Endocrine - Thymus Development

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Embryonic origins of the endocrine organs of the neck

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[1] 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.

Endocrine Links: Introduction | BGD Lecture | Science Lecture | Lecture Movie | pineal | hypothalamus‎ | pituitary | thyroid | parathyroid | thymus | pancreas | adrenal | endocrine gonad‎ | endocrine placenta | other tissues | Stage 22 | endocrine abnormalities | Hormones | Category:Endocrine
Historic Embryology - Endocrine  
1903 Islets of Langerhans | 1903 Pig Adrenal | 1904 interstitial Cells | 1908 Pancreas Different Species | 1908 Pituitary | 1908 Pituitary histology | 1911 Rathke's pouch | 1912 Suprarenal Bodies | 1914 Suprarenal Organs | 1915 Pharynx | 1916 Thyroid | 1918 Rabbit Hypophysis | 1920 Adrenal | 1935 Mammalian Hypophysis | 1926 Human Hypophysis | 1927 Adrenal | 1927 Hypophyseal fossa | 1930 Adrenal | 1932 Pineal Gland and Cysts | 1935 Hypophysis | 1935 Pineal | 1937 Pineal | 1935 Parathyroid | 1940 Adrenal | 1941 Thyroid | 1950 Thyroid Parathyroid Thymus | 1957 Adrenal

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Immune Links: immune | blood | spleen | thymus | lymphatic | lymph node | Antibody | Med Lecture - Lymphatic Structure | Med Practical | Immune Movies | vaccination | bacterial infection | Abnormalities | Category:Immune
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1909 Lymph glands | 1912 Development of the Lymphatic System | 1918 Gray's Lymphatic Images | 1916 Pig Lymphatics | 1919 Chicken Lymphatic | 1921 Spleen | 1922 Pig Stomach Lymphatics | 1932 Cat Pharyngeal Tonsil | Historic Disclaimer

Some Recent Findings

Human Embryo (week 6 - 8)[2]
  • What do we know about the structure of human thymic Hassall's corpuscles?[3] "Hassall's corpuscles are the most prominent structures in the human thymus. However, relatively few analyses have been performed to determine their function and cellular origins during development. In this study, we evaluated the cellular microenvironment of human thymic Hassall's corpuscles using histochemistry, immunohistochemistry, and transmission electron microscopy. ... Considerable variation in the sizes, shapes, and numbers of Hassall's corpuscles was observed, even amongst children of the same age. ... Immunohistochemical staining and electron microscopy revealed that Hassall's corpuscles resemble other types of stratified squamous epithelia."
  • Dynamics of thymus organogenesis and colonization in early human[4] "The thymus is the central site of T-cell development and thus is of fundamental importance to the immune system, but little information exists regarding molecular regulation of thymus development in humans. ... In addition, we provide molecular evidence that the human thymic epithelium derives solely from the third pharyngeal pouch, as in the mouse, in contrast to previous suggestions."
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Search term: Thymus Embryology | Endocrine Thymus Development | Endocrine Thymus Embryology | thymic epithelial cells

Thymus Hormones

Thymus produces self-hormones

  • thymulin
  • thymosin
  • thymopentin
  • thymus humoral factor

Thymus Development

Developing Human (stage 22)
Developing Human Thymus (stage 22)
  • 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
Stage 13 image 058.jpg Stage 22 image 071.jpg
B2 Pharyngeal Arch Pouches 3 and 4 (stage 13) D1 Developing Human Thymus (stage 22)

Thymus Vasculature

Like all endocrine organs the thymus is eventually richly vascularised, development has been previously summarised.[6]

  • 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.

Thymus Involution

A postnatal process defined as a decrease in the size, weight and activity of the gland with advancing age. In a recent review[7], 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.



Anderson G, Jenkinson EJ & Rodewald HR. (2009). A roadmap for thymic epithelial cell development. Eur. J. Immunol. , 39, 1694-9. PMID: 19582736 DOI.

Rodewald HR. (2008). Thymus organogenesis. Annu. Rev. Immunol. , 26, 355-88. PMID: 18304000 DOI.

Nowell CS, Farley AM & Blackburn CC. (2007). Thymus organogenesis and development of the thymic stroma. Methods Mol. Biol. , 380, 125-62. PMID: 17876091 DOI.

Holländer G, Gill J, Zuklys S, Iwanami N, Liu C & Takahama Y. (2006). Cellular and molecular events during early thymus development. Immunol. Rev. , 209, 28-46. PMID: 16448532 DOI.


Itoi M, Tsukamoto N, Yoshida H & Amagai T. (2007). Mesenchymal cells are required for functional development of thymic epithelial cells. Int. Immunol. , 19, 953-64. PMID: 17625108 DOI.

Blackburn CC & Manley NR. (2004). Developing a new paradigm for thymus organogenesis. Nat. Rev. Immunol. , 4, 278-89. PMID: 15057786 DOI.

Rodewald HR, Paul S, Haller C, Bluethmann H & Blum C. (2001). Thymus medulla consisting of epithelial islets each derived from a single progenitor. Nature , 414, 763-8. PMID: 11742403 DOI.

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Historic Images

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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

Adult Histology


  • 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

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© Dr Mark Hill 2021, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G