Difference between revisions of "Hormone" - New World Encyclopedia

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[[Image:Adrenaline_chemical_structure.png|right|thumb|[[Epinephrine]] (adrenaline), a [[catecholamine]]-type hormone |200px]]
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[[Image:Thyroxine-2D-skeletal.png|right|thumb|200px|The chemical structure of ''thyroxine'', a hormone that stimulates the breakdown of glucose, fats, and proteins.]]  
A '''hormone''' (from [[Greek language|Greek]] ''όρμή'' - "to set in motion") is a [[chemical compound|chemical]] messenger from one [[cell (biology)|cell]] (or group of cells) to another. All [[multicellular organism]]s produce hormones (including [[Plant hormone|plants]] - ''see [[phytohormone]]'').
 
  
The function of hormones is to serve as a signal to the target cells; the action of hormones is determined by the pattern of secretion and the [[signal transduction]] of the receiving tissue.
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'''Hormones''' are chemical messengers secreted by [[cell (biology)|cells]] (including [[tissue]]s and [[organ (anatomy)|organs]]) in one part of a multicellular organism that travel to and coordinate the activities of different cells, providing a value to the whole organism. An enormous range of chemicals are used for this type of cell-to-cell communication, including [[peptide]]s (chains of [[amino acid]]s) and [[steroid]]s (a type of fat-soluble organic compound).
The best-known animal hormones are those produced by [[endocrine gland]]s of [[vertebrate]] animals, but hormones are produced by nearly every [[organ (anatomy)|organ]] ''system and'' [[Biological tissue|tissue]] type in a multicellular organism.  
 
  
[[endocrine system|Endocrine]] hormone [[molecule]]s are secreted (released) directly into the [[bloodstream]], while [[exocrine gland|exocrine hormones]] (or ectohormones) are secreted directly into a duct, and from the duct they either flow into the bloodstream or they flow from cell to cell by [[diffusion]] in a process known as [[paracrine signalling]].
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Hormones reveal a remarkable coordination in the body. Although produced by particular cells in one part of the body, they travel to another part of the body, impact target cells, and produce important functions. They only register on particular targets, not all cells, and the concentration of the hormones is tightly regulated.
  
Hormone effects vary widely, but can include:
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The term hormone (from the Greek “to spur on”) was first used by [[William Bayliss]] and [[Ernest Starling]] in 1904, to describe the action of [[secretin]]. Their research generated three key concepts:
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#Hormones are molecules synthesized by specific tissues ([[gland]]s).
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#They are secreted directly into the [[blood]], which carries them to their sites of action.
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#They specifically alter the activities of responsive cells (called "target cells"), which have receptors for the signaling molecules.
  
* stimulation or inhibition of growth,
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This traditional definition has been expanded to include similar regulatory molecules that are distributed by diffusion across cell membranes instead of by a [[circulatory system]]. For example, ''neurohormones,'' produced by [[neurosecretory]] cells primarily in the [[brain]], are distinguished from classical [[neurotransmitter]]s in that, acting as hormones, they are able to affect cells distant from their source.
* induction or suppression of [[apoptosis]] (programmed cell death)
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{{toc}}
* activation or inhibition of the [[immune system]]
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Although scientific research has focused on the function of hormones in vertebrates, hormones play important roles in other multicellular organisms. The [[insect]] hormone ''ecdysome,'' for example, triggers the [[metamorphosis]] of larvae to adults. Plants produce a variety of hormones involved in processes such as cell growth and differentiation ''([[auxin]]s)'', stem elongation ''(gibberellins)'', and fruit ripening ''(ethylene)''.
* regulating [[metabolism]]
 
* preparation for a new activity (e.g., [[fighting]], [[fleeing]], [[mating]])
 
* preparation for a new phase of life (e.g., [[puberty]], [[caring for offspring]], [[menopause]])
 
* controlling the [[reproductive cycle]]
 
  
In many cases, one hormone may regulate the production and release of other hormones
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==Overview==
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[[Image:Illu endocrine system.png|right|thumb|150px|The eight major endocrine glands in humans ([[male]], left; [[female]], right.) '''1.''' [[Pineal gland]] '''2.''' [[Pituitary gland]] '''3.''' [[Thyroid gland]] '''4.''' [[Thymus]] '''5.''' [[Adrenal gland]] '''6.''' [[Pancreas]] '''7.''' [[Ovary]] '''8.''' [[Testes]]]]
  
Many of the responses to hormone signals can be described as serving to [[homeostasis|regulate]] metabolic activity of an organ or tissue.  
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Broadly conceived, the role of hormones is to help maintain the [[homeostasis]] of a living organism: That is, to regulate its internal environment. Hormonal effects vary widely and may include:
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*Stimulation or inhibition of growth and development
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*Activation or inhibition of the [[immune system]]
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*Regulation of [[metabolism]] (the breakdown or synthesis of biological molecules)
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*Preparation for a new activity in response to environmental stimuli (such as, fighting, fleeing, mating)
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*Preparation for a new phase of life (for example, [[puberty]], caring for offspring, [[menopause]])
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*Control of the [[reproduction|reproductive cycle]].
  
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In [[vertebrate]]s, most hormones belong to the [[endocrine system]], a control system of ductless glands and single cells. In [[human]]s, there are eight main glands that generally are considered part of the endocrine system. Other organs of the body also produce and secrete hormones, but are generally not considered part of the endocrine system; these include the heart, kidney, liver, skin, and placenta. The endocrine system works in close relation with the [[nervous system]], and, as noted above, neurohormones are produced by specialized [[neuron]]s.
  
