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| ''A draft page designed to be used only for testing.''
| | {{Header}} |
| | | [[File:Amniocentesis.jpg|300px|right]] |
| ----
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| [[File:SHsmall.jpg]] | |
| ==Introduction== | | ==Introduction== |
| [[File:Lymphatic-system-overview.jpg|thumb|400px|Lymphatic system]]
| | '''How and why do things go wrong in development?''' |
| While the structure of the lymphatic system (''lympha'' = clear water) is well described, there is much to still learn about the complex development and function of this "system".
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| # Immune - “monitor” of body surfaces, internal fluids
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| # Extracellular fluid - returns interstitial fluid to circulation
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| # Gastrointestinal tract - carries fat and fat-soluble vitamins
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| ===Aim===
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| This lecture will provide an overview of the histology of key lymphoid organs, including the lymph nodes, spleen and thymus, as well as extranodal lymphoid tissues including mucosal associated lymphoid tissues (MALT)
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| Note - Immunity is covered in detail elsewhere in the course, current lecture is limited to Lymphoid Organ structure/location.
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| ===Key Concepts===
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| # Lymphatic System
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| # Organs - Thymus, Spleen
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| # Lymph Nodes and Nodules
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| # Bone Marrow
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| # Extranodal Lymphoid Tissues
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| # Mucosal Associated Lymphoid Tissues (MALT)
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| <gallery>
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| File:Lymphatic-system-tonsil.jpg|Tonsil
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| File:Lymphatic-system-tonsil-MALT.jpg|Tonsil and MALT
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| File:Lymphatic-system-thymus.jpg|Thymus
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| File:Lymphatic-system-spleen.jpg|Spleen
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| File:Lymphatic-system-bone-marrow.jpg|Bone marrow
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| </gallery>
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| ==Textbook References==
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| * [[SH_Practical_-_Lymphatic_Structure_and_Organs|SH Laboratory Support]]
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| * '''Janeway’s Immunobiology''' (see in [[SH_Lecture_-_Lymphatic_Structure_and_Organs#Additional_Information|additional information]]) [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=imm.TOC&depth=2 NCBI Bookshelf]
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| * '''Histology and Cell Biology''' - A.L. Kiersenbaum (2001) Chapter 6: Blood, Chapter 10: Immune-Lymphatic
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| * Previous Lectures: [http://embryology.med.unsw.edu.au/embryology/index.php?title=SH_Lecture_-_Lymphatic_Structure_and_Organs&oldid=115513 2012] | [http://php.med.unsw.edu.au/cellbiology/index.php?title=2010_Society_and_Health_-_Lymphatic_organs_histology 2010] | [http://cellbiology.med.unsw.edu.au/units/medicine/SHlymph.htm 2008]
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| ----
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| {{Immune Links}}
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| ==Two Cellular Systems==
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| * '''Mononuclear Phagocytic System''' (MPS, also called Lymphoreticular System or Reticuloendothelial System, RES) - circulating monocytes of peripheral blood and non-circulating (fixed) tissue macrophages found throughout the body
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| * '''Lymphoid System''' - three major types of lymphocytes (T, B, and NK), tissues, organs and vessels
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| [[File:Hematopoietic_and_stromal_cell_differentiation.jpg|600px]]
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| {| class="wikitable collapsible collapsed"
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| ! Blood Cells
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| | The blood cell information shown below in the table is shown to identify the relative proportions of different cell types in the circulating blood. This information is provided in the lecture as additional information for reference purposes only.
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| {{Blood Cell Number Table}}
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| |}
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| ===Mononuclear Phagocytic System===
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| (Mononuclear Phagocytic System MPS, also called Lymphoreticular System or Reticuloendothelial System, RES)
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| [[File:Monocyte 01.jpg|300px]] [[File:Liver- Kupffer cell and reticular fibre.jpg]]
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| Mononuclear Phagocytes 2 types:
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| # Circulating '''monocytes''' of peripheral blood (monocytes entering the connective tissue differentiate into '''macrophages''')
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| # Non-circulating (fixed) tissue '''macrophages''' (MΦ) found throughout the body (Liver (Kuffler cells), spleen and other tissues)
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| ===Lymphoid System===
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| Functional cells are the '''lymphocytes'''.
