Editing Thymus (section)

== Structure ==
The thymus is an organ that sits behind the [[Human sternum|sternum]] in the upper front part of the chest, stretching upwards towards the neck. In children, the thymus is pinkish-gray, soft, and lobulated on its surfaces.<ref name="Grays2008" /> At birth, it is about 4–6&nbsp;cm long, 2.5–5&nbsp;cm wide, and about 1&nbsp;cm thick.<ref name="Grays2016" /> It increases in size until puberty, where it may have a size of about 40–50&nbsp;g,<ref name="Robbins9th" /><ref name="Wheaters2013" /> following which it decreases in size in a process known as [[Involution (medicine)|involution]].<ref name="Wheaters2013" />

The thymus is located in the [[anterior mediastinum]].<ref>{{Cite journal |last1=Nasseri |first1=Farbod |last2=Eftekhari |first2=Farzin |date=March 2010 |title=Clinical and Radiologic Review of the Normal and Abnormal Thymus: Pearls and Pitfalls |url=http://pubs.rsna.org/doi/10.1148/rg.302095131 |journal=RadioGraphics |language=en |volume=30 |issue=2 |pages=413–428 |doi=10.1148/rg.302095131 |pmid=20228326 |issn=0271-5333}}</ref> It is made up of two lobes that meet in the upper midline, and stretch from below the [[thyroid]] in the neck to as low as the cartilage of the fourth rib.<ref name=Grays2008>{{cite book| editor-first1 = Susan | editor-last1 = Standring | editor-first2 = Neil R. | editor-last2 = Borley | name-list-style = vanc |title=Gray's Anatomy: The Anatomical Basis of Clinical Practice |date=2008|publisher=Churchill Livingstone|location=London|isbn=978-0-8089-2371-8|edition=40th|display-editors=1}}</ref> The lobes are covered by a capsule.<ref name="Robbins9th" /> The thymus lies behind the sternum, rests on the [[pericardium]], and is separated from the [[aortic arch]] and [[great vessels]] by a layer of [[fascia]]. The left [[brachiocephalic vein]] may even be embedded within the thymus.<ref name=Grays2008 /> In the neck, it lies on the front and sides of the [[vertebrate trachea|trachea]], behind the [[sternohyoid muscle|sternohyoid]] and [[sternothyroid muscle]]s.<ref name=Grays2008 />

===Microanatomy===
The thymus consists of two lobes, merged in the middle, surrounded by a capsule that extends with blood vessels into the interior.<ref name=Grays2016>{{cite book |title=Gray's Anatomy: The Anatomical Basis of Clinical Practice | editor-first1 = Susan | editor-last1 = Standring | editor-first2 = Henry | editor-last2 = Gray | name-list-style = vanc  |isbn= 9780702052309 |edition=41st |location=Philadelphia |oclc=920806541 |year=2016 |pages=983–6 }}</ref> The lobes consist of an outer {{wt|en|cortex}} rich with cells and an inner less dense {{wt|en|medulla}}.<ref name="Wheaters2013" /> The lobes are divided into smaller lobules 0.5-2 mm diameter, between which extrude radiating insertions from the capsule along {{wt|en|septa}}.<ref name=Grays2008 />

The cortex is mainly made up of [[thymocyte]]s and epithelial cells.<ref name="Robbins9th" /> The thymocytes, immature [[T cells]], are supported by a network of the finely-branched [[epithelial reticular cells]], which is continuous with a similar network in the medulla. This network forms an [[adventitia]] to the blood vessels, which enter the cortex via septa near the junction with the medulla.<ref name=Grays2008/> Other cells are also present in the thymus, including [[macrophage]]s, [[dendritic cell]]s, and a small amount of [[B cell]]s, [[neutrophil]]s and [[eosinophil]]s.<ref name="Robbins9th" />

In the medulla, the network of epithelial cells is coarser than in the cortex, and the lymphoid cells are relatively fewer in number.<ref name=Grays2008 /> Concentric, nest-like bodies called [[Hassall's corpuscles]] (also called ''thymic corpuscles'') are formed by aggregations of the medullary epithelial cells.<ref name="Robbins9th" /> These are concentric, layered whorls of [[epithelial cell]]s that increase in number throughout life.<ref name=Grays2008 /> They are the remains of the epithelial tubes, which grow out from the third [[pharyngeal pouch (embryology)|pharyngeal pouches]] of the embryo to form the thymus.<ref>{{cite book |vauthors = Larsen W |title=Human Embryology |date=2001 |publisher=Elsevier |isbn=978-0-443-06583-5 |pages=366–367 |edition=3rd}}</ref>

<gallery>
File:Thymus.JPG|[[Micrograph]] showing a lobule of the thymus. The cortex (deeper purple area) surrounds a less dense and lighter medulla.
File:Thymic corpuscle.jpg|Micrograph showing a Hassall's corpuscle, found within the medulla of the thymus.
</gallery>

===Blood and nerve supply===
The [[Artery|arteries]] supplying the thymus are branches of the [[internal thoracic artery|internal thoracic]], and [[inferior thyroid artery|inferior thyroid arteries]], with branches from the [[superior thyroid artery]] sometimes seen.<ref name="Grays2016" /> The branches reach the thymus and travel with the septa of the capsule into the area between the cortex and medulla, where they enter the thymus itself; or alternatively directly enter the capsule.<ref name="Grays2016" />

The [[vein]]s of the thymus, the [[thymic veins]], end in the [[left brachiocephalic vein]], [[internal thoracic vein]], and in the [[inferior thyroid vein]]s.<ref name="Grays2016" /> Sometimes the veins end directly in the superior vena cava.<ref name="Grays2016" />

[[Lymphatic vessel]]s travel only away from the thymus, accompanying the arteries and veins. These drain into the brachiocephalic, tracheobronchial and parasternal [[lymph node]]s.<ref name="Grays2016" />

The [[nerve]]s supplying the thymus arise from the [[vagus nerve]] and the cervical [[sympathetic chain]].<ref name="Grays2016" /> Branches from the [[phrenic nerve]]s reach the capsule of the thymus, but do not enter into the thymus itself.<ref name="Grays2016" />

===Variation===
The two lobes differ slightly in size, with the left lobe usually higher than the right. Thymic tissue may be found scattered on or around the gland, and occasionally within the thyroid.<ref name="Grays2016" /> The thymus in children stretches variably upwards, at times to as high as the thyroid gland.<ref name="Grays2016" />