Hormonal signaling across this hierarchy involves the following:
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Systems involving hormones are so complex and finely tuned that some have speculated that it is irreducibly complex—that the system could not have evolved over time (and certainly not through the non-purposeful, non-progressive agency of [[natural selection]]) because all parts would have had to exist at the same time. However, other scientists have reported findings tracing the evolution of vertebrate steroid hormones from hundreds of millions of years ago, suggesting scenarios for such an [[evolution#Theory of descent with modification|evolution by common descent]] (Bridgham et al. 2006).  
#'''[[Protein biosynthesis|Biosynthesis]]''' of a particular hormone in a particular tissue.
 
#'''Storage and [[Cellular secretion|secretion]]''' of the hormone.  
 
#'''Transport''' of the hormone to the target cell(s).
 
#'''Recognition''' of the hormone by an [[membrane protein | associated cell membrane]] or [[intracellular]] [[receptor (biochemistry)|receptor]] protein.
 
#'''Relay and amplification''' of the received hormonal signal via a [[signal transduction]] process. This then leads to a cellular response. The reaction of the target cells may then be recognized by the original hormone-producing cells, leading to a [[down-regulation]] in hormone production. This is an example of a [[homeostasis | homeostatic]] [[negative feedback loop]].  
 
#'''Degradation''' of the hormone.
 
  
As can be inferred from the hierarchical diagram, hormone biosynthetic cells are typically of a specialized cell type, residing within a particular [[endocrine system | endocrine gland]] (e.g. the [[thyroid gland]], [[ovary|ovaries]] or [[testes]]). Hormones may exit their cell of origin via [[exocytosis]] or another means of [[cell membrane|membrane transport]].  
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Pollution represents a potentially serious problem. Some present-day chemical pollutants, such as PVC, can interfere with hormones. Humanity has a responsibility to carefully manage actions that might negatively impact the environment and disrupt systems that have been developed over many millions of years.
  
==Classification==
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==Signaling==
Most hormones initiate a cellular response by initially combining with either a specific [[intracellular]] or [[membrane protein| cell membrane associated]] [[receptor (biochemistry)|receptor]] protein. A cell may have several different receptors that recognize the same hormone and activate different [[signal transduction]] pathways, or alternatively different hormones and their receptors may invoke the same biochemical pathway.
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===Types of signaling===
 
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In [[animal]]s, there are three types of signaling by secreted molecules—[[endocrine]], [[paracrine]], and [[autocrine]]—based on the distance over which the signal acts.
For many hormones, including most [[protein hormone]]s, the receptor is membrane associated and embedded in the [[plasma membrane]] at the surface of the cell. The interaction of hormone and receptor typically triggers a cascade of secondary effects within the [[cytoplasm]] of the cell, often involving [[phosphorylation]] or dephosphorylation of various other cytoplasmic proteins, changes in [[ion channel]] permeability, or increased concentrations of intracellular molecules that may act as [[second messenger | secondary messengers]] (e.g. [[cyclic AMP]]). Some [[protein hormone]]s also interact with [[intracellular]] receptors located in the [[cytoplasm]] or [[nucleus]] by an [[intracrine]] mechanism.
 
  
For hormones such as [[Steroid hormone|steroid]] or [[Thyroid hormone|thyroid]] hormones, their receptors are located [[intracellular | intracellularly]] within the [[cytoplasm]] of their target cell. In order to bind their receptors these hormones must cross the cell membrane. The combined hormone-receptor [[protein complex | complex]] then moves across the nuclear membrane into the nucleus of the cell, where it binds to specific [[DNA sequences]], effectively amplifying or suppressing the action of certain [[genes]], and affecting [[protein synthesis]].<ref name=beato>{{cite journal | author =Beato M, Chavez S and Truss M | title = Transcriptional regulation by steroid hormones | journal = Steroids | year=1996 | volume=61 | issue=4 | pages=240-251 | id = PMID 8733009}}</ref> However, it has been shown that not all steriod receptors are located [[intracellular | intracellularly]], some are [[plasma membrane]] associated.<ref name=hammes>{{cite journal | author =Hammes SR | title = The further redefining of steroid-mediated signaling | journal = Proc Natl Acad Sci USA  | year=2003 | volume=100 | issue=5 | pages=21680-2170 | id = PMID 12606724}}</ref>
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Hormones belong to the first type: They act on target cells distant from their site of synthesis by cells of the endocrine organs. In animals, an endocrine hormone is usually carried by the blood from its site of release to the target cell.
  
[[Vertebrate]] hormones fall into three chemical classes:
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Paracrine signaling molecules only affect target cells in close proximity (an example is the conduction of an [[impulse]]), while autocrine cells respond to substances that they themselves release.
  
* [[Amine]]-derived hormones are derivatives of the [[amino acid]]s [[tyrosine]] and [[tryptophan]]. Examples are [[catecholamine]]s and [[thyroxine]].
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However, the designations above are not so clear-cut, as some compounds can participate in two or even three types of signaling. For example, certain small peptides (called ''neurohormones'') function both as neurotransmitters (paracrine signaling) and as hormones (endocrine signaling).
* [[Peptide hormone]]s consist of chains of amino acids. Examples of small peptide hormones are [[thyrotropin-releasing hormone|TRH]] and [[vasopressin]]. Peptides composed of scores or hundreds of amino acids are referred to as [[protein]]s. Examples of protein hormones include [[insulin]] and growth hormone. More complex protein hormones bear [[carbohydrate]] side chains and are called [[glycoprotein]] hormones. [[Luteinizing hormone]], [[follicle-stimulating hormone]] and [[thyroid-stimulating hormone]] are glycoprotein hormones.
 
