Histology of Lymphoid Tissue PDF

Histology of Lymphoid Tissue Amir Mhawi, DVM, PhD [email protected] Learning Objectives • Recognize the primary and secondary lymphoid organs, and know which are capsulated. • Identify the general structure of each of the lymphoid organs (spleen, lymph node, thymus, MALT including tonsil and appen...

Histology of Lymphoid Tissue Amir Mhawi, DVM, PhD [email protected] Learning Objectives • Recognize the primary and secondary lymphoid organs, and know which are capsulated. • Identify the general structure of each of the lymphoid organs (spleen, lymph node, thymus, MALT including tonsil and appendix), point out the distinguishing features of each, and describe (in general) what happens in each location. • Describe the locations and functions of the following cell types: epithelial reticular cell, reticular cell, dendritic cell, follicular dendritic cell, thymocyte, M cell, stave cell. • Explain the functions, general structure, and locations of the following structures: HEVs (high endothelial venules), PALs (periarteriolar lymphatic sheaths), primary and secondary lymphatic nodules, germinal centre, Hassall’s corpuscles, MALT (mucosa associated lymphoid tissue), Peyer’s patches, red pulp, white pulp, blood thymus barrier. • Describe the paths taken by lymph as it flows through the lymph nodes and blood as it flows through the spleen. • Explain the clinical significance of DiGeorge Syndrome, thymic involution, swollen lymph nodes, and splenectomy. • Recognize labelled structures shown in the histological sections in the lecture. The Lymphatic System Includes the following tissues: • Primary (central) lymphatic tissue: • Bone marrow (site for B lymphocytes maturation) • Thymus (site for T lymphocytes maturation) • Sites where lymphocytes mature (“educated”) and become able to recognize and respond to antigens − i.e., become immunocompetent but NAIVE • Secondary (peripheral) lymphatic tissue: • Sites where mature (“educated”) lymphocytes become stimulated to respond to antigens − − − − diffuse lymphatic tissue lymphatic nodules (follicles) lymph nodes spleen Thymus • Site where stem lymphocytes (common lymphoid progenitor, CLP) differentiate into immunocompetent T lymphocytes (also called thymocytes) • Contains epithelial cells (of oropharyngeal origin). • Called epithelioreticular (erc) cells (AKA Thymus Epithelial Cells {TEC} in immunology textbooks). • 6 types (I-VI): 3 in cortex and 3 in medulla • Form a cytoplasmic reticulum within parenchyma of the thymus. • Serve as framework for T cells development. • Correspond to reticular cells and their fibers in other lymph organs. • RETICULAR FIBERS NOT PRESENT IN THYMUS. I II III IV V VI Details of this image FYI, except for the type and location of epithelioretucular cells Thymus cont’d. young • Structural features include: I • Capsule • Thin irregular dense connective tissue • Extends trabeculae into the parenchyma of the organ. • Capsule and trabeculae contain blood vessels, nerves, efferent lymphatic vessels. (BUT NO AFFERENT; WHY?) no lymph bla lymph will bring born − Trabeculae establish domains antigens disturb education – Called thymic lobules. process – Students may mistake thymic lobule for lymphatic nodule with germinal center (covered later) unless other features are examined. + • Cortex • Outer basophilic portion T ↓ • Numerous densely packed lymphocytes • Medulla • Inner portion • Fewer lymphocytes cort Thymus capsule /medulla trabeculae Cortex Trabeculae establish domains called thymic lobules. Trabeculae Thymus: Cortex • Extremely basophilic in H&E • Packed T lymphocytes • Numerous macrophages I III • Phagocytosis of T cell that do not fulfill thymic education requirements • Contains type I, II, and III epithelioreticular cells (erc) II IV V VI Cortex contains: • Type I erc: • Cells are squamous • Separate the cortical parenchyma from the CT of the capsule, trabeculae, and blood vessels. • Join each other by occluding junction. • Barrier between parenchyma and CT • Type II erc: • Cells are large • Antigen-presenting cells • Their cytoplasmic processes form meshwork to support developing T lymphocytes. • Have large euchromatic nucleus. • Connect to each other by desmosomes. Type II erc • Type III erc: • • • Squamous Located between the cortex and the medulla. Connect to each other by occluding junctions. • Barrier