Histology of the Lymphatic System 2024 PDF

Nov-24  LYMPHATIC SYSTEM  LYMPHATIC ORGANS  THYMUS  LYMPH NODE  SPLEEN  BURSA OF FABRICIUS  LYMPHATIC TISSUES  TONSIL  PEYER’S PATCHES  MALT, GALT, BALT To be able to describe the histology structure of the above organs / tissues...

Nov-24  LYMPHATIC SYSTEM  LYMPHATIC ORGANS  THYMUS  LYMPH NODE  SPLEEN  BURSA OF FABRICIUS  LYMPHATIC TISSUES  TONSIL  PEYER’S PATCHES  MALT, GALT, BALT To be able to describe the histology structure of the above organs / tissues 1  Nov-24 LYMPHATIC SYSTEM  Comprise of lymphatic organs or the collections of lymphocytes as diffuse, dense, or nodular forms  Dense accumulations of lymphocytes  Widely distributed; typically at sites that provide a possible route of entry of pathogens and/or sites that are liable to infections Primary lymphatic organs  Primary lymphatic organs are where lymphocytes are formed and mature. They provide an environment for stem cells to divide and mature into B- and T- cells  There are two primary lymphatic organs: the red bone marrow and the thymus gland.  Both T-cell and B-cells are formed in the bone marrow.  B cells also mature in the bone marrow; but T-cells have to migrate to the thymus, which is where they mature in the thymus. 2  Nov-24 Secondary lymphatic organs/ tissues  Secondary lymphoid tissues: arranged as a series of filters monitoring the contents of the extracellular fluids, i.e. lymph, tissue fluid and blood.  The lymphoid tissue filtering each of these fluids is arranged in different ways.  Secondary lymphoid tissues are also where lymphocytes are activated.  These include: lymph nodes, tonsils, spleen, Peyer's patches and mucosa associated lymphoid tissue (MALT). LYMPHATIC ORGANS  lymphatic organs or as diffuse, dense, or nodular collections of lymphocytes, and it collectively constitutes the lymphatic system.  A lymphatic organ is a mass of lymphatic tissue that is surrounded by a connective tissue capsule or covered by an epithelium.  Lymphatic organs include (1) lymph nodes, (2) thymus (3) spleen, (4) tonsil (conflict of organ vs tissues).  All lymphatic organs have efferent lymph vessels, but only lymph nodes have afferent lymph vessels. 3  Nov-24 LYMPHATIC / LYMPHOID TISSUES  Epithelium limit the ‘contact’ with the exterior  Lymphoid tissues often located near them  Epithelium-associated lymphoid tissues  Depending on location: mucosa-associated lymphoid tissue (MALT) e.g. tonsil, Payer’s patches bronchus-associated lymphoid tissue (BALT) in the lungs; (GALT) in gut  represent the sites of proliferation and differentiation of lymphocytes LYMPHATIC / LYMPHOID TISSUES  Lymphatic tissue is called:  diffuse lymphatic tissue if only a few lymphocytes are present  dense lymphatic tissue if many lymphocytes are present. Dense lymphatic tissue may appear as cords or nodules A nodule is a spherical mass of lymphocytes, measuring 2-3 mm in diameter.  An active nodule is one that is producing lymphocytes and consists of two zones.  The outer zone  The inner zone 4  Nov-24 OZ LYMPHATIC TISSUES  Outer zone (peripheral zone or corona) IZ IZ  stains dark  crowded with small lymphocytes. OZ  Inner zone (germinal center, central zone) IZ= Inner zone; OZ= Outer zone  appears pale  contains several cell types: large & medium lymphocytes, reticular cells, and macrophages  An inactive nodule is not actively producing lymphocytes  does not contain a germinal center  primarily of small lymphocytes.  a nodule is not permanent, and it may disappear and reappear as well as fluctuate between periods of activity and inactivity. CELLS OF THE IMMUNE SYSTEM GRANULOCYTIC DENDRITIC LYMPHOCYTES PHAGOCYTIC CELLS CELLS CELLS T-lymphocytes Monocytes Neutrophils Macrophages - B lymphocytes Basophils Alveolar: lungs CT: Histiocyte NK cells Liver: Kupffer cells Eosinophils Kidney: Messangial cells Brain: Microglial cells Bone: Osteoclast Skin: Langerhans cells 5  Nov-24 IMMUNE CELLS TYPES - Functions  Lymphocytes (B and T).  Plasma cells - which secrete antibodies, derived from B-cells  Macrophage - which engulfs micro-organisms and presents antigens on its surfaces to lymphocytes.  