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Lymphatic tissue.

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Presentation on theme: "Lymphatic tissue."— Presentation transcript:

1 Lymphatic tissue

2 Components of lymphoid system
Lymph and lymph nodes Lymph – fluid scavenged from the intersitium Collected in thin walled lymphatic vessels Returned to general circulation Contains leukocytes Filtered through small specialized organs (lymph nodes)

3 Lymph Capillaries lose 2-4 L of fluid and plasma protein
Lymphatic system absorbs excess fluid and returns it to the bloodstream Makes sure circulatory system has enough blood to work properly Decreases edema

4 Lymph– Lymphatic vessels Clear colorless fluid—like plasma
Less proteins Has bacteria, virus, cell debris Lipid content high Lymphocytes content high Many in capillary bed Lymphatic vessels Take fluid from tissue back to the bloodstream—veins Usually along side of vein Similar to vein (low pressure, use SK muscle and respiratory pump to help flow, valves also)

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7 Lymphoid Cells Both types originate from stem cells in bone marrow
The two main varieties are T cells and B cells Both types originate from stem cells in bone marrow T cells – cell mediated immunity B cells – humoral/ antibody mediated immunity Small and large lymphocytes in peripheral blood Large are the activated B lymphocytes Antigen – anything the body perceives as foreign Bacteria and their toxins; viruses Mismatched RBCs or cancer cells

8 Lymphocytes T cells Manage the immune response
Attack and destroy foreign cells B cells Produce plasma cells, which secrete antibodies Plasma cells are derived from activated B lymphocytes that have left the blood stream and taken up residence in connective tissue. SPOKE wheel appearance Antibodies immobilize antigens

9 Other Lymphoid Cells Macrophages – phagocytize foreign substances and help activate T cells Dendritic cells – spiny-looking cells with functions similar to macrophages Reticular cells – fibroblast like cells that produce a stroma, or network, that supports other cell types in lymphoid organs

10 Lymphoid Tissue Lymphatic follicles (nodules)
Solid, spherical bodies consisting of tightly packed reticular elements and cells Have a germinal center composed of dendritic and B cells Found in isolation and as part of larger lymphoid organs

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13 Introduction Lymphoid system consists of cells, tissues & organs that contain aggregates of lymphocytes. The lymphoid organs are: Primary lymphoid organs – bone marrow & thymus Secondary lymphoid organs – lymph nodes, spleen, tonsils, Peyer’s patches, MALT (mucosa associated lymphoid tissue)

14 Lymphoid Organs Central Origin and main differentiation Thymus
Bone marrow Spleen Lymphoid nodes Solitary nodules Tonsils Appendix Peyer’s patches Peripheral Proliferation and additional differentiation Immune response

15 Lymphoid Organs Encapsulated Thymus Lymph nodes Spleen Unencapsulated
Diffuse Tonsils Diffuse lymphatic tissue is found in many regions of the body, especially beneath moist mucosal surface e.g trachea and intestine. There is absent of capsule and the lymphatic tissue merges with the surrounding connective tissue.

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17 Thymus A bilobed organ that secrets hormones (thymosin and thymopoietin) that cause T lymphocytes to become immunocompetent The size of the thymus varies with age In infants, it is found in the inferior neck and extends into the mediastinum where it partially overlies the heart It increases in size and is most active during childhood It stops growing during adolescence and then gradually atrophies

18 Thymus The thymus differs from other lymphoid organs in important ways It functions strictly in T lymphocyte maturation It does not directly fight antigens Function: production of immunocompetent T lymphocytes production of mature but naïve T cells for peripheral tissues and circulation immunological self-tolerance regulation of T cell maturation, proliferation and function via secretion of hormones

19 Internal Anatomy of the Thymus
Thymic lobes contain an outer cortex and inner medulla The cortex contains densely packed lymphocytes and scattered macrophages The medulla contains fewer lymphocytes and thymic (Hassall’s) corpuscles

20 Thymus

21 Thymic Cortex Consists of proliferating lymphocytes dispersed among macrophages & epithelial reticular cells. These epithelial cells/ reticular cells form the support structure for the developing T cells but also play an important role in isolating the T cells from foreign anitgens during their development They are of endodermal origin & do not secrete reticular fibers.

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24 Epithelial Reticular Cells
Endodermally derived. Stellate eosinophilic cells with an oval nucleus. Joined to adjacent reticular cells by desmosomes. Enclose the developing lymphocytes within them. Help in the formation of a blood-thymus barrier - prevents antigenic stimulation of the lymphocytes & allows for proper differentiation. Secrete various hormones (thymulin, thymopoietin, thymosin & thymic humoral factor) that regulate T cell differentiation.