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| [[File:lymphocyte 01.jpg|200px]] [[File:Lymphocyte_02.jpg|400px]]
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| {| class="wikitable collapsible collapsed"
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| ! Lymphocyte Electron Micrographs
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| | Histologically, there is little difference in appearance between T and B lymphocytes until activated.
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| <gallery>
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| File:T and B lymphocytes EM10.jpg|T and B Lymphocyte
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| File:T_and_B_lymphocytes_EM09.jpg|T and B Lymphocyte
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| File:Plasma_cell_EM06.jpg|Plasma cell (B)
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| File:T2_lymphocyte_EM13.jpg|Cytotoxic (T)
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| </gallery>
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| |}
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| ==Central/Peripheral Lymphoid Organs==
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| * Central - Lymphocytes develop from precursor cells in bone marrow. (see blood marrow image)
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| * Peripheral - Lymphocytes respond to antigen lymph nodes or spleen.
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| ==Lymph==
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| [[File:Lymphatic_vasculature_04.jpg|thumb|400px|Vasculature]]
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| * Fluid portion of lymphatic circulation
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| * blood plasma will leave blood vessels into surrounding tissues
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| * adds to normal tissue interstitial fluid
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| * surplus of liquid needs to be returned to circulation
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| * Lymph vessels provide unidirectional flow of this liquid
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| <gallery>
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| File:Lymphatic capillary.jpg|Lymph capillary
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| File:Lymphatic_vasculature_03.jpg|Lymphangion
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| File:Intestine_histology_001.jpg|Jejunum lacteal
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| </gallery>
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| ==Lymph Vessels==
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| [[File:Gray0599.jpg|thumb|Thoracic and right lymphatic ducts]]
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| Three types based on size and morphology
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| * '''Lymph capillaries''' begin as blind-ending tubes in connective tissue, larger than blood capillaries, very irregularly shaped
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| * '''Lymph collecting vessels''' larger and form valves, morphology similar to lymph capillaries
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| * '''Lymph ducts''' 1 or 2 layers of smooth muscle cells in wall
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| (Remember anatomy acronym - '''NAVL''' = Nerve, Artery, Vein and Lymph)
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| ==Lymphocyte Circulation==
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| [http://www.ncbi.nlm.nih.gov/books/NBK26921/figure/A4442 MBoC Figure 24-14. The path followed by lymphocytes as they continuously circulate between the lymph and blood]
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| * The circulation through a lymph node is shown.
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| * Microbial antigens are carried into the lymph node by dendritic cells, which enter via afferent lymphatic vessels draining an infected tissue.
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| * T and B cells, by contrast, enter the lymph node via an artery and migrate out of the bloodstream through postcapillary venules.
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| * Unless they encounter their antigen, the T and B cells leave the lymph node via efferent lymphatic vessels, which eventually join the thoracic duct.
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| * The thoracic duct empties into a large vein carrying blood to the heart.
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| * A typical circulation cycle takes about 12–24 hours.
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| '''Links:''' [http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mboc4&part=A4419 MBoC Chapter 24 - The Adaptive Immune System] | [http://www.ncbi.nlm.nih.gov/books/NBK26921/figure/A4442 MBoC Figure 24-14. The path followed by lymphocytes as they continuously circulate between the lymph and blood] | [http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=imm Immunobiology]
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| ==Additional Information==
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| ''The following is not part of the lecture and is for reference purposes only.''
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| [[File:SHsmall.jpg|left]] [[SH Practical - Lymphatic Structure and Organs|'''SH Practical - Lymphatic Structure and Organs''']] associated practical support page. Note that virtual slides will be used in the associated practical class and this linked page is provided for student self-directed learning of concepts from the virtual slides.
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| {| class="wikitable collapsible collapsed"
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| ! Additional Images
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| <gallery>
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| File:Adult_lymphatic_system.jpg|Adult lymphatic system
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| File:Lymph_node_cartoon.jpg|Lymph node cartoon
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| File:Lymph_nodes_head_neck_superficial.jpg|Lymph nodes - head neck superficial
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| </gallery>
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| |}
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| {| class="wikitable collapsible collapsed"
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| ! Janeway’s Immunobiology
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| | [[File:Mark_Hill.jpg|left|50px]] A useful resource textbook for further reading on '''Lymphatic Structure and Organs''' is [http://www.ncbi.nlm.nih.gov/books/NBK10757/ Immunobiology] 5th edition The Immune System in Health and Disease Charles A Janeway, Jr, Paul Travers, Mark Walport, and Mark J Shlomchik. Open links in a new tab if you wish to refer back to this lecture page.