* [[Lipid]] and [[phospholipid]]-derived hormones derive from lipids such as [[linoleic acid]] and [[arachidonic acid]] and phospholipids. The main classes are the [[steroid hormones]] that derive from [[cholesterol]] and the [[eicosanoid]]s. Examples of [[steroid hormones]] are [[testosterone]] and [[cortisol]]. [[Sterol]] hormones such as [[calcitriol]] are a [[homology (biology)|homologous]] system. The [[adrenal cortex]] and the [[gonad]]s are primary sources of steroid hormones. Examples of [[eicosanoid]]s are the widely studied [[prostaglandin]]s.
 
  
An important consideration, dictating the level at which cellular [[signal transduction]] pathways are activated in response to a hormonal signal is the effective [[concentration]] of hormone-receptor complexes that are formed. Hormone-receptor complex concentrations are effectively determined by three factors:
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===How hormones transmit signals===
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Hormonal signaling typically involves the following six steps:
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#'''[[Protein biosynthesis|Biosynthesis]]''' of the hormone in a specialized tissue.
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#'''Storage and [[Cellular secretion|secretion]]''' of the hormone.
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#'''Transport''' of the hormone to the target cell(s), often via the bloodstream.
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#'''Recognition''' of the hormone by an associated [[cell membrane]] or [[intracellular]] [[receptor (biochemistry)|receptor]] protein.
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#'''Relay and amplification''' of the received hormonal signal via a [[signal transduction]] process.
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#Removal of the signal, which often involves '''degradation''' of the hormone, to terminate the cellular response.
  
#The number of hormone molecules available for complex formation
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Once the hormone reaches the target cell, it binds to, or “fits,” a site on the receptor protein. Binding creates a ''ligand-receptor complex,'' causing a [[conformation]]al change (a change in the molecule's structural arrangement) that ultimately leads to a change in cellular function.
#The number of receptor molecules available for complex formation and
 
#The [[dissociation constant#protein ligand binding | binding affinity]] between hormone and receptor.
 
 
The number of hormone molecules available for complex formation is usually the key factor in determining the level at which [[signal transduction]] pathways are activated. The number of hormone molecules available being determined by the concentration of circulating hormone, which is in turn influenced by the level and rate at which they are secreted by biosynthetic cells. The number of receptors at the cell surface of the receiving cell can also be varied as can the [[dissociation constant#protein ligand binding | affinity]] between the hormone and its receptor.
 
  
==Regulation==
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Different cells respond differently to the same ligand. In addition, different ligand-receptor complexes can induce the same biochemical response in some cell types. For example, the hormones [[glucagon]] and [[epinephrine]] both stimulate increased [[glucose]] breakdown in liver cells.
Hormonal regulation of some physiological activities involves a hierarchy of cell types acting on each other either to stimulate or modulate the release and action of a particular hormone. The secretion of hormones from successive levels of [[endocrine system | endocrine cell]]s is stimulated by chemical signals originating from cells higher up the hierarchical system. The master coordinator of hormonal activity in [[mammal]]s is the [[hypothalamus]] acting on input it receives from the [[central nervous system]].
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Some hormones bind to receptors embedded in the [[plasma membrane]] at the surface of the cell, while others are able to interact with receptors inside the cell (either in the [[nucleus]] or the [[cytoplasm]]). The former require the aid of molecules called ''second messengers,'' such as [[cyclic AMP]], which convey the signal within the cell.
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==Major classes of vertebrate hormones and their function==
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Vertebrate hormones may be classified by their ''chemical make-up''. Alternatively, they may be grouped by their solubility and mode of action (i.e., whether they bind to intracellular receptors or to receptors on the cell surface).  
  
Other hormone secretion occurs in response to local conditions, such as the rate of secretion of [[parathyroid hormone]] by the [[parathyroid]] cells in response to fluctuations of ionized [[calcium]] levels in [[extracellular fluid]].
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According to this latter schema, there are three categories of vertebrate hormones:
  
However, the hierarchical model is an over simplification of the hormonal signaling process. Typically cellular recipients of a particular hormonal signal may be one of several cell types that reside within a number of different tissues, as is the case for [[insulin]], which triggers a diverse range of systemic physiological effects. Different tissue types may also respond differently to the same hormonal signal. Because of this, hormonal signaling is elaborate and hard to dissect.
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*Small ''lipophilic'' ([[lipid]]-soluble) molecules that are able to diffuse across the plasma membrane of the target cell and interact with intracellular receptors of the cytoplasm or nucleus. The resulting complexes bind to [[transcription]]-control regions in [[DNA]], affecting expression of specific genes. The [[steroid]] hormones and [[thyroxine]] are two examples of this type.
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*Lipophilic molecules that bind to cell-surface receptors, such as the [[eicosanoid]]s.
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*''Hydrophilic'' (water-soluble) molecules that bind to receptors on the cell surface because they cannot diffuse across cell membrane. There are two subgroups: (a) ''peptide hormones,'' such as [[insulin]], [[growth hormone]], and [[glucagon]], which range in size from a few amino acids to protein-size compounds; and (b) small charged molecules, such as epinephrine and [[histamine]], derived from amino acids, which function as both hormones and neurotransmitters.
  
