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HISTOLOGY OF THYMUS
BY
DR. MUDASSAR ALI ROOMI (MBBS, M. PHIL)
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THYMUS-INTRODUCTIONMay2,2012
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A bilobed organ
Location: Anterior superiorMediastinum
Unlike the liver, kidney andheart, the thymus is at itslargest in children.
Like bone marrow and B cells,the thymus is considered acentral or primary lymphoidorgan because T lymphocytesform there
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DEVELOPMENTOFTHYMUSMay2,2012
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Whereas all other lymphoidorgans originate exclusivelyfrom mesenchyme(mesoderm), the thymushas a dual embryonicorigin.
Its precursor lymphoblastsoriginate in the bonemarrow (mesodermalorigin), but then move toinvade a unique epitheliumthat developed from the
endoderm of the embryo'sthird and fourth pharyngealpouches.
So thymus is a lympho-epithelial organ.
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HISTOLOGYOFTHYMUSMay2,2012
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The thymus has a connectivetissue capsule that penetratesthe parenchyma and divides itinto incomplete lobules, withcontinuity between the cortex
and medulla of adjoininglobules .
Each lobule has a peripheraldarkly stained zone known asthe cortex and a central lightzone called the medulla.
The cortex is richer in smalllymphocytes than the medullaand therefore it stains moredarkly.
Medulla is rich in epithelial cells
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HISTOLOGYOFTHYMUS
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CELLSOFTHYMUS
1. Epithelial cells***
2. Lymphocytes***
3. Macrophages
4. Plasma cells
5. Mast cells
6. adipocytes
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1:EPITHELIAL CELLSMay2,2012
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Six forms of these cells
Type II cells: these cellsappear as stellate cells withslender processes. Thesecells form cytoreticulum inwhich T-cells are tightly
packed. Processes of thesecells are joined together bydesmosomes. No fibers areassociated with theseepithelioreticular cells
Type I cells: these epithelialcells form the blood-thymusbarrier.
Type VI cells: these epithelialcells form the thymiccorpuscles (Hassalscorpuscles)
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HASSALSCORPUSCLES (*****)
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Rounded or oval whorl-likestructures
20-100 um in diameter
Found in the medullaryregions of the thymic
lobules Each consists of
concentrically arranged flat,eosinophilic epithelial cells
These cells contain keratinfilaments
Central cells lose theirnucleus and makekeratinized aggregations.
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EPITHELIALCELLSOFTHYMUS
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2: T-LYMPHOCYTES
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95 % of T-lymphocytes are in thecortex and 5 % in the medulla.
10 % of the total thymiclymphocytes are present in theperipheral cortex. these representlarge lymphocytes (stem cells).These proliferate very actively andproduce small lymphocytes.
Deeper cortex contains smalllymphocytes (thymocytes). Theseare maturing cells and constitute85 % of the total thymic
lymphocytes.
Most of the T-cells (which dontshow immune tolerance) die inthe cortex and are phagocytosedby macrphages.
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THYMUSMa
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HOWTODRAWIT?Ma
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BLOOD-THYMUS BARRIER
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It exits in the cortex only making it an
immunologically protected region
Macromolecules cannot pass from thecapillaries to the thymic parenchyma inthe cortical region of thymic lobules.
It protect the cortical lymphocytes fromthe circulating antigens
Structural basis of blood-thymusbarrier:
1. Tight junctions between theendothelial cells of the capillaries.
2. Basal lamina of endothelial cells.
3. C.T with macrophages.
4. basal lamina of thymic epithelial cells.5. Perivascular sheath of thymic
epithelial cells.
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INVOLUTIONOFTHYMUSMa
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TYPES OF INVOLUTION:
1. Age involution:2. Accidental involution:
The thymus continues to grow betweenbirth and puberty and then begins toatrophy; this thymic involution isdirected by the high levels of circulatinghormones. Proportional to thymic size,thymic activity (T-cell output) is mostactive before puberty.
Upon atrophy, the size and activity aredramatically reduced, and the organ isprimarily replaced with fat (aphenomenon known as "organinvolution").
The atrophy is due to the increasedcirculating level of sex hormones, andchemical or physical castration of anadult results in the thymus increasing insize and activity.
Involution of the thymus has beenlinked to loss of immune function in theelderly, susceptibility to infection and tocancer.
WEIGHT OF THYMUS ATDIFFERENT AGES:
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COMPARISONOFNORMALANDINVOLUTEDTHYMUSMa
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FUNCTIONSOFTHYMUSMa
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Production of T-
lymphocytes: aftertheir production T-cellsare carried by blood to
their thymus-dependent zones in thespleen, lymph nodesand other lymphoid
organs. Production of
hormones byepithelial cells
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HORMONESACTINGONTHETHYMUS
1. Thymosin: tranforms stem lymphocytes to T-lymphocytes.
2. Thymopoitin: induces T-cells maturation
3.
Thymic humoral factor: essential for T-cellsdiffentiation.
4. Thyroxine
5. Adrenal steroids: depress the T-cells formation in
the thymus.
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CLINICAL IMPORTANCE OF THYMUS
Accelerated involution (by stress, steroids andinfection)
Neonatal thymectomy: severely impairs the cellmediated and humoral immunity.
Adult thymectomy: causes less severeimpairment of the immunity
Congenital thymic aplasia (DiGeorgeSyndrome): absence of thymus and parathyroidglands. Leads to hypocalcemia and recurrentinfections.
AIDS: destruction of CD4+ helper T-cells.
Autoimmune diseases: because of failure ofimmune tolerance. e.g. myasthenia gravis.
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IDENTIFICATION POINTS: THYMUS
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Characteristic lobuleswith cortex andmedulla
Hassalls corpuscles
No lymph sinuses
No lymphatic nodules
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COMPARISON
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