Dendritic cell - presents antigens to lymphocytes on its surface.  Neutrophil - phagocytic cell  Reticular cell - also a dendritic cell, which presents antigens. https://www.histology.leeds.ac.uk/lymphoid/B_T_lymphocytes.php IMMUNE CELLS TYPES - Functions  Mast Cell - derived from the bone marrow, release histamine, heparin, etc. Have receptors for IgE antibodies on their surface. Involved in allergic reactions  Histiocyte: tissue resident macrophages  Langerhans cells: macrophage (in origin) found in skin tissues; dendritic-cell like phenotype 6  Nov-24 LYMPHOCYTES  Lymphocytes: 20-50% of circulating leucocytes.  Most are small (6-9µ)  About 3% are large (about 12-15µ) and have more cytoplasm - these are mostly activated B-lymphocytes which are on their way to tissues where they will become antibody secreting plasma cells.  The two types of lymphocytes are:  B-cells, which are made and matured in the bone marrow  T-cells, which are made in the bone marrow, but have to migrate and reside in the Thymus before they are matured T-lymphocytes  T-cells make up 65 to 85% of the peripheral blood lymphocytes.  Do not make antibodies, the antigen receptors on their surfaces are different to those on B-cells.  have markers on their surfaces (antigens) called CD antigens (CD for cluster of differentiation). T-cells start to express these markers during their maturation in the thymus.  The T-lymphocytes also populate the peripheral lymphoid tissues (lymph nodes, M.A.L.T. and spleen).  The different types of T-cell and B-cell can be distinguished by immunostaining for the different cell surface markers that these cells express. 7  Nov-24 Three Types of T-cell Cytotoxic Helper Regulatory /Supressor destroy their targets by regulate immune downregulate both humoral releasing perforin, makes the responses by releasing and cell-mediated immune target permeable and kill it cytokines responses. Express CD8 Express CD4 Express CD4 Suppress CD4 and CD8 in CMI B-cells  B-cell is pre-programmed to respond to a particular antigen. This Ag will stimulate the B-cells, only if they are also stimulated by cytokines, produced by T-helper cells, that stimulate the B-cells to divide.  An activated B-cell divides many times and produces many progeny that have the same antigenicity as the original B-cell. (known as a clone of cells). These B-cells start to secrete antibody.  Some of these B-cell develop into plasma cells, that are specialized to produce lots of antibody.  The rest remain as small lymphocytes, and persist in secondary lymphoid organs as long lived memory B-cells, that can respond quickly when challenged with the same Ag. 8  Nov-24 https://www.histology.leeds.ac.uk/lymphoid/B_T_lymphocytes.php 9  Nov-24 THYMUS  thyme leaf shape  bilobed mass of lymphatic tissue  located in close association with the great vessels at the base of the heart (cranial mediastinum @ sternum)  Role: in the development of the immune system although the exact mechanism by which it does this is still being sought  Site of terminal differentiation & selection of T lymphocytes THYMUS https://www.dartmouth.edu/~anatomy/Histo/lab_6/lymphoid/DMS117/popup.html 10  Nov-24 THYMUS the thymus is composed of lobules rather than nodules lacks afferent lymph vessels and lymph sinuses has a framework of cytoreticular cells http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Lymphoid1/lymph1.htm#Thymus THYMUS  Capsule  connective tissue, which surrounds the thymus  Trabeculae  projections of connective tissue, which extend inward from the capsule  divide the thymus into incomplete lobules.  Blood vessels course in the trabeculae  Lobules  These are the units of the thymus and they are roughly rectangular in shape. http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Lymphoid1/lymph1.htm#Thymus 11  Nov-24 THYMUS Cortex  is the peripheral portion of each lobule.  It contains many small lymphocytes (thymocytes)  stains darkly http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Lymphoid1/lymph1.htm#Thymus THYMUS  Medulla  central portion of each lobule  Although the medulla is pale, it is not called a germinal center  The medullary tissue of one lobule is continuous with that of other lobules of the lobe.  