Most cells are capable of producing one or more molecules, which act as signaling molecules to other cells, altering their growth, function, or metabolism. The classical hormones produced by [[endocrine gland]]s mentioned so far in this article are cellular products, specialized to serve as regulators at the overall organism level. However, they may also exert their effects solely within the tissue in which they are produced and originally released.
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=== Lipophilic molecules that diffuse across the plasma membrane===
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[[Cholesterol]] is an important precursor of the ''steroid hormones,'' which produce their physiological effects by binding to steroid hormone receptor proteins inside the [[cytoplasm]] of the cell. The combined hormone-receptor complex then moves into the nucleus of the cell, where it binds to specific DNA sequences, causing changes in [[gene]] transcription and cell function (Beato 1996). It has been shown, however, that some steroid receptors are membrane-associated rather than intracellular (Hamme, 2003).
  
The rate of hormone biosynthesis and secretion is often regulated by a [[Homeostasis|homeostatic]] [[negative feedback]] control mechanism. Such a mechanism depends on factors which influence the [[metabolism]] and [[excretion]] of hormones. Thus, higher hormome concentration alone cannot trigger the negative feedback mechanism. Negative feedback must be triggered by overproduction of an "effect" of the hormone.
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[[Image:Steroid-nomenclature.png|thumb|right|250px|Steroids, such as cholesterol and the steroid hormones, are characterized by a carbon skeleton with four fused rings. They are distinguished by the functional groups attached to the rings.]]
  
Hormone secretion can be stimulated and inhibited by:
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The five major classes of steroids are as follows:
*Other hormones (''stimulating''- or ''releasing''-hormones)
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*[[Androgens]] (such as [[testosterone]]) are responsible for the development of male secondary sex characteristics.
*Plasma concentrations of ions or nutrients, as well as binding [[globulin]]s
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*Glucocorticoids enable animals to respond to [[Stress (medical)|stress]]. They regulate many aspects of [[metabolism]] and [[immune system|immune function]], and are often prescribed by doctors to reduce [[inflammation|inflammatory conditions]] like [[asthma]] and [[arthritis]].
*[[Neuron]]s and mental activity
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*Mineralocorticoids help maintain blood volume and control [[kidney|renal]] excretion of electrolytes.
*Environmental changes, e.g., of light or temperature
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*[[Estrogen]]s and progestagens are two classes of sex steroids, a subset of the hormones that produce sex differences or support [[reproduction]].
  
In order to release active hormones quickly into the [[circulation]], hormone biosynthetic cells may produce and store biologically inactive hormones in the form of [[prehormone | pre-]] or [[prohormone | prohormones]]. These can then be quickly converted into their active hormone form in response to a particular stimulus.
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''Thyroxine,'' produced by [[thyroid]] cells, also binds to internal receptors. Thyroid hormones stimulate the breakdown of glucose, fats, and proteins by increasing the levels of many [[enzyme]]s that catalyze these metabolic reactions.
  
After a hormone has been secreted and has exerted its effect, its action must be terminated. This is accomplished by enzymes, which degrade, or breakdown, the hormone into metabolites. The metabolites are excreted along with bile and/or urine. Enzymes may be present in the blood or within the cell itself. Endocytosis of the receptor-hormone complex can also terminate hormone action. As noted above, the rate at which a hormone is broken down in the bloodstream is called the half life of the hormone. It is the amount of time needed to reduce the hormone concentration by one-half. This rate gives a measure of the period of time a hormone is active in the body.
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===Lipophilic molecules that bind to cell-surface receptors===
The endocrine system regulates hormone release and concentration through the negative feedback loop. Increases in hormone activity decrease the production and secretion of that hormone. Similarly, a decrease in activity of a hormone prompts an increase in the production and release of that hormone. The immune system as well as other factors contribute as control factors of hormone secretion. Together, these various mechanisms of control regulate the levels of hormones within the body.
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''Eicosanoids'' are 20-carbon [[fatty acid]]s derived from [[arachidonic acid]]; the group includes prostaglandins, prostacyclins, thromboxanes, and leukotrienes. Eicosanoids are considered local hormones because they are short-lived; they alter the activities in cells where they are synthesized (autocrine signaling) and in nearby cells (paracrine signaling). Prostaglandins may stimulate inflammation, regulate blood flow, control transport, and induce sleep. [[Aspirin]], for example, works  as an anti-inflammatory agent by inhibiting the synthesis of prostaglandin.
  
Peptide hormones are made and stored in vesicles within cells until they receive a signal for secretion. Initially, peptide hormones are made as preprohormones, which are inactive, and then are converted into prohormones. These prohormones are then cut into active hormones and peptide pieces, which are all secreted together. Because of their hydrophilic (water loving) nature, peptide hormones travel freely in the blood as they dissolve. They experience short half lives and tend to bind to surface cell receptors to initiate quick cellular responses. Peptide hormones can cause the synthesis of new proteins.
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=== Hydrophilic molecules that bind to cell-surface receptors===
Steroid hormones, on the other hand, are made on demand. Because they are derived from cholesterol, they are hydrophobic (water fearing) and tend to travel in the blood with protein carriers. Consequently, they have a longer half life. Receptors for steroid hormones are traditionally found inside the target cell. Responses include the turning on and off of genes and the direct synthesis of fresh proteins. Overall, the cell responses with steroid hormones are slower than those with peptide hormones.
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*[[Peptide hormone]]s are composed of amino-acid chains. Examples of small peptide hormones are [[thyrotropin-releasing hormone|TRH]] and [[vasopressin]]. Peptides composed of scores or hundreds of amino acids, such as [[insulin]] and growth hormone, are referred to as [[protein]] hormones. More complex protein hormones bear [[carbohydrate]] side chains and are called [[glycoprotein]] hormones. [[Luteinizing hormone]], [[follicle-stimulating hormone]], and [[thyroid-stimulating hormone]] are glycoprotein hormones.
  