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| I have included some links in this table below to specific notes and there is also available a [[Talk:SH_Lecture_-_Lymphatic_Structure_and_Organs#Immunobiology_3|complete list of contents]].
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| {{External Links}}
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| [http://www.ncbi.nlm.nih.gov/books/NBK10757/ Immunobiology] 5th edition The Immune System in Health and Disease Charles A Janeway, Jr, Paul Travers, Mark Walport, and Mark J Shlomchik.
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| '''Part I. An Introduction to Immunobiology and Innate Immunity''' Chapter 1. Basic Concepts in Immunology
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| * [http://www.ncbi.nlm.nih.gov/books/NBK27092/ The components of the immune system]
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| ** [http://www.ncbi.nlm.nih.gov/books/NBK27092/figure/A40 Figure 1.3 All the cellular elements of blood, including the lymphocytes of the adaptive immune system, arise from hematopoietic stem cells in the bone marrow]
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| ** [http://www.ncbi.nlm.nih.gov/books/NBK27092/figure/A41 Figure 1.4 Myeloid cells in innate and adaptive immunity]
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| ** [http://www.ncbi.nlm.nih.gov/books/NBK27092/figure/A42 Figure 1.5 Lymphocytes are mostly small and inactive cells]
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| ** [http://www.ncbi.nlm.nih.gov/books/NBK27092/figure/A43 Figure 1.6 Natural killer (NK) cells]
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| ** [http://www.ncbi.nlm.nih.gov/books/NBK27092/figure/A45 Figure 1.7 The distribution of lymphoid tissues in the body]
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| ** [http://www.ncbi.nlm.nih.gov/books/NBK27092/figure/A47 Figure 1.8 Organization of a lymph node]
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| ** [http://www.ncbi.nlm.nih.gov/books/NBK27092/figure/A48 Figure 1.9 Organization of the lymphoid tissues of the spleen]
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| ** [http://www.ncbi.nlm.nih.gov/books/NBK27092/figure/A49 Figure 1.10 Organization of typical gut-associated lymphoid tissue]
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| ** [http://www.ncbi.nlm.nih.gov/books/NBK27092/figure/A51 Figure 1.11 Circulating lymphocytes encounter antigen in peripheral lymphoid organs]
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| * [http://www.ncbi.nlm.nih.gov/books/NBK27092/#A52 Summary to Chapter 1]
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| '''Part III. The Development of Mature Lymphocyte Receptor Repertoires''' Chapter 7. The Development and Survival of Lymphocytes
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| * [http://www.ncbi.nlm.nih.gov/books/NBK27123/ Generation of lymphocytes in bone marrow and thymus]
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| ** [http://www.ncbi.nlm.nih.gov/books/NBK27123/figure/A803 Figure 7.3 The early stages of B-cell development are dependent on bone marrow stromal cells]
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| ** [http://www.ncbi.nlm.nih.gov/books/NBK27123/figure/A806 Figure 7.5 The development of a B-lineage cell proceeds through several stages marked by the rearrangement and expression of the immunoglobulin genes]
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| ** [http://www.ncbi.nlm.nih.gov/books/NBK27123/figure/A809 Figure 7.7 The cellular organization of the human thymus]
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| ** [http://www.ncbi.nlm.nih.gov/books/NBK27123/figure/A818 Figure 7.13Thymocytes at different developmental stages are found in distinct parts of the thymus]
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| * [http://www.ncbi.nlm.nih.gov/books/NBK27150/ Survival and maturation of lymphocytes in peripheral lymphoid tissues]
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| * [http://www.ncbi.nlm.nih.gov/books/NBK27123/#A819 Summary to Chapter 7]
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| |}
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| {| class="wikitable collapsible collapsed"
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| ! Blood Cells
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| | [[File:Mark_Hill.jpg|left|50px]] Blood cell information shown in the table below is also additional information for reference purposes.
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| {{Blood Cell Number Table}}
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| |}
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| {| class="wikitable collapsible collapsed"
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| ! Anatomy of the Human Body (1918) - Lymphatics
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| | [[File:Mark_Hill.jpg|left|50px]] [[Anatomy_of_the_Human_Body_by_Henry_Gray|Anatomy of the Human Body]] Gray (1918) Historic anatomy is good, though there are there are some functional inaccuracies.
