Thymus Development
Introduction
The thymus has a key role in the development of an effective immune system as well as an endocrine function.
The mature thymus epithelium has two main cell types: cortical thymic epithelial (cTECs) and medullary thymic epithelial cells (mTECs) or stromal cells. These thymic stromal cells provide signals for T cell differentiation.
Links: original Endocrine Development - Thymus page
Some Recent Findings
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Development Overview
The thymus and parathyroid are derived from 3rd pharyngeal pouches.
Development is a series of epithelial/mesenchymal inductive interactions between neural crest-derived arch mesenchyme and pouch endoderm. There is also the possibility that the surface ectoderm of 3rd pharyngeal clefts participates in thymus development.
Hassall's bodies form between 6 and 10 lunar months in humans. They appear after lymphopoiesis has been established and the cortex, medulla and the cortico-medullary junction are able to select of T lymphocytes undergoing progressive maturation. (Text modified from Bodey and Kaiser, 1997)
Experimental studies have shown that a neural crest contribution is also required during early thymic organogenesis.
MBoC Figure 24-6. The development and activation of T and B cells
Figure 24-7. Electron micrographs of nonactivated and activated lymphocytes
Development Changes
Changes with age Overall Size
- birth 10-15 g
- puberty 30-40 g
- after puberty - involution
- Replaced by adipose tissue
- middle-aged 10 g
Thymus Anatomy
- Superior mediastinum, anterior to heart
- Bilobed lymphoepithelial organ
- Contains reticular cells but no fibers
- Stem lymphocytes
- proliferate and differentiate
- forms long-lived T- lymphocytes
Thymus Cells
- Reticular cells
- 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)
- Macrophages
- cortex and medulla
- difficult to distinguish from reticular cells in H&E
- Lymphocytes
- cortex and medulla - more numerous (denser) in cortex
- majority of them developing T-lymphocytes (= thymic lymphocytes or thymocytes)
Fetal/Young Thymus
Young medulla | Young cortex |
Thymic corpuscle
Hassall’s corpuscle - Mass of concentric epithelioreticular cells
Adult Thymus
- Cortical lymphoid tissue is replaced by adipose tissue
- Increase in size of thymic corpuscles
Links: Blue Histology - Thymus
Hassall's Bodies
Hassall's bodies, also called Hassall's corpuscles, form between 6 and 10 lunar months in humans. They appear after lymphopoiesis has been established and the cortex, medulla and the cortico-medullary junction are able to select of T lymphocytes undergoing progressive maturation.Within the thymus their number increases until puberty, then decreases. These features are named after Arthur Hill Hassall (1817-1894) a British physician and chemist.
- Hassall's corpuscles express thymic stromal lymphopoietin (TSLP), suggesting that Hassall's corpuscles have a critical role in dendritic-cell-mediated secondary positive selection of medium-to-high affinity self-reactive T cells, leading to the generation of CD4(+)CD25(+) regulatory T cells within the thymus.[1]
- There has been one report showing changes in Hassall's bodies morphology associated with congenital heart defects.[2]
Molecular Development
Cited2
Cited2 deletion in the mouse is embryonic lethal with cardiovascular malformations, adrenal agenesis, cranial ganglia fusion, exencephaly, and left-right patterning defects.[3]
- "Examination of Lmo4-deficient embryos revealed partially penetrant cardiovascular malformations and hypoplastic thymus. Examination of Lmo4;Cited2 compound mutants indicated that there is a genetic interaction between Cited2 and Lmo4 in control of thymus development. Our data suggest that this may occur, in part, through control of expression of a common target gene, Tbx1, which is necessary for normal thymus development."
Eva and Six
Both Eva and Six have been implicated in thymus development.[4]
- Eya - human homolog of the Drosophila 'eyes absent' (Eya) gene.
- Six - vertebrate genes which are homologs of the Drosophila 'sine oculis' (so) gene.
References
Reviews
<pubmed>18403191</pubmed> <pubmed>16448532</pubmed> <pubmed>12969307</pubmed> <pubmed>11292256</pubmed>
Articles
<pubmed>11857615</pubmed>
Search PubMed: Thymus Development
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Cite this page: Hill, M.A. (2024, May 3) Embryology Thymus Development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Thymus_Development
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G