==Hormone effects==
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[[Image:Adrenaline_chemical_structure.png|right|thumb|The structure of [[epinephrine]] (adrenaline), a neurohormone involved in stimulating the "fight-or-flight" response.|150px]]
One special group of hormones is the [[trophic hormone]]s that stimulate the hormone production of other [[endocrine system|endocrine glands]].  For example, [[thyroid-stimulating hormone]] (TSH) causes growth and increased activity of another endocrine gland, the [[thyroid]], which increases output of [[thyroid hormone]]s.
 
  
A recently-identified class of hormones is that of the "hunger hormones" - [[ghrelin]], [[orexin]] and [[PYY 3-36]] - and "satiety hormones" - e.g., [[leptin]], [[obestatin]], [[nesfatin-1]].
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*Some water-soluble hormones are derived from a single amino acid. ''Histamine'', a hormone and [[neurotransmitter]] derived from the amino acid histidine, is involved in the dilation of blood vessels. The ''catecholamines'', chemical compounds derived from the [[amino acid]] [[tyrosine]], may act as neurohormones. The most abundant catecholamines are [[epinephrine]] (adrenaline), [[norepinephrine]] (noradrenaline), and [[dopamine]]. Released by the [[adrenal glands]] in situations of stress, catecholamines cause general physiological changes that prepare the body for physical activity (the fight-or-flight response). Some typical effects are increases in heart rate, [[blood pressure]], [[blood glucose]] levels, and a general reaction of the [[sympathetic nervous system]].
  
==Plant hormones==
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The table below provides some examples of water-soluble hormones that bind to cell-surface receptors. The size of the hormone is given in amino acids (note that some hormones have two polypeptide chains of varying lengths, which are designated either A and B or alpha and beta).
'''Plant hormones''' (or '''plant growth regulators''', or '''PGRs''') are internally-secreted chemicals in [[plant]]s that are used for regulating the plants' growth. According to a standard definition, plant hormones are signal molecules produced at specific locations, that occur in very low concentrations, and cause altered processes in target cells at other locations.
 
  
It is accepted that there are five major classes of plant hormones:
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{| class="wikitable" class="wikitable"
#[[abscisic acid]] (ABA)
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|  | Type  || Name || Size  ||  Origin || Major effects
#[[auxin]]s
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|-
#[[cytokinin]]s(CKs)
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| Peptide  || [[Follicle-stimulating hormone]] (FSH) ||  alpha: 92, beta: 118 ||  anterior pituitary  || Stimulates growth of oocytes and ovarian follices
#[[ethylene]]
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|-
#[[gibberellin]]s (GAs)
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| Peptide  || [[Glucagon]] || 29 || pancreas alpha cells ||  Stimulates glucose synthesis
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|-
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|  Peptide || [[Insulin]]  || A: 21, B: 30 || pancreas beta cells || Regulates glucose uptake; stimulates cell proliferation
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|-
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| Peptide  || [[Luteinizing hormone]] (LH) || 10, beta chain 115 || anterior pituitary  || maturation of oocyte; stimulates estrogen and progesterone secretion by ovarian follices
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|-
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| Growth factor  || nerve growth factor (NGF)  ||  118  || all tissues innervated by sympathetic neurons || growth and differentiation of sympathetic neurons
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|-
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| Growth factor  || Epidermal growth factor (EGF) || 53 || salivary and other glands? || Growth of epidermal and other body cells
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|-
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| Growth factor  || Platelet-derived growth factor || A: 125, B: 109  || platelets and cells in many other tissues  || Proliferation of fibroblasts and other cell types; wound healing 
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|-
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| Neurohormone  || [[Oxytocin]] || 9 || posterior pituitary gland || Stimulation of smooth muscle contraction 
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|-
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| Neurohormone  || [[Vasopressin]] || 9  || posterior pituitary gland  || Stimulation of water reabsorption in the kidney
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|-
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|}
  
Non-traditional plant hormones include
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==Regulation==
#[[salicylic acid]]
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===By rate of synthesis===
#[[jasmonates]]
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Organisms must be able to respond instantly to many changes in their internal or external environment; such rapid responses are mediated primarily by '''peptide hormones''' and '''catecholamines.''' Signaling cells that produce them store these hormones in secretory vesicles just under the [[plasma membrane]]. All peptide hormones, including [[insulin], are synthesized as part of a longer ''propolypeptide,'' which is cleaved (split) by specific [[enzymes]] to generate the active molecule just after it is transported to a secretory vesicle. Because of their hydrophilic (water-loving) nature, peptide hormones travel freely in the blood as they dissolve. Peptide hormones mediate short responses that are terminated by their own breakdown.
#[[oligosaccharins]]
 
#[[brassinolides]]
 
#[[small extracellular signalling peptides]]  
 
  
*[http://www.plant-hormones.info/ Excellent plant hormone site on the Web.]
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In contrast, '''steroid-producing cells,''' like those in the [[adrenal gland|adrenal cortex]], store only a small supply of hormone precursor; when stimulated, they are converted to active hormone, which then diffuses across the [[cell membrane]] into the blood. Because cells store little of the active hormone, release takes from hours to days. Steroids are hydrophobic (water-fearing), so they are transported by carrier proteins, and are not rapidly degraded. Thus, responses to thyroxine and steroid hormones take awhile to occur but effects last much longer than those triggered by peptide hormones.
  
==The endocrine system==
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===By feedback control===
text
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The rate of hormone biosynthesis and secretion is often regulated by feedback circuits, in which changes in the level of one hormone affect the levels of other hormones. This type of control is especially important in coordinating the complex processes of cell growth and differentiation.
  