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| <gallery>
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| File:Gray0592.jpg|592 Primary lymph sacs.
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| File:Gray0593.jpg|593 Lymph capillaries of the human conjunctiva.
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| File:Gray0594.jpg|594 Lymph capillaries from the human scrotum.
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| File:Gray0595.jpg|595 Lymph capillaries of the sole of the human foot.
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| File:Gray0596.jpg|596 Section through human tongue.
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| File:Gray0597.jpg|597 Lymph gland (Node).
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| File:Gray0598.jpg|598 Lymph gland tissue.
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| File:Gray0599.jpg|599 Thoracic and right lymphatic ducts.
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| File:Gray0600.jpg|600 Modes of origin of thoracic duct.
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| File:Gray0601.jpg|601 Terminal collecting trunks of right side.
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| File:Gray0602.jpg|602 Lymph glands of the head.
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| File:Gray0603.jpg|603 Lymphatics of pharynx.
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| File:Gray0604.jpg|604 Lymphatics of the face.
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| File:Gray0605.jpg|605 Lymphatics of the Tongue.
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| File:Gray0606.jpg|606 Lymph glands of the upper extremity.
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| File:Gray0607.jpg|607 Lymphatics of the mamma.
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| File:Gray0608.jpg|608 Lymphatic vessels of the dorsal hand surface.
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| File:Gray0609.jpg|609 Lymph glands of popliteal fossa.
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| File:Gray0610.jpg|610 Superficial lymph glands and vessels of the lower extremity.
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| File:Gray0611.jpg|611 Parietal lymph glands of the pelvis.
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| File:Gray0612.jpg|612 Iliopelvic glands.
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| File:Gray0613.jpg|613 Lymphatics of stomach.
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| File:Gray0614.jpg|614 Lymphatics of stomach.
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| File:Gray0615.jpg|615 Lymphatics of cecum and vermiform process.
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| File:Gray0616.jpg|616 Lymphatics of cecum and vermiform process.
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| File:Gray0617.jpg|617 Lymphatics of Colon.
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| File:Gray0618.jpg|618 Lymphatic of the Bladder.
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| File:Gray0619.jpg|619 Lymphatics of the Prostate.
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| File:Gray0620.jpg|620 Lymphatics of the Uterus.
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| File:Gray0621.jpg|621 Lymphatics of the thorax and abdomen.
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| File:Gray0622.jpg|622 Tracheobronchial Lymph Glands
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| </gallery>
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| |}
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| == Terms ==
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| A few key terms associated with the Lymphoid system.
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| * '''adenoid''' - (Greek " +''-oeides ''<nowiki>= in form of) in the form of a gland, glandular; the pharyngeal tonsil. </nowiki>
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| * '''Afferent lymph''' - vessel carrying lymph towards a node.
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| * '''Antibody mediated immunity''' - the immune function of plasma cells (active B lymphocytes) secreting antibody which binds antigen.
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| * '''antibodies''' - mammals have five classes (IgA, IgD, IgE, IgG, and IgM)
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| * '''antigen''' - any substance that is recognised by the immune system and stimulates antibody production.
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| * '''appendix''' - is a gut-associated lymphoid tissue located at the beginning of the colon. The anatomy is as a finger-like structure that arises from the cecum. The length (2.5-13 cm) is longer in both infants and children and also has more abundant lymphatic tissue in early life. The wall structure is similar to the small intestine (though with no villi), nor plicae circularis. Lymph nodules surround the lumen of the gastrointestinal tract and extend from the mucosa into the submucosa.
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| * '''B lymphocyte (cell)''' - historically named after a structure called the '''b'''ursa of Fabricius in birds, a source of antibody-producing lymphocytes. These cells develop in the bone marrow. (More? [[Immune_System_Development#Adult_Lymphocyte_Histology|Electron micrographs of nonactivate and activated lymphocytes]])
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| * '''BALT''' - Bronchus Associated Lymphoid Tissue
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| * '''band cell''' - (band neutrophil or stab cell) seen in bone marrow smear, a cell undergoing granulopoiesis, derived from a metamyelocyte, and leading to a mature granulocyte. Also occasionally seen in circulating blood.