25 Blood Thymus barrier Prevents premature exposure of lymphocytes to antigen (to inhibit tolerance). The integrity of the  space within the epithelial cell framework is extremely important because it prevents the premature stimulation of T cells by antigens Non-fenestrated endothelium with occluding junctions (major component of the barrier) Endothelium Thick basal lamina Pericytes Macrophages Type I epithelioreticular cells

26 Blood Thymus barrier

27 Thymic Cortex Contains maturing lymphocytes, macrophages & reticular cells. Many lymphocytes die by apoptosis & are removed by macrophages. Outer cortex is populated by immature lymphocytes & as they mature they migrate to the deeper part of the cortex.

28 Maturation of T cell in thymus
CORTEX Epithelio reticular cell (RE) MHC I MHC II Immature T cell CD4 CD8 First T cell has to acquire markers CD4 and CD8 Cell without marker is destroyed RE cell teaches the immature T cell to recognize the MHC markers Some identify MHC I and some MHC II T cell who don’t learn to identify either are destroyed Immature T cell CD4 CD8 MHC I MEDULLA T cells that recognize MHC I are destroyed or they lose CD4 from their surface CD4 +ve T cell Immature T cell CD4 CD8 MHC II CD8 +ve T cell

29 Thymic Medulla Hassall’s corpuscles concentrically arranged, flattened epithelial reticular cells that become filled with keratin filaments, degenerate & may become calcified. They appear in the fetus & their number gradually increases after birth.

30 QUESTION The production of new T lymphocytes in the thymus occurs in which of the following regions? Superficial cortex Corticomedullary junction Thymic nodules Deep medulla Thymic corpuscles

31 Lymph Node A filter: Removes: Initiation of immune response
purifies lymph before return to venous circulation Removes: debris pathogens 99% of antigens Initiation of immune response Activation & proliferation of lymphocytes

32 Function of lymph nodes
filter debris and microorganisms via phagocytosis by fixed macrophages facilitate the interaction between antigen presenting cells and circulating lymphocytes to initiate an immune response B lymphocytes: activation and proliferation; plasma cell formation and antibody production in response to antigens T lymphocytes: activation to become T helper and T cytotoxic cells

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34 Lymphnode structure Afferent vessels – convey lymph towards the node
Efferent vessels – drain lymph away from the node at the hilum Capsule Trabeculae Reticular tissue

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36 Lymph node Capsule Subcapsular sinus

37 Structure of a Lymph Node (Cortex)
Superficial cortex: The cortex contains follicles with germinal centers, heavy with dividing B cells Dendritic cells nearly encapsulate the follicles Deep cortex houses T cells in transit T cells circulate continuously among the blood, lymph nodes, and lymphatic stream

38 Lymph node

39 Lymphoid follicle Germinal Center (GC) -  contains pale-staining cells. The open, pale-staining nature of the nuclei of these cells indicate that they are B lymphocytes undergoing active proliferation. Other cells include: Follicular dendritic cells that present antigen to the B cells Macrophages that engulfed dead B cells that have died by apotosis

40 Lymphoid follicle cortex
Germinal center - Functional aspects:  resting B cells enter the lymph node parenchyma though the high endothelial venules and if they encounter an antigen with which they can react, they then enter the cycle of blast transformation to produce clones of plasma cells and B memory cells. This production of clones occurs in the germinal centers of lymphoid follicles. Mantle zone (corona) – The germinal center is surrounded by a ring of darker-staining cells.  The condensed nature of their nuclei indicates that these  are resting B cells. Also present in the mantle zone are T helper cells, macrophages and dendritic cells.

41 Paracortical zone / deep cortex
Paracortical zone - deeper regions of the cortex contain primarily T lymphocytes that do not form into follicles.  T lymphocytes (helper and cytotoxic/suppresor) arrive through the circulation, enter the lymph node parenchyma through the high endothelial venules and take up residence in the paracortical zone.  If activated, the T lymphocytes undergo active proliferation to produce expanded clones of activated T lymphocytes.   Functional aspects: T lymphocytes that arrive at the lymph node via the arterial blood stream gain access to the parenchyma of the lymph node through the wall of the high endothelial venules located in the paracortical zone.  These blood vessels contain endothelial cells that are expressing specific lymphocyte binding molecules called addressins.  These surface molecules are available to bind to lymphocytes that recognize them, the lymphoctyes bind to the surface of the endothelium, then cross the vessel wall and enter the lymph node parenchyma.

42 Homing of lymphocytes A process by which lymphocytes return to lymph node by crossing walls of blood vessels, High endothelial venules- present in lymph nodes.