Cells of the medulla include cytoreticular cells lymphocytes of various sizes http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Lymphoid1/lymph1.htm#Thymus few macrophages and plasma cells. 12  Nov-24 RETICULAR CELLS  The cytoreticular cells are stellate in shape  their processes form a network in which thymocytes (thymic lymphocytes derived from mesoderm) are embedded  Degenerating reticular cells will become Hassal’s corpuscle. Hassall's corpuscles (Thymic corpuscles)  These are aggregates of degenerating cytoreticular cells, arranged concentrically  located in the medulla and are unique to the thymus  They are acidophilic and their centers may appear hyalinized (glassy)  Most corpuscles are 20-50 m in diameter, but some may be much larger  Unknown function http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Lymphoid1/lymph1.htm#Thymus 13  Nov-24 THYMUS http://education.vetmed.vt.edu/Curriculum/VM8054/Hp/LECLIST.HTM BLOOD-THYMUS BARRIER  in the cortex of the lobule there are thick walls capillaries.  believed to constitute a blood-thymus barrier, which prevents antigenic macromolecules of the blood from reaching the thymus  Layers of the barrier are: the continuous layer of endothelium, which lines the capillary the thick basement membrane of the capillary the layer of cytoreticular cells closely applied to the basement membrane of the capillary Impermeable to protein (Ag) *Significance: developing lymphocytes are prevented from exposure to blood borne antigens; if so the immature T-lymphocytes will undergo apoptosis. https://medizzy.com/feed/16665886 14  Nov-24 INVOLUTION OF THE THYMUS  Maximum developed immediately after birth  Undergoes involution after attaining its greatest size at puberty  Continue produce lymphocytes until old age  Replaced with adipose tissues http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Lymphoid1/lymph1.htm#Thymus 15  Nov-24 LYMPH NODE  Round, oval or bean shaped  located along the course of blood vessels  may enlarge during infection or disease  theyare easily palpated  compulsory to know in various species: the palpable LN http://www.vetmed.wsu.edu/ClientED/anatomy/lymph.aspx LYMPH NODE  FUNCTIONs:  produce lymphocytes and antibodies  filter the lymph (remove dead bacteria and other foreign bodies)  serve as a defense mechanism to prevent the spread of malignant cells  represent potential haemopoetic organs http://www.vetmed.wsu.edu/ClientED/anatomy/lymph.aspx 16  Nov-24 LYMPH NODE Efferent lymph vessels Afferent lymph vessels http://www.vetmed.wsu.edu/ClientED/anatomy/lymph.aspx LYMPH NODE http://www.vetmed.wsu.edu/ClientED/anatomy/lymph.aspx 17  Nov-24 LYMPH NODE  Capsule  The dense irregular CT : collagenous fibers, elastic fibers, fibroblasts & an occasional smooth muscle fiber  Much fat and loose connective tissue lies outside the capsule  Afferent lymph vessels  carry lymph to the lymph node.  thin-walled tubes lined by epithelium & surrounded by a thin layer of CT http://www.vetmed.wsu.edu/ClientED/anatomy/lymph.aspx  They contain valves LYMPH NODE  Subcapsular sinus  lymphatic sinus lying just below the capsule (between the capsule and the lymphatic nodules)  represents the site of termination of the afferent lymph vessels  A loose network of cells and fibers (diffuse lymphatic tissue) extends across the sinus http://www.vetmed.wsu.edu/ClientED/anatomy/lymph.aspx 18  Nov-24 LYMPH NODE  Hilus  slight indentation on one side of the node  It contains blood vessels, nerves, and efferent lymph vessels  Efferent lymph vessels  These begin as open-ended, thin-walled tubes within the lymph node  They carry lymph from the lymph node and are fewer in number (but larger in diameter) than the afferent lymph vessels  They pierce the hilus as they leave the node. Valves are present in these vessels http://www.vetmed.wsu.edu/ClientED/anatomy/lymph.aspx LYMPH NODE  Trabeculae  projections of CT, which extend from the capsule into the substance of the lymph node  straight in the cortex and branched in the medulla  separate the cortex of the node into incomplete compartments  Blood vessels present in the trabeculae  Peritrabecular sinuses  also called trabecular sinuses  can be identified as areas of diffuse lymphatic tissue, which lie next to the trabeculae. http://www.vetmed.wsu.edu/ClientED/anatomy/lymph.aspx 19  Nov-24 LYMPH NODE  Cortex  outer zone of the lymph node  having lymphatic nodules  Nodules may be active (possess a corona and a germinal center) or inactive.  