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| * '''cecum''' - (caecum, Latin, ''caecus'' = "blind") within the gastrointestinal tract a pouch that connects the ileum with the ascending colon of the large intestine.
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| * '''cell''' - has a specific cell biology definition, but is often used instead of "lymphocyte" when describing B and T cells.
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| * '''Cell-mediated immunity''' - the immune function of T lymphocytes.
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| * '''CD''' - (cluster of differentiation) identifies immunological surface markers on cells.
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| * '''CD4+''' - (T helper cells) refers to T lymphocytes that express CD4 (glycoprotein of the immunoglobulin superfamily) on their surface.
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| * '''CD8+''' - (cytotoxic T cells) refers to T lymphocytes that express CD8 (glycoprotein of the immunoglobulin superfamily) on their surface.
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| * '''"clockface"''' - a term used to describe the appearance of plasma cell nuclei due to the clumping of the chromatin at the nucleus periphery. More clearly seen in tissue plasma cells that the bone marrow smear, where they are sometimes confused with the basophilic erythroblasts.
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| * '''cords of Billroth''' - spleen cellular columns located in red pulp. surrounded by splenic sinusoids. Cords contain reticular cells, macrophages, lymphocytes, plasma cells and erythrocytes.
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| * '''cortex''' - outer layer, used in association with medulla (innner layer or core) a general description that can be applied to describing an organ with a layered structure.
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| * '''dendritic cells''' - (DCs) immune cells that function to process antigen and present it on their surface to other immune cells.
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| * '''Effector cells''' - the immune functioning (active) B and T lymphocytes.
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| * '''Efferent lymph''' - vessel carrying lymph away from a node.
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| * '''GALT''' - Gut Associated Lymphatic Tissue
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| * '''haemopoiesis''' (hemopoiesis) formation of blood cells.
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| * '''Hassall's corpuscle''' - thymic corpuscle.
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| * '''IgA''' - the main class of antibody in secretions (saliva, tears, milk, and respiratory and intestinal secretions).
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| * '''IgD''' - the immunoglobulin B cell starts to produce as a cell-surface molecule after leaving the bone marrow.
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| * '''IgE''' - bind Fc receptors (surface of mast cells in tissues and basophils in the blood).
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| * '''IgG''' - the major class of immunoglobulin in the blood.
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| * '''IgM''' - the first class of antibody made by a developing B cell, which may switch to making other classes of antibody.
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| * '''immunodeficiency''' - when one or more components of the immune system is defective. (More? [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&rid=imm.section.1494 Immunobiology - immunodeficiency])
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| * '''involution''' - in the Thymus refers to the replacement, mainly in the cortex, of cells by adipose tissue. (More? [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&term=thymus+involution&doptcmdl=Books PubMed- thymus involution]) | [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&rid=cmed6.section.23856#23857 Cancer Medicine - Thymomas and Thymic Tumors])
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| * '''lamina propria''' - a layer of loose connective tissue found underneath the epithelium of mucosa.
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| * '''Leukocyte-''' (Greek, lukos= clear, white) white blood cell.
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| * '''lingual'''- related to the tongue.
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| * '''lymph node''' - connective tissue encapsulated lymphoid organ (1mm - 2cm in size), positioned in the pathway of lymph vessels.
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| * '''M cell''' - (microfold cell) found in the follicle-associated epithelium of the Peyer's patch. Function to transport gut lumen organisms and particles to immune cells across the epithelial barrier.
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| * '''macrophage''' - a large highly motile white blood cell which engulfs foreign material (bacteria etc) and both degenerating cells and cell fragments. Found in many different tissues and locations. (More? [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=imm.figgrp.1508 Immunobiology - Defects in phagocytic cells are associated with persistence of bacterial infection])
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| * '''MALT''' - Mucosa Associated Lymphoid Tissue
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| * '''medulla''' - inner layer or core, used in association with cortex (outer layer) a general description that can be applied to describing an organ with a layered structure.
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| * '''Memory Cell''' - effector T cell (lymphocyte)
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| * '''NK cell''' - (Natural killer cell, large granular lymphocytes) are a type of cytotoxic lymphocyte, responding rapidly to virally infected and tumor cells.
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| * '''normoblast''' - seen in bone marrow smear, a developing erythroblast (red blood cell) that still retains a nucleus.