43 Lymph node architecture
HEV – specialized Post-capillary venules. Cuboidal endothelium Concentrates lymph Receptors for antigen primed lymphocytes *HEV- High endothelial venules

44 Structure of a Lymph Node (Medulla)
Medullary cords extend from the cortex and contain B cells, T cells, and plasma cells The medullary cords are composed of  plasma cells producing  antibodies, their precursors, macrophages and T helper cells. The most prominent cell in the cord is the precursor to plasma cells or immunoblasts that came from the germinal centers of the lymphoid follicles in the cortex of the node. In the medullary cords, the plasma cells undergo final maturation and secrete antibodies into the lymph that is collected by efferent lymphatic vessels in the node and eventually carried to the general circulation.  Plasma cells may also get into the general circulation in this manner

45 Structure of a Lymph Node (Medulla)
The medullary sinuses are composed primarily of reticular fibers (RF) providing the support framework, reticular cells (fibroblast-like cells that secrete the reticulin) (RC) and  macrophages. 

46 Lymph node – medullary cords

47 Medullary cords – reticulum stain

48 Lymphatic Disorders: Elephantiasis: parasite, roundworm
Lives in lymph nodes Prevents fluid flow back to circulatory system Chronic edema (fluid swelling) Leads to thickening of skin Autoimmune diseases Failure of self tolerance Immune system fails to recognize self antigens from foreign ones and attacks body’s own tissues e.g Lupus Young women more impacted usually Autoimmune disease against DNA

49 Elephantiasis

50 Spleen Part of the circulatory system with secondary immune system function Functions Reserve blood storage Removal of aged cells Immune-surveillance (hematogenous exposure to antigens) Filters particulates from the blood (secondary to hepatic filter)

51 Structure of the Spleen
Fibrous capsule Trabeculae that extend inward and contains lymphocytes, macrophages, and huge numbers of erythrocytes Two distinct areas of the spleen are: White pulp : area containing mostly lymphocytes suspended on reticular fibers and involved in immune functions Red pulp : remaining splenic tissue concerned with disposing of worn-out RBCs and bloodborne pathogens

52 Spleen architecture

53 Spleen – Capsule Collagenous connective tissue
Smooth muscle (most prominent in ruminants and horses) Allows contraction of the spleen Large network of septa/trabeculae Septa are framework through which vessels reach interior of spleen Delicate reticular stroma

54 Splenic circulation

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56 White Pulp – lymphiod Tissue
Periarteriolar lymphoid tissue (PALS) Central artery coated by sheath of lymphocytes Forms nodular lymphoid tissue Has germinal centers Supported by a fine specialized stroma

57 White pulp The white pulp of the spleen is characterized by a parenchyma that consists of two types of lymphocytes, i.e., B cells and T cells located in two different areas of the spleen.   B cells are located in the lymphoid follicles scattered throughout the organ. In younger animals, a germinal center can be seen as seen in lymph nodes. In fact, this type of white pulp functions much in the manner that lymphoid follicles of lymph nodes function, i.e., initiation of immune responses by B cells to foreign antigens in the blood. T cells are located around the central arteries and form a kind of sheath.  This site is called the periarteriolar lymphoid sheath.

58 In the micrograph above, white pulp of the spleen is seen in both locations: (1) a lymphoid follicle, outlined by the dashed line and (2) the PALS located around the central artery in which T cells are found.  The lymphoid follicle has a pale-staining germinal center (GC) in which B cells are proliferating.  Note the presence of a mantle zone (ManZ) that contains small lymphocytes and an outer marginal zone that contains larger lymphocytes that are less densely packed than cells in the mantle zone.   Outside the marginal zone is the red pulp.

59 In the micrograph above, white pulp of the spleen is seen in both locations: (1) a lymphoid follicle, outlined by the dashed line and (2) the PALS located around the central artery in which T cells are found.  The lymphoid follicle has a pale-staining germinal center (GC) in which B cells are proliferating.  Note the presence of a mantle zone (ManZ) that contains small lymphocytes and an outer marginal zone that contains larger lymphocytes that are less densely packed than cells in the mantle zone.   Outside the marginal zone is the red pulp.

60 Splenic follicle Same structure as cortical follicle in lymph node.
Germinal center Mantle zone Marginal zone Form adjacent to PALS.

61 Splenic Cords and Sinuses
Make up the red pulp Venous sinuses Lined by phagocytic endothelial cells Contain circulating blood Cords are all the tissue inbetween Contain RBCs, macrophages, lymphocytes, occasional neutrophils Site where filtering function occurs High number of nuclei compared to the sinuses

62 Red pulp The red pulp of the spleen is characterized by a parenchyma (PN) that consists of macrophages of the sheathed capillaries as well as other macrophages and blood cells.  Occupied by numerous venous sinuses (VS).  Sinuses are very open and can be easily traversed by blood cells.  Their lining consists of long endothelial cells (arrows) oriented along the longitudinal axis of the vessel.  Large spaces occur between adjacent endothelial cells and the underlying basement membrane is discontinuous, thus blood cells can easily pass between the endothelial cells and gain access to the bloodstream on the venous side.  Few fibroblasts responsible for producing the reticulin fibers; special stains are required to visualize the reticular network.

