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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.jpg|400px]]
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| |-
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| | adult thymus - bilobed, superior mediastinum, anterior to heart
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| | infant thymus - large
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| |}
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| {|
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| ! Thymus Involution
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| |-
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| | [[File:Fetal thymus weight growth graph.jpg|400px]]
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| | valign=top|'''Overall Size Changes with age'''
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| * birth 10-15 g
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| * puberty 30-40 g
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| * after puberty - involution
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| ** replaced by adipose tissue
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| * middle-aged 10 g
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| |}
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| ===Thymus Functions===
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| {| class="wikitable mw-collapsible mw-collapsed"
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| ! colspan=2|Thymus Functions
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| | valogn=top|
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| * specialised thymus microenvironments allow the production of self-tolerant T-cells (T lymphocytes) from immature precursors.
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| ** immature precursors enter the thymus
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| ** differentiate and undergo selection by thymic epithelial cell (TEC) subtypes
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| ** mature release into circulation of these cells
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| * destruction of cells that recognise self antigens
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| * T-cells kill infected and oncogenic cells
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| T Cells maturation within the thymus.
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| # T cell progenitors enter the thymus at the cortex/medulla border via post–capillary venules
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| # migrate toward the capsule in response to chemokine signalling.
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| # In the cortex, thymocytes undergo positive selection by cTECs then migrate to the medulla
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| # In the medulla, thymocytes are screened for reactivity to tissue-restricted self antigens expressed by mTECs.
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| # Mature T cells exit the thymus via blood or lymphatic vessels in response to a sphingosine-1-phosphate (S1P) gradient.
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| | [[File:Thymus structure and function cartoon01.jpg|600px]]
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| |-
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| | [[File:T and B lymphocytes EM09.jpg|400px]]
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| T and B lymphocytes
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| |}
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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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| |-
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| | [[File:Thymus histology 01.jpg|800px]]
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|
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| |}
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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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| |-
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| * Capsule with trabeculae (dense connective tissue)
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| [[File:Thymus histology 01.jpg|600px]]
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| Infant thymus
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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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| 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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| |}
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