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| ==Spleen==
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| ===Thymus Anatomy===
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| {|
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| ! Thymus Anatomy
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| |-
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| | [[File:Thymus cartoon.jpg|400px]]
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| | [[File:Gray1178.jp|400px]]
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| |-
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| | adult thymus
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| | infant thymus
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| |}
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| ===Thymus Functions===
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| {| class="wikitable mw-collapsible mw-collapsed"
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| ! Thymus Functions
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| # '''Immune''' - filters blood in much the way that the lymph nodes filter lymph.
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| ## '''Lymphocytes''' in the spleen react to pathogens in the blood and attempt to destroy them.
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| ## '''Macrophages''' then engulf the resulting debris, the damaged cells, and the other large particles.
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| # '''Red Blood Cell Removal''' - spleen (and liver) removes old and damaged erythrocytes from the circulating blood.
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| # '''Blood Reservoir''' - The sinuses in the spleen also act as a reservoir for blood. In emergencies (haemorrhage) smooth muscle in the vessel walls and in the capsule of the spleen contracts, squeezes blood out of the spleen into the general circulation.
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| |}
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| ===Thymus Structure===
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| {| class="wikitable mw-collapsible mw-collapsed"
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| ! Thymus Structure
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| | [[File:Spleen structure cartoon 01.jpg|800px]]
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| * '''afferent splenic artery''' branches into central arterioles, which are sheathed by white pulp areas.
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| * '''white pulp''' areas consist of the T-cell zone (also known as the periarteriolar lymphoid sheath, PALS), arterioles and B-cell follicles.
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| * arterioles end in cords in the '''red pulp''', from where the blood runs into venous sinuses that collect into the '''efferent splenic vein'''.
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| * larger arteries and veins run together in connective-tissue trabeculae, which are continuous with the capsule that surrounds the spleen.
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| |}
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| ===Thymus Histology===
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| {| class="wikitable mw-collapsible mw-collapsed"
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| ! Thymus Histology
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| * Capsule with trabeculae (dense connective tissue)
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| [[File:Spleen histology 06.jpg|600px]]
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| {|
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| ! Overview - red and white pulp
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| ! Overview - blood vessels
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| ! Red pulp
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| |-
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| | [[File:Spleen_histology_01.jpg|300px]]
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| | [[File:Spleen_histology_02.jpg|300px]]
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| | [[File:Spleen_histology_03.jpg|300px]]
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| |}
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| '''Reticular Fibers''' (type III collagen) act as supporting meshwork (can be seen in Silver stained preparations)
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| {|
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| | [[File:Spleen_histology_05.jpg|300px]]
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| | [[File:Spleen_histology_04.jpg|300px]]
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| |}
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| {{Thymus Histology}}
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| |}
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| ==Thymus==
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| [[File:Thymus cartoon.jpg|thumb|Adult Thymus]]
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| [[File:Gray1178.jpg|thumb|Fetal thymus anatomy]]
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| Gross Anatomy
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| * Superior mediastinum, anterior to heart
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| * Bilobed lymphoepithelial organ
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| {|
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| | Histology
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| * Contains reticular cells but no fibers
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| * Stem lymphocytes
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| ** proliferate and differentiate
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| ** forms long-lived T- lymphocytes
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| * removes auto-reactive T-lymphocytes (negative selection)
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| | {{Thymus Movie 1}}
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| |}
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| ===Thymus Cells===
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| {|
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| |-bgcolor="F5FAFF"
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| ! Reticular cells
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| ! Macrophages
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| ! Lymphocytes
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| |-
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| * Abundant, eosinophilic, large, ovoid and light nucleus 1-2 nucleoli
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| * sheathe cortical capillaries
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| * form an epitheloid layer
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| * maintain microenvironment for development of T-lymphocytes in cortex (thymic epitheliocytes)
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| * cortex and medulla
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| * difficult to distinguish from reticular cells in {{HE}}
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| * remove auto-reactive T-lymphocytes
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| * located in cortex and medulla
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| * more numerous (denser) in cortex
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| * majority are developing T-lymphocytes (= thymic lymphocytes or thymocytes)
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| |}
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| ===Development Changes===
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| {Changes with age
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| Overall Size
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| * birth 10-15 g
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| * puberty 30-40 g
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| * after puberty - decreases in size (involution)
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| * middle-aged 10 g, replaced by adipose tissue
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| Histology
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| <gallery>
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| File:Fetal thymus.jpg|Fetal thymus
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| File:Thymus - young 01.jpg|Young medulla
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| File:Thymus - young 02.jpg|Young cortex
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| </gallery>
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| ===Adult Thymus===
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| {|
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| | [[File:Thymus adult.jpg|300px]]
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| * Cortical lymphoid tissue is replaced by adipose tissue (involution)
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| * Increase in size of thymic corpuscles
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| * '''Thymic corpuscle''' - (Hassall’s corpuscle) mass of concentric epithelioreticular cells.
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| [[File:Thymus - young 01.jpg|300px]]
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| |}
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