==section on challenges to classical definition of hormones?==
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===By other hormones===
A '''neurohormone''' is any [[hormone]] produced by [[neurosecretory]] [[cells]], usually in the [[brain]]. Neurohormonal activity is distinguished from that of classical [[neurotransmitters]] as it can have effects on cells distant from the source of the hormone.
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The [[trophic hormone]]s are a special class of hormones that stimulate the hormonal production of [[endocrine system|endocrine glands]]. For example, [[thyroid-stimulating hormone]] (TSH) causes growth and increased activity of the thyroid, which in turn increases output of [[thyroid hormone]]s.
  
'''Neuroendocrine''' cells are a specialized group of nerve cells ([[neurons]]) that produce [[hormones]]. These hormones may be [[amines]], [[neuropeptides]], or specialized [[amino acids]]. They package the hormones in [[vesicle (biology)|vesicles]] and send these packages via long processes ([[axon|axons]]) to blood vessels. When stimulated (by [[hormones]] from the blood stream or other neurons) the neuroendocrine cells secrete the hormones into the blood stream. The hormones then travel to their target cells and may stimulate, inhibit or maintain function of these cells. The [[target cells]] may feed back information to these neurons that regulates further secretion.
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==Plant hormones==
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[[Image:IAAII.png|thumb|right|175px|A member of the auxin family, the first group of plant hormones discovered.]]
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[[Plant hormones]] are internally secreted molecules that typically coordinate the responses of plant tissues to environmental signals, such as light or infection.  
  
Specialized groups of neuroendocrine cells can be found at the base of the [[third ventricle]] in the brain (in a region called the [[hypothalamus]]). This area controls most anterior [[pituitary]] cells and thereby regulates functions in the entire body, like responses to stress, cold, sleep, and the reproductive system. The neurons send processes to a region connecting to the pituitary stalk and the hormones (called releasing or [[inhibiting hormones]]) are released into the blood stream. They are carried by portal vessels to the pituitary cells where they may stimulate, inhibit, or maintain the function of a particular cell type. Many of the projects in the neuroendocrine group focus on the regulatory circuitry in this pathway.
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Plant hormones are traditionally divided into five major groups, although several additional plant hormones have recently been discovered:
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*[[Auxin]] was the first plant hormone to be identified; early experiments leading to its discovery were conducted by [[Charles Darwin]] in the 1880s. Auxins regulate various aspects of plant development, including cell division and differentiation.
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*[[Abscisic acid]] (ABA) was originally thought to play a major role in the abscission (shedding) of fruits and in bud dormancy. Some of its confirmed effects (which are mostly inhibitory) include stimulating the closure of [[stomata]] in dry conditions, inhibiting shoot growth, and inducing seeds to synthesize proteins.
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*[[Cytokinin]]s (CKs) are active in promoting [[cell division]], and are also involved in cell growth and differentiation.  
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*[[Ethylene]] acts at trace levels throughout the life of the plant by stimulating or regulating the ripening of [[fruit]], the opening of [[flower]]s, and the shedding of [[leaves]].
 +
*[[Gibberellin]]s (GA) are involved in promoting [[stem elongation]] and mobilizing [[food]] reserves in [[seed|seeds]]. The absence of gibberellins results in the [[dwarfism]] of some plant varieties.
  
The endocrine system works in close relation with the nervous system. It links the brain to the organs that control various aspects of the body. In addition, neurohormones are released by specialized groups of neurons in the brain. These function similarly to hormones and are often categorized into three major groups: catecholamines; hypothalamic neurohormones that monitor hormone release from the anterior pituitary; and hypothalamic neurohormones that monitor hormone release from the posterior pituitary. Neuroendocrinology is an area of medicine that focuses on the overlapping fields between the nervous and endocrine systems.
+
Non-traditional plant hormones include [[brassinolide]], a plant-specific ''steroid hormone'' involved in developmental processes.
  
==Pharmacology and cell bio research==
+
==The role of hormones in pharmacology==
Many hormones and their [[analog (chemistry)| analogue]]s are used as [[medication]]. The most commonly-prescribed hormones are [[estrogen]]s and [[progestagen]]s (as methods of [[hormonal contraception]] and as [[Hormone replacement therapy|HRT]]), [[thyroxine]] (as [[levothyroxine]], for [[hypothyroidism]]) and [[steroid]]s (for [[autoimmune disease]]s and several [[pulmonology|respiratory disorders]]). [[Insulin]] is used by many [[diabetes mellitus|diabetics]]. Local preparations for use in [[otolaryngology]] often contain [[pharmacology|pharmacologic]] equivalents of [[adrenaline]], while [[steroid]] and [[vitamin D]] creams are used extensively in [[dermatology|dermatological]] practice.
+
Many hormones and their [[analog (chemistry)|analogue]]s are used as [[medication]]:
 +
*The most commonly prescribed hormones are estrogens and progestagens (as methods of [[hormonal contraception]] and as [[hormone replacement therapy]]); other steroids (for [[autoimmune disease]]s and several [[pulmonology|respiratory disorders]]); and thyroxine (as [[levothyroxine]], for [[hypothyroidism]]).
 +
*[[Insulin]] is used by many [[diabetes mellitus|diabetics]].  
 +
*Local preparations for use in [[otolaryngology]] (the diagnosis and treatment of ear, nose, and throat disorders) often contain [[pharmacology|pharmacological]] equivalents of [[adrenaline]].
 +
*[[Steroid]] and [[vitamin D]] creams are used extensively in [[dermatology|dermatological]] practice.
  