between cortex and medulla Type V erc A Type VI erc BB Type II erc Type II ERCs: - Located in the cortex. - Large - Euchromatic nuclei - Cytoplasmic processes form network and join to other ERCs processes by Desmosomes. Thymus: Medulla • Medulla stains lighter. • Due to fewer and larger lymphocytes • Contains type IV, V, and VI epithelioreticular cells. • Type IV cells: located between cortex and medulla. • Possess occluding junction. • Type V cells: • Similar features of type II epithelioreticular cells • Antigen-presenting cells • Type VI cells: form Hassall’s corpuscles. • Dendritic cells Thymus: Hassall’s Corpuscles • Hassall’s (AKA thymic) corpuscles • • • • Isolated masses of type VI epithelioreticular cells Packed together. Concentrically arranged. Center may appear keratinized (due to oropharyngeal epithelial origin). • Produce thymosin and thymopoietin. • Required for the T lymphocytes maturation. Thymic medulla Arrows point to type V epithelioreticul ar Cells. Hassall’s corpuscle Thymic Involution • After birth, thymus gradually involutes and replaced by fat (yet still producing some new lymphocytes), • B cells are short lived and continually replenished from the bone marrow (60 billion B cells produced every day). Young thymus • The organ can be restimulated under conditions that demand rapid T lymphocyte proliferation. Involuted thymus Blood-Thymus Barrier • Thymus gland has NO afferent lymphatics. • Protect against lymph-borne antigens • Blood vessels of cortex are impermeable to blood-borne antigens. • Due to the presence of series of layers around the blood capillaries (refer to next slides). • Create “privileged” areas for T cell development. Blood-Thymus Barrier cont’d. • Components of barrier – starting from vessel lumen outward: • Capillary endothelium • Major structural component of barrier • Extensive tight junctions • Continuous (non-fenestrated) • Thick endothelial basal lamina • Thin perivascular connective tissue sheath and residing macrophages • Basal lamina of epithelioreticular cell • Epithelioreticular cell sheath (made of Type I epithelioreticular cells) Thymic Education Positive and Negative Selection of T Cells • The immature lymphocytes emerge from the bone marrow and enter the thymus via the cortex where they begin their “education.” In the cortex: positive selection • They are exposed to thymic antigens presented by type II epithelioreticular cells (erc) on MHC (Major Histocompatibility Complex) molecules. • Responsive cells (those who recognize the antigens) are considered as immunocompetent and are positively selected and may proceed to the medulla. • Non-responsive cells are deleted by apoptosis and are removed by phagocytosis. In the medulla: negative selection • Immunocompetent lymphocytes are presented with selfantigen in the medulla by type V erc and those that are selfreactive are negatively-selected and are deleted by apoptosis. Type II erc Type V erc/DC This process of selection ensures that surviving lymphocytes are responsive to appropriate stimuli and are not self-reactive. Only about 5% of entering lymphocytes emerge as mature T-cells and enter the peripheral circulation. Clinical Correlation: DiGeorge Syndrome • • • • • Inherited immunodeficiency Caused by deletion of genes from chromosome 22. Epithelioreticular (thymic epithelial) cells fail to develop. T cell precursors cannot differentiate. Thymus and parathyroid glands are rudimentary or absent. • Patients may have: • Hypoparathyroidism • Cleft palate and lip Secondary Lymphatic Tissues/Organs • The secondary lymphoid tissues/organs are the sites where naïve, immunocompetent cells interact with antigens and each other and become activated. • Two types • Uncapsulated Lymphtic nodules Tonsil • Diffuse lymphatic tissue (MALT) • Found in the lamina propria LNAfferent lymphatics • Lymphatic nodules/follicles (LN) • Found in tonsils (upper image), GIT, respiratory tract, genitourinary tract • Capsulated • Lymph nodes (lower image) • Spleen Lymphtic nodules LN Lymph node Diffuse Lymphatic Tissue (MALT) • Uncapsulated • Accumulation of cells of the immune system (lymphocytes, macrophages, plasma cells, dendritic cells) • Located in lamina propria of • Digestive tract (GALT) • Respiratory passages (BALT) • Genitourinary tract • Cells arrive from the blood. • Guards the body against pathogens. • Leave via