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.
Named after Arthur Hill Hassall (1817-1894) a British physician and chemist.
Molecular Development
Both Eva and Six have been implicated in thymus development.[1]
- Eya - human homolog of the Drosophila 'eyes absent' (Eya) gene.
- Six - vertebrate genes which are homologs of the Drosophila 'sine oculis' (so) gene.
Xu PX, Zheng W, Laclef C, Maire P, Maas RL, Peters H, Xu X. Eya1 is required for the morphogenesis of mammalian thymus, parathyroid and thyroid. Development. 2002 Jul;129(13):3033-44.
References
- ↑ Patterning of the third pharyngeal pouch into thymus/parathyroid by Six and Eya1. Zou D, Silvius D, Davenport J, Grifone R, Maire P, Xu PX. Dev Biol. 2006 May 15;293(2):499-512. Epub 2006 Mar 10. PMID: 16530750
Reviews
<pubmed>16448532</pubmed>
Articles
<pubmed>11857615</pubmed>
Search PubMed: Thymus Development
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Cite this page: Hill, M.A. (2024, June 16) 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