Endocrine - Thymus Development
|Embryology - 6 May 2021 Expand to Translate|
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The thymus has two origins for the lymphoid thymocytes and the thymic epithelial cells. The thymic epithelium begins as two flask-shape endodermal diverticula that form from the third pharyngeal pouch and extend lateralward and backward into the surrounding mesoderm and neural crest-derived mesenchyme in front of the ventral aorta. The immune system T cells are essential for responses against infections and much research concerns the postnatal development of T cells within the thymus.
Stieda in 1881 was the ﬁrst to observe that the thymus gland originated from a visceral (pharyngeal) pouch (endoderm).
This current page relates to the endocrine role of the thymus, for more detailed description of this organ development see Thymus Development.
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Some Recent Findings
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Thymus produces self-hormones
- thymus humoral factor
- Endoderm - third pharyngeal pouch
- Week 6 - diverticulum elongates, hollow then solid, ventral cell proliferation
- Thymic primordia - surrounded by neural crest mesenchyme, epithelia/mesenchyme interaction
- Thymus - bone-marrow lymphocyte precursors become thymocytes, and subsequently mature into T lymphocytes (T cells)
- Thymus hormones - thymosins stimulate the development and differentiation of T lymphocytes
|B2 Pharyngeal Arch Pouches 3 and 4 (stage 13)||D1 Developing Human Thymus (stage 22)|
Like all endocrine organs the thymus is eventually richly vascularised, development has been previously summarised.
- GA week 10 - initial blood supply.
- GA week 12 - interlobular septa blood spaces late normoblasts and granulocytes increase, cortical and medullary vasculature increases.
- GA week 16 - nerve bundles accompany arteries and veins.
- GA week 20 to 24 - radial cortical capillaries drain into capsular venules. The arterioles give rise to a series of radial cortical capillaries and less regular vessels to the medulla.
- GA week 28 to 40 - vascular thymic supply markedly increases and cortical capillaries can anastomose.
A postnatal process defined as a decrease in the size, weight and activity of the gland with advancing age. In a recent review, thymic involution was described as a result of high levels of circulating sex hormones, in particular during puberty, and a lower population of precursor cells from the bone marrow and finally changes in the thymic microenvironment.
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Sudler, MT. The Development of the Nose and of the Pharynx and its Derivatives in Man. (1902) Amer. J. Anat 1:391–416. Thymus Gland
- Hassall's corpuscle - thymic corpuscle.
- Thymic corpuscle (=Hassall's corpuscle) a mass of concentric epithelioreticular cells found in the thymus. The number present and size tend to increase with thymus age. (see classical description of Hammar, J. A. 1903 Zur Histogenese und Involution der Thymusdriise. Anat. Anz., 27: 1909 Fiinfzig Jahre Thymusforschung. Ergebn. Anat. Entwickl-gesch. 19: 1-274.)
- thymic epitheliocytes - reticular cells located in the thymus cortex that ensheathe the cortical capillaries, creating and maintain the microenvironment necessary for the development of T-lymphocytes in the cortex.
- T lymphocyte (cell) - named after thymus, where they develop, the active cell is responsible for cell-mediated immunity. (More? Electron micrographs of nonactivate and activated lymphocytes)
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Cite this page: Hill, M.A. (2021, May 6) Embryology Endocrine - Thymus Development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Endocrine_-_Thymus_Development
- © Dr Mark Hill 2021, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G