lymphatics if not activated (by antigen). • Stay in the lamina propria if activated. Diffuse lymphatic tissue in the lamina propria of the large intestine Lymphatic Nodules (Follicles) • Uncapsulated • Discrete concentrations of lymphocytes supported by a meshwork of follicular dendritic cells (FDC). • Sharply defined. • Darkly stained mass of nuclei of lymphocytes • Two types: • Primary lymphatic nodule (no germinal center) • Secondary lymphatic nodule • Secondary lymphatic nodule develops a lightly stained germinal center. • Rich in B lymphocytes. • Surrounded by intensely stained mantle zone (corona). • Mantle zone is rich in T lymphocytes. • Germinal center is a sign of immune response to antigen. Diffuse lyjphatic tissue Primary lymphatic nodule No germinal center GC Secondary lymphatic nodules Germinal center GC Lymphatic Nodules (Follicles) cont. • Germinal center develops when B lymphocytes interact with T lymphocytes. • B lymphocytes proliferate. • Proliferation gives rise to large lymphoblasts (centroblasts). • Lymphoblasts are precursor of antigen-specific plasma cells. • Lighter GC is due to the presence of large centroblasts. microvilli in cross/oblique sections germinal center Lymphatic nodule in the lamina propria of the samll intestine Lymphatic Nodules/Follicle cont. • Lymphatic nodules usually found in structures associated with alimentary canal such as: • Tonsils − Pharyngeal − Palatine − Lingual Waldeyer’s ring • Ileum (Peyer’s patches) • Vermiform appendix • Microanatomy and function (of trapping antigens to activate lymphocytes) are generally similar. Tonsils pseudostratified columnar epithelium • Pharyngeal (AKA adenoids) • Roof of pharynx • Covered with pseudostratified columnar ciliated epithelium (respiratory). • Display crypts (folds of surface epithelium). • Pseudostratified columnar ciliated may change to stratified squamous. • Process is called metaplasia. • Occurs when disease induces sever repeated coughing. Lymphatic nodule with germinal center Peyer’s Patches • Aggregation of lymphatic nodules under the gut epithelium • Antigens enter Peyer’s patches via microfold (M) cells • Patches consists of: • T lymphocytes area • Dark blue • B lymphocyte area • Light germinal center Lymphatic Nodules Are Covered with M Cells • • • • M cells are modified epithelial cell Squamous Cover group of immune cells Transport sample by pinocytosis from the lumen of the intestine to the immune cells in the underlying pockets • M cells are NOT antigen presenting cells. • They are antigen transporting cells. B-cell Dendritic cell Tcell Dendritic cell B FDC Vermiform Appendix • The appendix is a blind pouch connected to the cecum of the large intestine that was once thought to be a vestigial organ. • Contains a large numbers of lymphatic nodules. • The exact function of the appendix remains unclear. Lymphatic nodules Lymph Nodes • Encapsulated organs • Located along lymphatic vessels. • Located everywhere especially in • Axilla • Groin • Mesenteries • Serve as filters for lymph. • Lymph enters lymph node via afferent lymphatic vessels and leaves via efferent lymphatic vessels. LN Afferent lymphatics LN LN LN LN General Architecture of the Lymph Node • Cortex • Outer cortex • Dense mass of lymphatic tissue • Organized onto nodules. Outer cortex • Inner Cortex (paracortex) Contains: • High endothelial venules (HEV) • Dendritic cells • T lymphocytes • Medulla • Consists of medullary cords (MC) • Contain B cells, plasma cells, and macrophages. • Medullary sinuses (MS) • Spaces between the medullary cords Inner cortex Medulla Lymph Nodes: High Endothelial Venules • Lymphocytes circulate through nodes. • • • • • 10% enter the lymph node via afferent lymphatic. 90% through the wall of the post-capillary venules. Located in the deep cortex. Called high endothelial venules (HEVs). Lined with cuboidal endothelial cells rather than the usual squamous cells that typically line blood vessels. • Possess receptors for lymphocytes. • Signal T and B lymphocytes to leave circulation and migrate into the lymph node parenchyma. • Except the spleen, they are also found in tonsils, Peyer’s patches, and the thymus. Arrowheads point to high endothelial cells. Arrows point to T and B Lymphocytes leaving the wall of the HEV. Students will see the animation In classroom 1- T lymphocytes leave the HEV, stay in the inner cortex, and multiply. 