A "pharmacologic dose" of a hormone is a medical usage referring to an amount of a hormone far greater than naturally occurs in a healthy body. The effects of pharmacologic doses of hormones may be different from responses to naturally occurring amounts and may be therapeutically useful. An example is the ability of pharmacologic doses of [[glucocorticoid]] to suppress [[inflammation]].
+
A "pharmacological dose" of a hormone is a medical usage referring to an amount of a hormone far greater than its natural occurrence in a healthy body. The effects of pharmacological doses of hormones may be different from responses to naturally occurring amounts and may be therapeutically useful. An example is the ability of pharmacological doses of [[glucocorticoid]] to suppress [[inflammation]].
  
==Important human hormones==
+
==Table of important human hormones==
[retain table?]
+
Spelling is not uniform for many hormones. For example, current North American and international usage is ''estrogen'' and ''gonadotropin,'' while British usage retains the Greek [[diphthong]] in ''oestrogen'' and the unvoiced aspirant h in ''gonadotrophin''.
Spelling is not uniform for many hormones. For example, current North American and international usage is estrogen, gonadotropin, while British usage retains the Greek [[diphthong]] in oestrogen and the unvoiced aspirant h in gonadotrophin.
 
  
 
{| class="wikitable" class="sortable wikitable"  
 
{| class="wikitable" class="sortable wikitable"  
Line 225: Line 252:
 
  | steroid - sex ([[estrogen|est]]) || [[Estrone]]  ||  || [[ovary]]  || [[granulosa cells]] || direct
 
  | steroid - sex ([[estrogen|est]]) || [[Estrone]]  ||  || [[ovary]]  || [[granulosa cells]] || direct
 
  |-  
 
  |-  
  | steroid - sex ([[estriol|est]]) || [[Estriol]] ||  || [[placenta]] || [[syncytiotrophoblast]] || direct
+
  | steroid - sex ([[estriol|est]]) || Estriol  ||  || [[placenta]] || [[syncytiotrophoblast]] || direct
 
  |-  
 
  |-  
 
  | steroid - sex ([[Progestagen|pro]]) || [[Progesterone]]  ||  || [[ovary]], [[adrenal glands]], [[placenta]]  || [[granulosa cells]] || direct
 
  | steroid - sex ([[Progestagen|pro]]) || [[Progesterone]]  ||  || [[ovary]], [[adrenal glands]], [[placenta]]  || [[granulosa cells]] || direct
Line 240: Line 267:
  
 
|}
 
|}
 +
<small>Code:
 +
*(and) = "Androgen is the generic term for any natural or synthetic compound, usually a steroid hormone, that stimulates or controls the development and maintenance of masculine characteristics in vertebrates by binding to androgen receptors."
 +
*(cat) = "Catecholamines are chemical compounds derived from the amino acid tyrosine containing catechol and amine groups."
 +
*(est) = "Estorgen, a group of steroid compounds, named for their importance in the estrous cycle, and functioning as the primary female sex hormone."
 +
*(pro) = "Progestagens (also spelled progestogens or gestagens) are hormones which produce effects similar to progesterone, the only natural progestagen." </small>
  
 
==References==
 
==References==
* Beato, M., Chavez, S., and M. Truss. 1996. Transcriptional regulation by steroid hormones. ''Steroids'' 61(4): 240-251. PMID 8733009
+
* Beato, M., S. Chavez, and M. Truss. 1996. Transcriptional regulation by steroid hormones. ''Steroids'' 61(4): 240-251.  
* Cooper, G. M., and R. E. Hausman. 2004. ''The Cell: A Molecular Approach,'' 3rd edition. Washington, D.C.: ASM Press & Sunderland, M.A.: Sinauer Associates. ISBN 0878932143  
+
* Bridgham, J. T., S. M. Carroll, and J. W. Thornton. 2006. Evolution of Hormone-Receptor Complexity by Molecular Exploitation. ''Science'' 312: 97-101.
* Hammes, S.R. 2003. The further redefining of steroid-mediated signaling. ''Proc Natl Acad Sci'' 100(5): 2168-70 PMID 12606724
+
* Cooper, G. M., and R. E. Hausman. 2004. ''The Cell: A Molecular Approach.'' Washington, D.C.: ASM Press. ISBN 0878932143
* Lodish, H., D. Baltimore, A. Berk, S. L. Zipursky, P. Matsudaira, and J. Darnell. 1995. ''Molecular Cell Biology'', 3rd edition. New York, NY: Scientific American Books. ISBN 0716723808.
+
* Hammes, S. R. 2003. The further redefining of steroid-mediated signaling. ''Proc Natl Acad Sci'' 100(5): 2168-70.
* Mathews, C.K. and K.E. van Holde. 1990. ''Biochemistry.'' San Francisco: Benjamin-Cummings. ISBN 0805350152
+
* Lodish, H., D. Baltimore, A. Berk, S. L. Zipursky, P. Matsudaira, and J. Darnell. 1995. ''Molecular Cell Biology.'' New York: Scientific American Books. ISBN 0716723808
* Stryer, L. 1995. ''Biochemistry'', 4th edition. New York: W.H. Freeman. ISBN 0716720094
+
* Mathews, C. K. and K. E. van Holde. 1990. ''Biochemistry.'' San Francisco: Benjamin-Cummings. ISBN 0805350152
 
+
* Stryer, L. 1995. ''Biochemistry,'' New York: W.H. Freeman. ISBN 0716720094
==See also==
 
* [[Endocrinology]]
 
* [[Endocrine system]]
 
* [[Neuroendocrinology]]
 
* [[Plant hormones]] or [[plant growth regulators]]
 
* [[Autocrine signaling]]
 
* [[Paracrine signaling]]
 
* [[Intracrine]]
 
* [[Cytokine]]
 
* [[Growth factor]]
 
