Talk:Draft 2016: Difference between revisions

Revision as of 16:55, 26 February 2016

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

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

EM - T and B lymphocytes (look the same)

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)

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)

Adult Thymus

Fetal thymus anatomy

Gross Anatomy

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

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

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.

@@ Line 60: / Line 60: @@
 ! Thymus Structure&nbsp;
 |-
-|  [[File:Thymus histology 01.jpg|800px]]
+|  '''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
+{|
+|-bgcolor="F5FAFF"
+! 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 {{HE}}
+* remove auto-reactive  T-lymphocytes
+|
+* located in cortex and medulla
+* more numerous (denser) in cortex
+* majority are developing T-lymphocytes (= thymic lymphocytes or thymocytes)
+|}
 |}