2- B lymphocytes leave the HEV and migrate to the outer cortex. 3- T lymphocytes migrate to the outer cortex and interact with B lymphocytes. 4- B lymphocytes are stimulated to divide and give rise to lymphoblasts (centroblasts), precursors of plasma cells. 5- Lymphoblasts are the precursors of plasma cells. - Plasma cells produce antigen-specific antibodies. Outer cortex B B BB B B T T TT T T Inner cortex HEV Medulla Lymph Filtration in the Lymph Node • Lymph is delivered to lymph node via afferent lymphatic vessels then sequentially move through the following lymphatic sinuses: Subcapsular sinus Trabecular sinuses Medullary sinuses Lymph also percolates through the spaces between the cells of the parenchyma. • Finally, lymph is drained by the efferent lymphatic vessels. capsule Afferent lymphatic vessel • • • • Subcapsular sinus Trabecular sinus Spleen Structure: • Connective tissue capsule • Trabeculae • Extends from the capsule. • Brings blood and nerves. • Splenic pulp: • White and red pulps • Based on color in fresh section. Unstained splenic tissue • White pulp • Tiny island designed to facilitate immune responses. • Contains lymphocytes. • Appears blue in H&E-stained section. • Red pulp • Designed for the removal of defected blood cells. • Contains RBCs, WBCs, and macrophages. • Appears red in fresh and H&E stained section. H&E Section of spleen showing its capsule sending trabeculae to the interior of the organ The red pulp occupies most of the parenchyma and the white pulp restricted to smaller areas. Note that the names of the splenic areas refer to their color in the fresh state: red pulp is filled with blood cells of all types; white pulp is lymphoid tissue. H&E. capsule red pulp trabecula white pulp red pulp red pulp Splenic Pulp: White Pulp • SPLENIC ARTERY exits capsule & trabeculae and enter the splenic pulp. • Now called CENTRAL ARTERY (AKA central arteriole in some textbooks). • Lymphocytes aggregate around central artery. • Constitute PERIARTERIAL LYMPHATIC SHEATH (PALS). red pulp Central artery PALS red pulp Splenic Pulp: White Pulp cont. • PALS may associate with lymphatic nodules (AKA splenic nodule). • Nodules appear when B lymphocytes encounter blood-borne antigens. • Germinal center appears due to proliferation of B lymphocytes. • Formation of the large centroblasts (lymphoblasts) • Expansion of the nodules displaces central artery to an eccentric position. central artery Splenic Pulp: Red Pulp • Has red appearance in live state and in histo sections. • Due to the presence of large number of red blood cells. • Consists of splenic sinuses. • Separated by splenic cords (AKA cords of Billroth). • Splenic cords of Billroth are • meshwork of reticular cells and fibers that contain large # of: − − − − − − Red blood cells Macrophages Lymphocytes Plasma cells Dendritic cells Granulocytes Unstained splenic tissue. Note the natural appearance of the white and red pulps Splenic Pulp: Red Pulp • Splenic sinuses are lined by rode-shaped endothelial cells (arrows) with large nuclei bulging into the sinusoidal lumens. • Called stave cells. • Separated by many gaps. • Supported by discontinuous basal lamina. • Allow ONLY healthy red blood cells in the splenic cords to enter the sinuses. inus inus Stave Cells ord ord inus inus Stave Cells Splenic Pulp: Red Pulp cont. • Macrophages are involved in the destruction of senescent (old) red blood cells trapped in the cord of Billroth. • Bone marrow or liver does this function when the spleen is removed (splenectomy). • Macrophages also extend cytoplasmic processes through the gaps between endothelial cells to monitor blood-borne antigen. G so Old red blood cells being phagocytosed by macrophages for destruction g Stave cells Blood Circulation of the Spleen Open circulation • Splenic artery branches into • central arteries • Give rise to penicillar arterioles. − Branches to sheathed capillaries. − Empty directly into the red pulp. Blood Circulation of the Spleen • Closed circulation • Sheathed capillaries open directly into the splenic sinus. • Experimental and morphologic evidences • support the open circulation model • More efficient exposure of red cells to macrophages of red pulp Suggested Reading Junquiera’s Basic Histology, Chapter 14 The Immune System and Lymphoid Organs Additional Resource: https://digitalhistology.org/