63 Splenic Red Pulp

64 Splenic red pulp E=endothelial cell VS= venous sinuses

65 Splenic Macrophages Part of the MPS (macrophage phagocytic system)
Primary function is removal of “senescent” cells Some are also antigen presenting cells (dendritic cells)

66 Lymphoid Tissue (Unencapsulated)
Diffuse lymphatic tissue Scattered reticular tissue in every body organ Larger collections in the lamina propria of mucous membranes and lymphoid organs Mucosal associated lymphoid tissue (MALT) Gut-associated (GALT) Bronchial-associated (BALT)

67 Tonsils

68 Tonsils Simplest lymphoid organs; form a ring of lymphatic tissue around the pharynx Location of the tonsils Palatine tonsils – either side of the posterior end of the oral cavity Lingual tonsils – lie at the base of the tongue Pharyngeal tonsil – posterior wall of the nasopharynx Tubal tonsils – surround the openings of the auditory tubes into the pharynx

69 Tonsils Lymphoid tissue of tonsils contains follicles with germinal centers Tonsil masses are not fully encapsulated Epithelial tissue overlying tonsil masses invaginates, forming blind-ended crypts Crypts trap and destroy

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71 Aggregates of Lymphoid Follicles
Peyer’s patches – isolated clusters of lymphoid tissue, Found in the wall of the distal portion of the small intestine (Ileum) Similar structures are found in the appendix Appendix: Destroy bacteria, preventing them from breaching the intestinal wall Generate “memory” lymphocytes for long-term immunity

72 Peyer's patches - Ileum

73 MALT MALT – mucosa-associated lymphatic tissue is composed of:
Peyer’s patches, tonsils, and the appendix (digestive tract) Lymphoid nodules in the walls of the bronchi (respiratory tract) MALT protects the digestive and respiratory systems from foreign matter

74 Types of MALT and Functional aspects:
larger aggregates function much like lymph nodes smaller, scattered MALT are mostly T lymphocytes but also have B cells and plasma cells      - mostly IgA in the intestines and respiratory tract to   protect against pathogens that may gain access to underlying tissues      - IgG and IgM secreted into lamina propria to counteract pathogens that have gained access to connective tissue      - IgE secreted into lamina propria; mediates the release of histamine from mast cells single lymphocytes found within the lamina epithelial are mostly T lymphocytes MALT is drained by efferent lymphatic's but there are no afferent lymphatic's lymphocytes exposed in MALT regions go through regional lymph nodes then return to the MALT region after activation 

75 Organs of the Lymphatic System
Thymus Houses developing lymphocytes Secretes hormones that regulate lymphocyte activity Shrinks with age Lymph nodes Concentrated near ear, mandible, neck, armpit, breasts, trachea, aorta, iliac crest, groin Filter lymph, cleanse foreign matter Remove impurities from fluid before return to blood Spleen Many RBC, lymphocytes, macrophages Produces blood cells Blood reservoir Destroys old and bad RBC Filter like lymph nodes If removed, functions taken over by liver and bone marrow Tonsils Deal with ingested, foreign, inhaled pathogens Many lymphocytes in tonsils

76 Q 2 Macrophages are directly involved in immune responses in which of
the following ways? a. Production of IL-2 b. Presentation of antigen c. Specific killing of tumor cells d. Production of antibodies e. Inactivation of helper T cells

77 Q 3 The mechanism for lymphocyte circulation from the lymphoid compartment in the region marked with the asterisk to the blood involves which of the following?

78 A. Homing receptors on lymphocytes that recognize vascular addressins on high endothelial venule cells B. Lymphocyte binding to endothelial integrins followed by passage through endothelial cells lining the high endothelial postcapillary venules C. Lymphocyte passage through the zonulae occludentes by diapedesis after dissolution of the junctions by proteolytic enzyme release D. Lymphocyte passage from the efferent lymphatic vessel to the thoracic duct and subsequently the venous system E. Passage of lymphocytes through the discontinuous sinusoidal wall into the blood

79 Q 4 This organ, shown at low magnification (A) and high magnification (B) is which of the following?

80 a. A site of antibody production
b. The site of filtration of the lymph and blood c. Derived embryologically from the 3rd branchial arch d. The site of production of CD4+ and CD8+ cells e. A major site of red blood cell degradation and bilirubin recycling


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