Medulla  inner zone  Lymphatic nodules are absent. There is no sharp boundary between cortex and medulla.  Medullary cords  strands of dense lymphatic tissue, which extend from the lymphatic nodules of the cortex into the medulla. http://education.vetmed.vt.edu/Curriculum/VM8054/Hp/LECLIST.HTM LYMPHATIC NODULE http://education.vetmed.vt.edu/Curriculum/VM8054/Hp/LECLIST.HTM Lymph nodule within the cortex of the lymph node http://education.vetmed.vt.edu/Curriculum/VM8054/Hp/LECLIST.HTM 20  Nov-24 LYMPH NODE Medullary sinuses: areas of loose lymphatic tissue in the medulla, which lie between the medullary cords and the trabeculae. http://education.vetmed.vt.edu/Curriculum/VM8054/Hp/LECLIST.HTM 21  Nov-24 SPLEEN  Largest lymphoid organ  Functions:  Proliferation of T and B cells  Formation of antibodies of blood borne antigen  Filtration of blood Breaking down the old RBCs & recycle the iron  Ability to add erythrocyte & leukocyte population  Reservoir for RBC SPLEEN  Covered by capsule (dense irregular CT) + smooth muscle cells  No afferent lymph vessels  Parenchyma =reticular fibres but covered by the lymphocytes  Collagenous cords (trabeculae)  extend radially from capsule  Separate the parenchyma into compartments  Smooth muscle cells  Splenic arteries follow the route  Hilus  Splenic artey http://education.vetmed.vt.edu/curriculum/vm8054/labs/Lab13/Lab13.htm  Splenic vein 22  Nov-24 TRABECULAE  The trabeculae contain smooth muscle cells and connective tissue  The trabecular artery has its own muscular wall (pink) surrounded by connective tissue (stained blue)  The trabecular vein has a thin wall made up of endothelium only  The fibromuscular trabecular tissue gives support to the vessel. http://ocw.tufts.edu/Content/4/imagegallery/221002/221020 RED PULP & WHITE PULP  Grossly, the spleen is traditionally said to have the bulk of its parenchyma as red pulp, with isolated areas of white pulp interspersed through it.  red pulp = formed elements of the blood (mostly erythrocytes)  white pulp consists almost entirely of lymphocytes  little confusing when applied to microscopic sections  Microscopically:  "white pulp" = lymphocyte population of the spleen, in the form of the periarteriolar lymphocyte sheath or PALS  "Red pulp" is everything else, which means the splenic cords and the sinuses between them. http://education.vetmed.vt.edu/Curriculum/VM8054/Hp/LECLIST.HTM 23  Nov-24 RED PULP & WHITE PULP http://ocw.tufts.edu/Content/4/imagegallery/221002/221020 SPLENIC CIRCULATION Blood enter the spleen (via Splenic artery) Branching (as it is reaching parenchyma) Arterioles will be sheathed by the lymphocytes (PALS) Subdivide (in bundle) = Penicillar arteries (penicillium = hairs of paintbrush) Close theory Open theory Directly enter the Sinus of red pulp arteriolar side simply ends erythrocytes enter the sinuses from the cords 24  Nov-24 PALS of the Spleen http://education.vetmed.vt.edu/curriculum/vm8054/labs/Lab13/Lab13.htm http://education.vetmed.vt.edu/curriculum/vm8054/labs/Lab13/Lab13.htm 25  Nov-24 http://education.vetmed.vt.edu/curriculum/vm8054/labs/Lab13/Lab13.htm Germinal centre http://education.vetmed.vt.edu/curriculum/vm8054/labs/Lab13/Lab13.htm Central arteries located in the middle of the germinal centre 26  Nov-24 SPLENIC CORDS & SINUS  SINUS  Blood spaces (‘special’ endothelium lining)  filled with RBCs  Wall is fusiform (spindle-shaped); RBCs can move into or from the cords  When joined together Vein in red pulp larger vein (in septa hilus)  CORDS  Masses of cells (RBCs, macrophages) between the sinus  Phagocytosis of aged RBCs happened here  If stain with Iron: presence of hemosiderin  However, difficult to distinguish them SPLENIC CORDS & SINUS http://education.vetmed.vt.edu/curriculum/vm8054/labs/Lab13/Lab13.htm 27 Nov-24 28  Nov-24 Bursa of Fabricius  blind, sac-like structure on the caudodorsal side of the cloaca  The term "bursa" ="purse" and implies the blind "pocket book" gross structure of this organ.  