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I read on my textbook that there is debate about whether thymus is an endocrine gland. But they do have glandular cells no? Is producing T Cells the only point for thinking it not as a part of endocrine system?

So, Q: What's the reason behind this debate and has it reached any conclusion?

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In my mind as an immunologist, there is no debate. In order to be part of the endocrine system, a body structure needs to be a gland that secretes one or more hormones to induce signaling at distal targets. These hormones can have multiple chemical forms. However, the thymus does not do that. It was proposed beginning back in the 1960s that the thymus contained and produced something called thymosin, which was eventually separated in 40 separate substances, none of which acts as a hormone, and most of which are now known to be present in many other tissues. T cell production and maturation occur as a result of signaling induced by cell-cell interactions (ligands and cognate receptors on the cell surface) as well as the action of many cytokines and chemokines. These compounds are not classified as hormones, though.

It is possible that some hormones might be expressed in the thymus in very small quantities; however they are not secreted. This is due to a part of T cell development and maturation known as negative selection, where the thymus can ectopically express proteins typically only found in other tissues, such as the nervous system, pancreas, or lung, for example. This process allows for the removal of potentially auto-reactive T cells, which recognize self antigen.

Basically, knowing what we do about the function of the thymus in human and mouse, and how it does what it does, I honestly don't see how someone today could classify it as part of the endocrine system, even though histologically it may appear similar to glandular structures, and small traces of one hormone or another may be found in thymic extracts.

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  • $\begingroup$ In short, yes the debate has reached a conclusion (years ago), and no the thymus is not an endocrine organ as it does not produce any real hormone. $\endgroup$ – Armatus Oct 16 '16 at 20:41
  • $\begingroup$ @Armatus you are absolutely correct. Now, we just need to find an image to replace the one in the Endocrine system article on Wikipedia... $\endgroup$ – MattDMo Oct 16 '16 at 20:49
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    $\begingroup$ Well, I went and did it myself. Hooray for Photoshop! $\endgroup$ – MattDMo Oct 16 '16 at 21:18

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