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| * '''parenchyma''' - (Greek = ''enkeim'' "to pour in") cells forming the functional cells of an organ or tissue. These cells carry out the function of the organ at a cellular level, and are not the structural cells, connective tissue, extracellular matrix (stromal).
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| * '''periarterial lymphoid sheath''' - (PALS) in the spleen the white pulp that surrounds the central arteries. (T-lymphocytes,macrophages and plasma cells)
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| * '''Plasma Cell''' - active B cell (lymphocyte) which is secreting antibody. Located in either bone marrow or peripheral lymphoid tissues, these cells have and increased cytoplasmic volume (due to increase rough endoplasmic reticulum) in comparison to the inactive (non-secreting) lymphocyte.
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| * '''sentinel lymph node''' - the hypothetical first lymph node or group of nodes reached by metastasizing cancer cells from a primary tumour.
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| * '''splenic sinusoids''' - enlarged spleen capillary spaces located in red pulp and surrounding cords of Billroth.
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| * '''stroma''' - (Greek = "a cover, table-cloth, bedding") tissue forming the framework/support of an organ or tissue. That is the structural cells which form connective tissue and secrete extracellular matrix, rather than the functional cells (parenchymal). All organs can therefore be functionally divided into these 2 components, stromal/parenchymal.
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| * '''Subcapsular sinus''' (=marginal sinus) space lying under the connective tissue capsule which receives lymph from afferent lymphatic vessels.
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| * '''Thymic corpuscle''' (=Hassall's corpuscle) a mass of concentric epithelioreticular cells found in the thymus. The number present and size tend to increase with thymus age. (see classical description of Hammar, J. A. 1903 Zur Histogenese und Involution der Thymusdriise. Anat. Anz., 27: 1909 Fiinfzig Jahre Thymusforschung. Ergebn. Anat. Entwickl-gesch. 19: 1-274.)
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| * '''thymic epitheliocytes''' - reticular cells located in the thymus cortex that ensheathe the cortical capillaries, creating and maintain the microenvironment necessary for the development of T-lymphocytes in the cortex.
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| * '''T lymphocyte (cell)''' - named after '''t'''hymus, where they develop, the active cell is responsible for cell-mediated immunity. (More? [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.figgrp.4430 Electron micrographs of nonactivate and activated lymphocytes])
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| * '''thymus''' - thymus has a key role in the development of an effective immune system as well as an endocrine function. Immune system T cells are essential for responses against infections and research relates to the postnatal development of T cells within the thymus. [[Thymus Development]]
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| * '''tonsils''' - mucosal-associated lymphoid tissues consists of: 2 palatine tonsils (tonsilla palatina), adenoids (tonsilla pharyngealis) and 1 lingual tonsil (tonsilla lingualis)
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| * '''tonsillar ring''' - ring of lymphoid tissue (tonsils) around where the mouth and nasal cavity meet the throat.
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| *''' vermiform appendix''' - see appendix, anatomical region containing gut-associated lymphoid tissue located within the gastrointestinal tract at the beginning of the colon. The anatomy is as a finger-like structure that arises from the cecum. The length (2.5-13 cm) is longer in both infants and children and also has more abundant lymphatic tissue in early life. The wall structure is similar to the small intestine (though with no villi), nor plicae circularis. Lymph nodules surround the lumen of the gastrointestinal tract and extend from the mucosa into the submucosa.
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| {{Glossary}}
| | [[File:child smallpox.jpg|thumb|Mother with child, face and body showing smallpox scars.]] |
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| | These notes cover abnormalities that can occur during development ({{abnormal development}}) often described as {{congenital abnormalities}} or {{birth defects}}. There are many different ways that developmental abnormalities can occur the 3 major types are '''Genetic''' (inherited), '''Environmental''' (maternal) and '''Idiopathic''' (unknown, not determined) derived abnormalities. The environmental factors that cause or lead to any of these abnormalities are described as teratogens. Congenital abnormalities are classified under the system called the [[International Classification of Diseases]] ({{ICD-11}}) that version updated in 2018. |
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| {{Footer}}
| | Note that genetic and environmental effects can also interact and it is this and the group, now classified as idiopathic or "unknown causes", that require further research to place them in either or both of the two real categories. Furthermore developmental organizer genes, for example Hedgehog, have been recently implicated in adult carcinogenesis. |
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| [[Category:Immune]] [[Category:Histology]]
| | {{#pmid:19782018|reload}} |
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