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 | Pineal | Hypothalamus‎ | Pituitary | Thyroid | Parathyroid | Thymus‎ | Pancreas‎ | Adrenal‎ | Gonad‎ | Placenta‎ | Other Tissues | Stage 22 | Abnormalities | Hormones | Category:Endocrine
Historic Embryology - Endocrine  
1903 Islets of Langerhans | 1904 interstitial Cells | 1908 Pancreas Different Species | 1912 Suprarenal Bodies | 1914 Suprarenal Organs | 1915 Pharynx | 1916 Thyroid | 1918 Rabbit Hypophysis | 1920 Adrenal | 1935 Mammalian Hypophysis | 1926 Human Hypophysis | 1937 Pineal | 1938 Parathyroid | 1940 Adrenal | 1941 Thyroid | 1950 Thyroid Parathyroid Thymus | 1957 Adrenal
| Lecture - Head Development
Immune Links: Introduction | Blood | Spleen | Thymus | Lymphatic | Lymph Node | Antibody | Med Lecture - Lymphatic Structure | Med Practical | Immune Movies | Vaccination | Bacterial Infection | Abnormalities | Category:Immune
Historic Embryology  
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 development[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."
  • Thymus-associated parathyroid hormone has two cellular origins with distinct endocrine and immunological functions[2] "In mammals, parathyroid hormone (PTH) is a key regulator of extracellular calcium and inorganic phosphorus homeostasis. Although the parathyroid glands were thought to be the only source of PTH, extra-parathyroid PTH production in the thymus, which shares a common origin with parathyroids during organogenesis, has been proposed to provide an auxiliary source of PTH, resulting in a higher than expected survival rate for aparathyroid Gcm2⁻/⁻ mutants. However, the developmental ontogeny and cellular identity of these "thymic" PTH-expressing cells is unknown. ...Our data show conclusively that the thymus does not serve as an auxiliary source of either serum PTH or parathyroid function. We further show that the normal process of parathyroid organogenesis in both mice and humans leads to the generation of multiple small parathyroid clusters in addition to the main parathyroid glands, that are the likely source of physiologically relevant "thymic PTH."" Endocrine - Parathyroid Development
  • Decision checkpoints in the thymus[5]"The development of T cells in the thymus involves several differentiation and proliferation events, during which hematopoietic precursors give rise to T cells ready to respond to antigen stimulation and undergo effector differentiation."

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 Involution

A postnatal process defined as a decrease in the size, weight and activity of the gland with advancing age. In a recent review[6], 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.


  1. Stieda L (1881) Untersuchungen über die Entwickelung der Glandular Thymus, Glandular Thyreoidea, und Glandular carotidica. Leipzig, Engelmann p38.
  2. 2.0 2.1 Zhijie Liu, Alison Farley, Lizhen Chen, Beth J Kirby, Christopher S Kovacs, C Clare Blackburn, Nancy R Manley Thymus-associated parathyroid hormone has two cellular origins with distinct endocrine and immunological functions. PLoS Genet.: 2010, 6(12);e1001251 PubMed 21203493 | PLoS Genet.
  3. Renáta Mikušová, Veronika Mešťanová, Štefan Polák, Ivan Varga What do we know about the structure of human Thymic Hassall's corpuscles? A histochemical, immunohistochemical, and electron microscopic study. Ann. Anat.: 2017; PubMed 28279759
  4. Alison M Farley, Lucy X Morris, Eric Vroegindeweij, Marianne L G Depreter, Harsh Vaidya, Frances H Stenhouse, Simon R Tomlinson, Richard A Anderson, Tom Cupedo, Jan J Cornelissen, C Clare Blackburn Dynamics of thymus organogenesis and colonization in early human development. Development: 2013, 140(9);2015-26 PubMed 23571219
  5. Andrea C Carpenter, Rémy Bosselut Decision checkpoints in the thymus. Nat. Immunol.: 2010, 11(8);666-73 PubMed 20644572
  6. Victor Appay, Delphine Sauce, Martina Prelog The role of the thymus in immunosenescence: lessons from the study of thymectomized individuals. Aging (Albany NY): 2010, 2(2);78-81 PubMed 20354268


Graham Anderson, Eric J Jenkinson, Hans-Reimer Rodewald A roadmap for thymic epithelial cell development. Eur. J. Immunol.: 2009, 39(7);1694-9 PubMed 19582736

Hans-Reimer Rodewald Thymus organogenesis. Annu. Rev. Immunol.: 2008, 26;355-88 PubMed 18304000

Craig S Nowell, Alison M Farley, C Clare Blackburn Thymus organogenesis and development of the thymic stroma. Methods Mol. Biol.: 2007, 380;125-62 PubMed 17876091

Georg Holländer, Jason Gill, Saulius Zuklys, Norimasa Iwanami, Cunlan Liu, Yousuke Takahama Cellular and molecular events during early thymus development. Immunol. Rev.: 2006, 209;28-46 PubMed 16448532


Manami Itoi, Noriyuki Tsukamoto, Hisahiro Yoshida, Takashi Amagai Mesenchymal cells are required for functional development of thymic epithelial cells. Int. Immunol.: 2007, 19(8);953-64 PubMed 17625108

C Clare Blackburn, Nancy R Manley Developing a new paradigm for thymus organogenesis. Nat. Rev. Immunol.: 2004, 4(4);278-89 PubMed 15057786

H R Rodewald, S Paul, C Haller, H Bluethmann, C Blum Thymus medulla consisting of epithelial islets each derived from a single progenitor. Nature: 2001, 414(6865);763-8 PubMed 11742403

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Additional Images

Historic Images

Historic Disclaimer - information about historic embryology pages 
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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

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. 2017 Embryology Endocrine - Thymus Development. Retrieved August 23, 2017, from https://embryology.med.unsw.edu.au/embryology/index.php/Endocrine_-_Thymus_Development

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