen (rat) Red pulp : consists mainly of blood filled pulp spaces within a reticular network giving the tissue its red appearance. 50 µm

64. SLIDE 145 Spleen (rat) Red pulp : consists mainly of blood filled pulp spaces within a reticular network giving the tissue its red appearance. 50 µm smooth muscle cells in trabeculae lymphocytes red blood cells in pulp spaces or sinuses

65. SLIDE 144 Spleen (mouse) carbon injected Carbon particles have been phagocytosed by macrophages in this section of spleen from a mouse. The macrophages are most numerous in the red pulp. 250 µm

66. SLIDE 144 Spleen (mouse) carbon injected Carbon particles have been phagocytosed by macrophages in this section of spleen from a mouse. The macrophages are most numerous in the red pulp. 250 µm capsule W : white pulp W red pulp W

67. SLIDE 144 Spleen (mouse) carbon injected What role do the macrophages play in lymphoid tissue? 50 µm

68. SLIDE 144 Spleen (mouse) carbon injected 50 µm What role do the macrophages play in lymphoid tissue? Macrophages phagocytose particulate matter (e.g. bacteria). They are antigen presenting cells and also secrete cytokines. macrophages arrowed (not all shown) red pulp white pulp

69. SLIDE 144 Spleen (mouse) carbon injected 50 µm What additional role do the macrophages have in the spleen? macrophages arrowed (not all shown) red pulp white pulp

70. SLIDE 144 Spleen (mouse) carbon injected 50 µm What additional role do the macrophages have in the spleen? In the spleen they phagocytose old/damaged red blood cells. macrophages arrowed (not all shown) red pulp white pulp

71. SLIDE 144 Spleen (mouse) carbon injected What does the presence of megakaryocytes in this section of spleen indicate? 25 µm

72. What does the presence of megakaryocytes in this section of spleen indicate? Megakaryocytes are platelet precursors. In the foetus and neonate the spleen is a site for haematopoiesis. This would indicate a young animal. SLIDE 144 Spleen (mouse) carbon injected 25 µm megakaryocyte macrophages with ingested carbon particles

73. SLIDE 45 Duodenum (cat) The lymphatic tissue here shows an unorganised accumulation of scattered lymphocytes in the mucosal layer of the intestinal wall. 0.5 mm

74. SLIDE 45 Duodenum (cat) The lymphatic tissue here shows an unorganised accumulation of scattered lymphocytes in the mucosal layer of the intestinal wall. 0.5 mm lumen of duodenum intestinal villi submucosa tunica muscularis mucosa

75. SLIDE 45 Duodenum (cat) The lymphocytes here appear as dark staining small cells against the paler staining columnar epithelial cells lining the villi. 50 µm

76. SLIDE 45 Duodenum (cat) The lymphocytes here appear as dark staining small cells against the paler staining columnar epithelial cells lining the villi. 50 µm columnar epithelium lymphocytes lamina propria of villus

77. SLIDE 45 Duodenum (cat) Note that the lymphocytes are smaller and darker then the elongated nuclei of columnar epithelial cells. 25 µm

78. SLIDE 45 Duodenum (cat) Note that the lymphocytes are smaller and darker then the elongated nuclei of columnar epithelial cells. 25 µm lymphocytes goblet cell brush border of microvilli nuclei of epithelium (simple columnar)

79. SLIDE 142 Peyer’s patch At very low magnification identify Peyer’s patches represented by collections of lymphatic nodules present in ONLY a part of the wall of the intestine. If you view this slide in the laboratory you can see these without the microscope. The remainder of the intestinal wall lacks these swellings. 1.0 mm

80. SLIDE 142 Peyer’s patch At very low magnification identify Peyer’s patches represented by collections of lymphatic nodules present in ONLY a part of the wall of the intestine. If you view this slide in the laboratory you can see these without the microscope. The remainder of the intestinal wall lacks these swellings. 1.0 mm P P P P P : Peyer’s patches

81. SLIDE 142 Peyer’s patch View this area at higher magnification to confirm the presence of lymphatic nodules. Identify the structures of the Peyer’s patch. 250 µm

82. SLIDE 142 Peyer’s patch View this area at higher magnification to confirm the presence of lymphatic nodules. Identify the structures of the Peyer’s patch. 250 µm lymphatic nodule (follicle) germinal centre (pale staining) diffuse lymphatic tissue tunica muscularis epithelium

83. SLIDE 142 Peyer’s patch Where would you find the B and the T lymphocytes in Peyer’s patches? 50 µm Part of lymphatic nodule with paler staining germinal centre and corona mostly of dark staining B lymphocytes.

84. SLIDE 142 Peyer’s patch 50 µm Part of lymphatic nodule with paler staining germinal centre and corona mostly of dark staining B lymphocytes. Where would you find the B and the T lymphocytes in Peyer’s patches? B cells are in the follicles. T cells are scattered throughout the rest of the lymphoid tissue. germinal centre B lymphocytes

85. SLIDE 142 Peyer’s patch High magnification of part of lymphatic nodule of Peyer’s patch. Many small dark staining B lymphocytes can be seen. 25 µm

86. SLIDE 141 Pharyngeal tonsil (cat) The tonsil is a partly encapsulated lymphoid tissue characterised by the presence of crypts. Identify the main features of its structure. 1.0 mm

87. SLIDE 141 Pharyngeal tonsil (cat) The tonsil is a partly encapsulated lymphoid tissue characterised by the presence of crypts. Identify the main features of its structure. 1.0 mm oral cavity crypt N : nodular lymphatic tissue D : diffuse lymphatic tissue D N N D epithelium capsule

88. SLIDE 141 Pharyngeal tonsil (cat) What does the presence of a large number of follicles with active germinal centres tell you? 0.5 mm

89. SLIDE 141 Pharyngeal tonsil (cat) 0.5 mm What does the presence of a large number of follicles with active germinal centres tell you? This is commonly a site of immune reactivity stimulated by foreign organisms in the oropharynx. oral epithelium F : follicles with pale staining germinal centres F F F diffuse lymphatic tissue

90. SLIDE 141 Pharyngeal tonsil (cat) Characteristic crypt of tonsil. 250 µm

91. SLIDE 141 Pharyngeal tonsil (cat) Characteristic crypt of tonsil. 250 µm crypt follicle diffuse lymphatic tissueepithelium lining crypt

92. SLIDE 141 Pharyngeal tonsil (cat) What type of epithelium lines the pharyngeal tonsil? 100 µm

93. SLIDE 141 Pharyngeal tonsil (cat) 100 µm What type of epithelium lines the pharyngeal tonsil? Stratified squamous epithelium. epithelium (stratified squamous) diffuse lymphatic tissue crypt

94. SLIDE 141 Pharyngeal tonsil (cat) Compare the nature of the epithelium in adjacent areas. 100 µm stratified squamous epithelium layers clear stratified squamous epithelium layers unclear

95. SLIDE 141 Pharyngeal tonsil (cat) Why is the nature of the epithelium unclear in some places? 50 µm

96. Why is the nature of the epithelium unclear in some places? Due to infiltration of the epithelium by lymphocytes. SLIDE 141 Pharyngeal tonsil (cat) 50 µm lymphocytes stratified squamous epithelium ‘normal’ appearance

97. Lectures. Dr B Catchpole. Second Year Histology. 016. LYMPHOID TISSUE. J Bredl. 03-09-04. Gross Anatomy Correlates. Dr S Frean. Histology Slides and Stains. Tanya Hopcroft. Compressed version updated. 2006/7.