Its appearance is at first glance similar to the thymus  Similar function but for B cells  continues to grow through development in chickens & reaches its maximum size by 6 weeks.  After this it slowly regresses (involutes) until only a small remnant is present in the adult. Bursa of Fabricius  Mucosal folds (plicae) project into the lumen of the bursa  Numerous lymphoid nodules (follicles) are found M in the lamina propria of each fold. M M  Each nodule has a darker cortex with densely C packed small lymphocytes M  and a lighter staining, less dense medulla M containing cells of various sizes.  The lymphoid cells of the bursa are the B-cell C lineage. http://ocw.tufts.edu/Content/4/CourseHome/221002/221015 29  Nov-24 Pseudostratified columnar Bursa of Lymphocytes Fabricius http://education.vetmed.vt.edu/Curriculum/VM8054/Hp/LECLIST.HTM Bursa of Fabricius  The surface of the bursa fold is covered by a pseudostratified columnar epithelium.  This micrograph shows three lymphoid nodules with darker cortex and lighter medulla (Azure-Eosin staining) C M M C http://ocw.tufts.edu/Content/4/CourseHome/221002/221015 30  Nov-24 Bursa of Fabricius  The nodule has a darker staining cortex with densely packed small lymphocytes  The medulla: cells of various sizes including lymphocytes, macrophages, & reticular epithelial cells.  It may not be possible to identify all the cells types http://ocw.tufts.edu/Content/4/CourseHome/221002/221015 31  Nov-24 MALT (Mucosa-associated lymphatic tissues)  Diffuse lymphatic tissues e.g. all lymphatic tissue structures in the GIT or Respiratory tract  Scattered at the mucosa or submucosa layer  none capsulated  Germinal centre  Usually occur separately & isolated  However, at specific areas, can be in clusters e.g.:  GALT alongGIT = Peyer’s patches Caudal oral cavity = Tonsil  Resp. tract = BALT TONSIL  the association of lymphatic tissue and an overlying stratified squamous epithelium  anatomically well defined by the presence  germinal centers are normally present  The mucus type glands below the lymphatic tissue are small salivary glands whose ducts http://education.vetmed.vt.edu/curriculum/vm8054/labs/Lab13/Lab13.htm open onto the surface of the tonsil. 32  Nov-24 TONSIL  Tonsils are usually incompletely encapsulated (organ?)  by CT on the side away from the oral cavity, making them easy to remove if they become infected.  The cell types are the same as those seen in other lymphoid organs, including lymphocytes and macrophages and plasma cells.  They act as a front-line defense forming the initial immunological response to inhaled or ingested pathogens. TONSIL  3 types (by location):  Palatine (bilaterally, oropharynx junction)  Lingual (along dorsal surface of caudal tongue)  Pharyngeal (along the roof of nasopharynx)  All lined by stratified squamous except in pharyngeal = pseudostratified squamous http://education.vetmed.vt.edu/curriculum/vm8054/labs/Lab13/Lab13.htm 33  Nov-24 PEYER’S PATCHES  Prominent in GIT of most species  Sites of maturation & development of B-lymphocytes  Lined by simple squamous-like cells (M cells)  M = microfold cells  Germinal centres http://education.vetmed.vt.edu/Curriculum/VM8054/Hp/LECLIST.HTM 34  Nov-24 General view of the mucosa immunity in the intestine. Luminal antigens are captured by dome-shaped M cells present in the covering of Peyer's patches and transported to the subjacent region. Antigens are recognized by B lymphocytes and displayed to helper T lymphocytes by antigen-presenting cells (macrophages and dendritic cells). B lymphocytes are stimulated by helper cells to differentiate into IgA-secreting plasma cells. Many lgA molecules are transported to the intestinal lumen coupled with the secretory piece. Source: Junqueira et al; Basic Histology Text & Atlas; pp 265 35 Nov-24 36