Talk:Draft 2016

Spleen

Thymus Anatomy

Thymus Anatomy
adult thymus - bilobed, superior mediastinum, anterior to heart	infant thymus - large

Thymus Involution
	Overall Size Changes with age birth 10-15 g puberty 30-40 g after puberty - involution replaced by adipose tissue middle-aged 10 g

Thymus Functions

Thymus Functions
specialised thymus microenvironments allow the production of self-tolerant T-cells (T lymphocytes) from immature precursors. immature precursors enter the thymus differentiate and undergo selection by thymic epithelial cell (TEC) subtypes mature release into circulation of these cells destruction of cells that recognise self antigens T-cells kill infected and oncogenic cells T Cells maturation within the thymus T cell progenitors enter the thymus at the cortex/medulla border via post–capillary venules migrate toward the capsule in response to chemokine signalling. cortex - thymocytes undergo positive selection by cTECs then migrate to the medulla medulla - thymocytes are screened for reactivity to tissue-restricted self antigens expressed by mTECs. Mature T cells exit the thymus via blood or lymphatic vessels in response to a sphingosine-1-phosphate (S1P) gradient.
Macrophages phagocytosis of T cells  ‎‎Thymus 1 Page | Play	EM - T and B lymphocytes (look the same)

Thymus Structure

Thymus Structure

Structure Overview Connective tissue capsule (thin) with numerous trabeculae (septa) major blood vessels run in CT Lobules containing cortex and medulla regions medullary regions are continuous (connected together) NOT supplied by afferent lymph vessels Blood-Thymic Barrier Blood vessels are separated from thymus cortex by epithelioreticular cells. impermeable to most macromolecules. Barrier layers: capillary endothelium - endothelial basal lamina- perivascular CT sheath - basal lamina of epithelioreticular cells - epithelioreticular cell sheath Thymus Epithelioreticular cells (TEC) Macrophages Lymphocytes Abundant, eosinophilic, large, ovoid and light nucleus 1-2 nucleoli sheathe cortical capillaries form an epitheloid layer maintain microenvironment for development of T-lymphocytes in cortex (thymic epitheliocytes) cortex and medulla difficult to distinguish from reticular cells in (Stain - Haematoxylin Eosin) remove auto-reactive T-lymphocytes located in cortex and medulla more numerous (denser) in cortex majority are developing T-lymphocytes (= thymic lymphocytes or thymocytes)

Thymus Histology

Thymus Histology

Capsule with trabeculae (dense connective tissue) Infant thymus Overview - red and white pulp Overview - blood vessels Red pulp Reticular Fibers (type III collagen) act as supporting meshwork (can be seen in Silver stained preparations) Thymus Histology: Fetal Thymus overview | Fetal Thymus Medulla | Fetal Thymus Cortex | Adult Thymus | unlabeled fetal overview | unlabeled fetal medulla |unlabeled fetal thymic corpuscle |unlabeled fetal cortex | unlabeled adult overview | Category:Thymus | Immune System Development

Thymus

Adult Thymus

Fetal thymus anatomy

Gross Anatomy

• Superior mediastinum, anterior to heart
• Bilobed lymphoepithelial organ

Histology Contains reticular cells but no fibers Stem lymphocytes proliferate and differentiate forms long-lived T- lymphocytes removes auto-reactive T-lymphocytes (negative selection)

‎‎Thymus 1 Page | Play

Thymus Cells

Reticular cells	Macrophages	Lymphocytes
Abundant, eosinophilic, large, ovoid and light nucleus 1-2 nucleoli sheathe cortical capillaries form an epitheloid layer maintain microenvironment for development of T-lymphocytes in cortex (thymic epitheliocytes)	cortex and medulla difficult to distinguish from reticular cells in (Stain - Haematoxylin Eosin) remove auto-reactive T-lymphocytes	located in cortex and medulla more numerous (denser) in cortex majority are developing T-lymphocytes (= thymic lymphocytes or thymocytes)

Histology

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  Fetal thymus

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  Young medulla

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  Young cortex

Adult Thymus

Cortical lymphoid tissue is replaced by adipose tissue (involution) Increase in size of thymic corpuscles Thymic corpuscle - (Hassall’s corpuscle) mass of concentric epithelioreticular cells.