Lymphoid histology PDF

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Lymphoid organs Enumerate the types of lymphoid organs Compare and contrast the histological characteristics of diffuse and nodular lymphoid tissues Describe the localization and the cell types that compose diffuse lymphoid tissue Describe the histological organization of the primary lymphoid organs...

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Lymphoid organs 1. Enumerate the types of lymphoid organs 2. Compare and contrast the histological characteristics of diffuse and nodular lymphoid tissues 3. Describe the localization and the cell types that compose diffuse lymphoid tissue 4. Describe the histological organization of the primary lymphoid organs a. Bone marrow (covered in the blood cells lecture) b. Thymus 5. Describe the histological organization of secondary lymphoid organs a. MALT (mucosa associated lymphoid tissue): i. Peyer’s patches ii. tonsils b. encapsulated lymphoid organs: i. Lymph nodes ii. Spleen

We will not test over the immunology subject matter covered on pages 226-232 in your textbook (Basic Histology, 12th edition). Instead, we will focus on the histology of lymphoid tissue (beginning on page 233).

HISTOLOGY OF LYMPHOID ORGANS Lymphoid tissue is associated with many regions of the body (e.g. GI tract, respiratory and reproductive system), where the connective tissue is heavily populated by lymphocytes. Lymphoid organs are classified as primary (or central) lymphoid organs, as these sites are the primary source of lymphocytes, where cells undergo antigenindependent differentiation. The newly formed lymphocytes migrate from these primary organs to peripheral (or secondary) lymphoid organs, where they can react with foreign antigens and undergo antigen-dependent differentiation. 1. Enumerate the types of lymphoid organs: - Primary lymphoid organs: sites where lymphocytes are made - Secondary lymphoid organs: sites where lymphocytes are activated Primary lymphoid organs • Bone marrow: site of origin of o B-lymphocytes o NK o dendritic cells o precursors of T-lymphocytes • Thymus: site of maturation of T-lymphocytes Secondary lymphoid organs • Mucosa associated lymphoid tissue (MALT) • Lymph nodes • Spleen 2. Compare and contrast the histological characteristics of diffuse and nodular lymphoid tissues • •

Diffuse lymphoid tissue contains numerous of lymphocytes without apparent organization. In nodular lymphoid tissue the lymphocytes are arranged as spherical masses called lymphoid nodules (or follicles) and contain mainly B lymphocytes.

Activated lymphoid nodules contain germinal centers, areas with actively dividing cells that have more cytoplasm than mature lymphocytes and therefore, stain lighter. In the lymphoid tissues, cells are surrounded by a mesh of reticular fibers produced by reticular cells (except in the thymus).

3. Describe the localization and the cell types that compose diffuse lymphoid tissue Diffuse lymphoid tissue is found associated with mucous membranes (digestive respiratory and urinary tracts), close to areas that can be the entry points for pathogens. Diffuse lymphoid tissues include the following cell types: •

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Dendritic cells: o The most potent antigen presenting cell o Derived from bone marrow Lymphocytes: o White blood cell responsible for the specificity of the adaptive immune response o Can be classified as B, T or natural killers. Indistinguishable under the light or electron microscope, lymphocytes can be differentiated by immunohistochemistry IgA-secreting plasma cells: o B lymphocyte-derived, antibody secreting cells that produce IgA o IgA is an immunoglobulin that contains a secretory component that gives it resistance to enzymes; therefore it is active in enzyme-rich secretions where it contributes to the resistance to infections.

4. Describe the histological organization of the primary lymphoid organs a. Bone marrow (covered in the blood cells lecture) b. Thymus •

The thymus is a bilateral organ located in the mediastinum that attains peak development during youth

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Site where formed

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The capsule penetrates the parenchyma and divides it into incomplete lobules, each one with a cortex and a medulla:

T

lymphocytes

are

o The cortex is formed by dense population of T lymphoblasts or thymocytes (immature T cells) and macrophages in a stroma of epithelial reticular cells

Thymus

o The medulla contains differentiated T lymphocytes, epithelial reticular cells, and reticular fibrocytes. It also contains structures called thymic or Hassall’s corpuscles (of unknown function), consisting of epithelial reticular cells arranged concentrically and filled with keratin filaments. o The cortex is isolated from the circulation by a blood-thymus barrier that prevents most circulating antigens from leaving the microvasculature and entering the cortex. This thymus-blood barrier consists of three major elements: 

Capillary endothelium and its basal lamina

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Perivascular connective tissue occupied by macrophages

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Type I epithelioreticular cells with their basal lamina

These three layers provide protection to the developing immature T cells and separate them from the mature lymphocytes that circulate in the blood-stream. Mature T lymphocytes in the medulla, which lacks a blood-thymus barrier, exit the thymus via venules present at the corticomedullary junction.

