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I have just started reading about the endocrine system and I am having some difficulty understanding the basis of distribution of glands and associated hormones.

I am using multifuntionality to describe the amount of relatively independent input and output that are shared through a same organ or proximal space. Many of these organs have a heterogenous mixture of endocrine cells sharing the same glandular space.

There appear to be some glands that are considered "master glands" such that they take a variety of relatively independent input, apply some control logic via a diverse set of endocrine cells and tissue and secrete a diverse hormonal output. The pituitary and hypothalamus are the best examples, however the adrenal cortex and the pancreas also fit this definition to a certain extent. Many times the diversity of hormones and cells within an organ have no obvious connection (eg thyroid). Some glands appear more specialized with a relatively straightforward endocrine axis of input and output (eg testes).

Some of this multifunctionality seems unnecessary, in that insulin secretion could occur anywhere and be effective in the manner of an insulin pump, or contain risks due to excessive centralization as in a pituitary tumor or renal artery stenosis, so there must be some advantages I do not understand.

My textbook (Headley, Levine, 6th) seems to minimally cover multiplexing, signal cross-talk and competition between diverse endocrine cells sharing an organ. Its focus on discrete pathways and axes suggests these are evolutionarily unfavorable.

Are there synergistic advantages in shared endocrine infrastructure such as vascularity or metabolic pathways?

Is there some overlying logic that can be used to explain this distribution of glands and hormones regarding multifuntionality-specialization, or is it necessary to view the distribution of vertebrate endocrine system as the outcome of evolutionary contingencies?

Thanks

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    $\begingroup$ In this text, Pituitary is referred to as master gland. But in one of the sessions that I participated, referred Thyroid gland and Pancreas as master glands. So I am curious to know the answer here myself. Are you referring to the shape of the particular gland by "Spoke-Like"? $\endgroup$
    – bonCodigo
    Jul 3, 2015 at 9:07
  • $\begingroup$ No I mean centralized as in diverse endocrine cells and tissue with diverse inputs and outputs that may be unrelated sharing a same organ or proximal space. The testes appear to me to be strictly male reproductive organs with a relatively straightforward endocrine axis of androgen production of primarily testosterone based on LH input, although a role of FSH in sperm production. $\endgroup$
    – hkjairam
    Jul 3, 2015 at 9:08
  • $\begingroup$ @WYSIWYG "they also affect the development of the physical and metabolic characteristics that are distinct for a sex" : Totally. Specially in terms of females, these glands (ovaries) seems to be even more crucial. Isn't it? $\endgroup$
    – bonCodigo
    Jul 3, 2015 at 9:40
  • $\begingroup$ @WYSIWYG I misspoke and meant strictly reproductive and endocrine. I then attempted to describe how that organ has a straightforward endocrine axis. Testosterone has a diverse set of outcomes in cells, however I refer to the number and diversity of testes outputs, which is primarily testosterone. Testosterone injections in castrated chickens maintains a relatively stable phenotype with regard to endocrine status. $\endgroup$
    – hkjairam
    Jul 3, 2015 at 9:54
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    $\begingroup$ @WYSIWYG. True, fixed the ducts. My connotation of "centralized" conveys the structural proximity of specialized cells with unrelated tasks, better in a more network-centric fashion, but that is my subjective opinion. I do not understand what you mean by "cellular university". $\endgroup$
    – hkjairam
    Jul 3, 2015 at 11:04

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It would seem to me that in the examples that you have listed that proximity to necessary input is the overriding logic behind gland geography. Take the hypothalamus as the first example. This gland receives input from diverse regions of the brain from the amygdala and hippocampus to the retina and brainstem. The brain can tweak levels of various releasing hormones entering the hypothalamic-hypophyseal portal veins in order to produce a response that is appropriate to the situation perceived by the brain. It makes sense to have all of these co-located because they are all receiving input from the brain. Similarly, it makes sense to have all of cells that are uptaking the releasing hormones close-by to protect these vital peptides from degradation in the systemic circulation. Their collectively unique circumstance makes sharing a specialized vasculature a logical choice.

The pancreas is a good example of the importance of cross-talk within some endocrine tissue. Within the islet, alpha, beta, and delta cells are feeding back on each other to get the balance of insulin and glucagon just right. To whit, glucagon secretion provides a paracrine signal for insulin secretion. This makes sense because the body knows that once glucagon is released the blood sugar will soon rise. Ergo, it makes sense to trigger some insulin secretion as well in preparation. Their co-location makes that possible.

In short, I would say the overriding logic to the placement of each of these glands is their proximity to the necessary inputs and outputs that each of these glands interacts with.

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  • $\begingroup$ Nice answer but one correction....The pancreas, although in proximity of the duodenum, does not sample fluid from the duodenum that is absorbed. Nutrients absorbed from the duodenum are mostly transported to the hepatic portal circulation and then back into the peripheral venous circulation via the hepatic veins. These nutrients then flow to the arterial circulation and then back to the pancreas. The liver may be a better example of being in a good 'geographic' location for your answer as it 'sees' all portal nutrient flow and can buffer nutrient levels in the peripheral blood. $\endgroup$
    – Vance L Albaugh
    Dec 12, 2016 at 15:42
  • $\begingroup$ Thanks for the catch. I vaguely remember learning in a physio class that elevated local levels of glucose in the ecf can stimulate glucose release, but I can't find a source for it other than my notes. Since it can't be sourced I've removed that point. I would add the liver case because it's a good example of the importance of geography, but I can't find a good link to why it's necessary for endocrine function per se. $\endgroup$
    – kingfishersfire
    Dec 12, 2016 at 16:06

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