Editing Thymus (section)

==Development==
[[File:Gray1175.png|thumb|Scheme showing development of branchial epithelial bodies from the thoracic cavity of the fetus. I, II, III, IV. Branchial pouches.]]
The thymocytes and the epithelium of the thymus have different developmental origins.<ref name="Wheaters2013" /> The epithelium of the thymus develops first, appearing as two outgrowths, one on either side, of the third [[pharyngeal pouch (embryology)|pharyngeal pouch]].<ref name="Wheaters2013" /> It sometimes also involves the fourth pharyngeal pouch.<ref name="Robbins9th">{{cite book |title=Robbins and Cotran Pathologic Basis of Disease |isbn=9780323296397 |edition=9th (online)|section=Chapter 13. Diseases of White Blood Cells, Lymph Nodes, Spleen, and Thymus: Thymus.|last1=Kumar |first1=Vinay |last2=Abbas |first2=Abul K. |last3=Fausto |first3=Nelson |last4=Aster |first4=Jon C. | name-list-style = vanc |date=2014-08-27 |publisher=Elsevier Health Sciences }}</ref> These extend outward and backward into the surrounding [[mesoderm]] and [[neural crest]]-derived [[mesenchyme]] in front of the ventral [[aorta]]. Here the thymocytes and epithelium meet and join with connective tissue.  The [[pharynx|pharyngeal]] opening of each diverticulum is soon obliterated, but the neck of the flask persists for some time as a cellular cord.  By further proliferation of the cells lining the flask, buds of cells are formed, which become surrounded and isolated by the invading mesoderm.<ref>{{EmbryologySwiss|qblood/lymphat03}}</ref>

The epithelium forms fine lobules, and develops into a sponge-like structure. During this stage, [[hematopoietic]] bone-marrow precursors migrate into the thymus.<ref name="Wheaters2013" /> Normal development is dependent on the interaction between the epithelium and the hematopoietic [[thymocyte]]s. [[Iodine]] is also necessary for thymus development and activity.<ref>{{cite journal | vauthors = Venturi S, Venturi M | title = Iodine, thymus, and immunity | journal = Nutrition | volume = 25 | issue = 9 | pages = 977–9 | date = September 2009 | pmid = 19647627 | doi = 10.1016/j.nut.2009.06.002 }}</ref>

===Involution===
{{main|Thymic involution}}

The thymus continues to grow after birth reaching the relative maximum size by puberty.<ref name="Grays2016" /> It is most active in [[fetal]] and [[neonatal]] life.{{sfn|Davidson's|2018|p=67}} It increases to a mass of 20 to 50 grams by puberty.<ref name="Robbins9th" /> It then begins to decrease in size and activity in a process called [[thymic involution]].<ref name="Wheaters2013" /> After the first year of life the amount of T cells produced begins to fall.<ref name="Wheaters2013" /> Fat and connective tissue fills a part of the thymic volume.<ref name="Grays2016" />  During involution, the thymus decreases in size and activity.<ref name="Wheaters2013" /> Fat cells are present at birth, but increase in size and number markedly after puberty, invading the gland from the walls between the lobules first, then into the cortex and medulla.<ref name="Wheaters2013" /> This process continues into old age, where whether with a microscope or with the human eye, the thymus may be difficult to detect,<ref name=Wheaters2013>{{cite book |last1=Young |first1=Barbara |last2=O'Dowd |first2=Geraldine |last3=Woodford |first3=Phillip | name-list-style = vanc |title=Wheater's functional histology: a text and colour atlas.|publisher=Elsevier |location=Philadelphia |date=2013|isbn=9780702047473 |edition=6th|pages=204–6}}</ref> although typically weighs 5–15 grams.<ref name="Robbins9th" /> Additionally, there is an increasing body of evidence showing that age-related thymic involution is found in most, if not all, vertebrate species with a thymus, suggesting that this is an evolutionary process that has been conserved.<sup>[[doi:10.1016/j.it.2009.05.001|[40]]]</sup>

The atrophy is due to the increased circulating level of [[sex hormones]], and chemical or physical castration of an adult results in the thymus increasing in size and activity.<ref>{{cite journal | vauthors = Sutherland JS, Goldberg GL, Hammett MV, Uldrich AP, Berzins SP, Heng TS, Blazar BR, Millar JL, Malin MA, Chidgey AP, Boyd RL | display-authors = 6 | title = Activation of thymic regeneration in mice and humans following androgen blockade | journal = Journal of Immunology | volume = 175 | issue = 4 | pages = 2741–53 | date = August 2005 | pmid = 16081852 | doi = 10.4049/jimmunol.175.4.2741 | doi-access = free }}</ref> Severe illness or [[human immunodeficiency virus]] infection may also result in involution.<ref name="Robbins9th" />