Thymus-blood barrier

5. Describe the histological organization of secondary lymphoid organs a. MALT (mucosa associated lymphoid tissue): Peyer’s patches and tonsils b. encapsulated lymphoid organs: lymph nodes and spleen a. MALT MALT is the lymphoid tissue found in the gastrointestinal, respiratory and genitourinary tracts. It is composed of diffuse lymphoid tissue, as well as non-encapsulated or partially encapsulated organized aggregations, such as Peyer’s patches and tonsils. • Peyer’s patches o Non-encapsulated, large clusters of lymphoid follicles in the ileum of the small intestine o Localized in the lamina propria and submucosa, and often bulge into the gut lumen o The simple follicleassociated epithelium that covers these nodules contains specialized M cells, which transport antigens and microorganisms in the intestinal lumen to antigen presenting cells and lymphocytes. • Tonsils o Partially encapsulated lymphoid tissue lying Diagram of intestinal epithelium showing M cells, Peyer’s beneath and in contact patches, and plasma and antigen presenting cells. with the epithelium of the oral cavity o According to their location are called palatine, pharyngeal, or lingual tonsils o Contain numerous lymphoid follicles with germinal centers o The mucosa in some tonsils form invaginations called crypts, that contained desquamated epithelial cells, live and dead lymphocytes and bacteria

b. encapsulated secondary lymphoid organs: Encapsulated lymphoid organs are organized structures that contained immature and mature lymphocytes and accessory cells surrounded by a connective tissue capsule. •

Lymph nodes o Bean-shaped structures distributed throughout the body along the course of the lymphatic vessels o Contain lymphocytes, macrophages and other antigen presenting cells, Lymph node plasma cells, and reticular cells o The capsule surrounds the lymph node and sends trabeculae into its interior.

Three areas can be distinguished based on the cells present: o The cortex, situated under the capsule and consisting of:  Reticular cells, macrophages, antigen-presenting cells, and lymphocytes  Lymphoid nodules with or without germinal centers. Germinal centers are sites in which antigen stimulation results in B cell proliferation and activation  Subcapsular sinuses, immediately beneath the capsule and formed by reticular fiber meshes  Cortical sinuses, which run between the lymphoid nodules and communicate with the subcapsular sinuses o The medulla, composed of two structures:  Medullary cords, which contain primarily B lymphocytes, plasma cells, and macrophages  Medullary sinuses, spaces that contain lymph, lymphocytes, and macrophages o The paracortex, does not have precise boundaries with the cortex and the medulla. The paracortex can be identified by the absence of B cells, the accumulation of T cells and the presence of postcapillary venules (or high endothelial capillaries), lined by tall endothelial cells. Antigens, accessory cells and lymphocytes enter the lymph node via the afferent lymphatics, which drain into the subcapsular sinuses and, from there, into the cortical sinuses. These antigens, accessory cells and lymphocytes may then enter

the cortex or remain in the sinuses and leave the lymph node via the efferent lymphatic vessels. Valves in the lymphatic vessels assure one-way flow of lymph. •

Spleen o Largest single accumulation of lymphoid tissue involved in filtration of blood, destruction of aged erythrocytes, and production of antibodies and activated lymphocytes o The capsule is formed by dense connective tissue from which emerge trabeculae that partially subdivide the parenchyma or splenic pulp o The splenic pulp is composed of reticular tissue containing reticular cells, lymphocytes and other blood cells, macrophages, and antigen presenting cells. It has two components, named based on the color of fresh sections:  White pulp, consisting of lymphoid nodules and periarteriolar lymphoid sheaths (PALS). It is densely populated by T and B lymphocytes.  Red pulp, composed almost entirely of splenic cords (cords of Bilroth) and venous sinusoids, which contain blood cells. Blood enters the spleen in the splenic artery, which branches to form the trabecular arteries that, in turn, give rise to a series of central arteries surrounded by T cells of the white pulp. Blood then enters the red pulp through a series of specialized vessels, (penicillary arteries and sheathed capillaries, the latter surrounded by macrophages). Blood flow through the splenic red pulp can take two routes: o Closed circulation: penicillar arterioles and capillaries connect directly to the sinusoids, so that the blood is always enclosed by the vascular endothelium. Macrophages extend through extracellular clefts between adjacent endothelial cells to contact circulating cells, binding to and destroying aged erythrocytes. Blood flow in the spleen o Open circulation: penicillar arterioles are open ended, emptying their content into the stroma of the splenic cords. Here, the blood is exposed to the macrophages, which phagocytose fragments of destroyed aged or abnormal

erythrocytes. Intact red cells then leave the system via trabecular veins and enter the splenic vein. In the white pulp, small arterioles lead to marginal sinuses that surround the lymphoid sheaths. In this area, blood comes into contact with antigen presenting cells, where foreign antigens can be trapped and presented to lymphoid cells....