* [[Hormone disruptor]]
 
  
==External links==
 
* {{MeshName|Hormones}}
 
* [http://hormone-imbalance.org Hormone Imbalance]
 
* [http://video.google.com/videoplay?docid=2730429945740914542&q=living+smart%3A+Hormones+and+Nutrition&hl=en Hormones and Nutrition]
 
  
 
{{Hormones}}
 
{{Hormones}}
{{credit|135430677}}
+
{{credit|Hormone|135430677|Catecholamine|138682679|Androgen|134550683|Estrogen|138643378|Progestagen|133282186|Plant_hormone|133717786|Thyroxine|137492983|Histamine|138341253}}
 
[[Category:Life sciences]]
 
[[Category:Life sciences]]
 +
[[Category:Biochemistry]]

Latest revision as of 15:30, 6 March 2014

The chemical structure of thyroxine, a hormone that stimulates the breakdown of glucose, fats, and proteins.

Hormones are chemical messengers secreted by cells (including tissues and organs) in one part of a multicellular organism that travel to and coordinate the activities of different cells, providing a value to the whole organism. An enormous range of chemicals are used for this type of cell-to-cell communication, including peptides (chains of amino acids) and steroids (a type of fat-soluble organic compound).

Hormones reveal a remarkable coordination in the body. Although produced by particular cells in one part of the body, they travel to another part of the body, impact target cells, and produce important functions. They only register on particular targets, not all cells, and the concentration of the hormones is tightly regulated.

The term hormone (from the Greek “to spur on”) was first used by William Bayliss and Ernest Starling in 1904, to describe the action of secretin. Their research generated three key concepts:

  1. Hormones are molecules synthesized by specific tissues (glands).
  2. They are secreted directly into the blood, which carries them to their sites of action.
  3. They specifically alter the activities of responsive cells (called "target cells"), which have receptors for the signaling molecules.

This traditional definition has been expanded to include similar regulatory molecules that are distributed by diffusion across cell membranes instead of by a circulatory system. For example, neurohormones, produced by neurosecretory cells primarily in the brain, are distinguished from classical neurotransmitters in that, acting as hormones, they are able to affect cells distant from their source.

Although scientific research has focused on the function of hormones in vertebrates, hormones play important roles in other multicellular organisms. The insect hormone ecdysome, for example, triggers the metamorphosis of larvae to adults. Plants produce a variety of hormones involved in processes such as cell growth and differentiation (auxins), stem elongation (gibberellins), and fruit ripening (ethylene).

Overview

The eight major endocrine glands in humans (male, left; female, right.) 1. Pineal gland 2. Pituitary gland 3. Thyroid gland 4. Thymus 5. Adrenal gland 6. Pancreas 7. Ovary 8. Testes

Broadly conceived, the role of hormones is to help maintain the homeostasis of a living organism: That is, to regulate its internal environment. Hormonal effects vary widely and may include:

  • Stimulation or inhibition of growth and development
  • Activation or inhibition of the immune system
  • Regulation of metabolism (the breakdown or synthesis of biological molecules)
  • Preparation for a new activity in response to environmental stimuli (such as, fighting, fleeing, mating)
  • Preparation for a new phase of life (for example, puberty, caring for offspring, menopause)
  • Control of the reproductive cycle.

In vertebrates, most hormones belong to the endocrine system, a control system of ductless glands and single cells. In humans, there are eight main glands that generally are considered part of the endocrine system. Other organs of the body also produce and secrete hormones, but are generally not considered part of the endocrine system; these include the heart, kidney, liver, skin, and placenta. The endocrine system works in close relation with the nervous system, and, as noted above, neurohormones are produced by specialized neurons.

Systems involving hormones are so complex and finely tuned that some have speculated that it is irreducibly complex—that the system could not have evolved over time (and certainly not through the non-purposeful, non-progressive agency of natural selection) because all parts would have had to exist at the same time. However, other scientists have reported findings tracing the evolution of vertebrate steroid hormones from hundreds of millions of years ago, suggesting scenarios for such an evolution by common descent (Bridgham et al. 2006).

Pollution represents a potentially serious problem. Some present-day chemical pollutants, such as PVC, can interfere with hormones. Humanity has a responsibility to carefully manage actions that might negatively impact the environment and disrupt systems that have been developed over many millions of years.

Signaling

Types of signaling

In animals, there are three types of signaling by secreted molecules—endocrine, paracrine, and autocrine—based on the distance over which the signal acts.

Hormones belong to the first type: They act on target cells distant from their site of synthesis by cells of the endocrine organs. In animals, an endocrine hormone is usually carried by the blood from its site of release to the target cell.

Paracrine signaling molecules only affect target cells in close proximity (an example is the conduction of an impulse), while autocrine cells respond to substances that they themselves release.

However, the designations above are not so clear-cut, as some compounds can participate in two or even three types of signaling. For example, certain small peptides (called neurohormones) function both as neurotransmitters (paracrine signaling) and as hormones (endocrine signaling).

How hormones transmit signals

Hormonal signaling typically involves the following six steps:

  1. Biosynthesis of the hormone in a specialized tissue.
  2. Storage and secretion of the hormone.
  3. Transport of the hormone to the target cell(s), often via the bloodstream.
  4. Recognition of the hormone by an associated cell membrane or intracellular receptor protein.
  5. Relay and amplification of the received hormonal signal via a signal transduction process.
  6. Removal of the signal, which often involves degradation of the hormone, to terminate the cellular response.

Once the hormone reaches the target cell, it binds to, or “fits,” a site on the receptor protein. Binding creates a ligand-receptor complex, causing a conformational change (a change in the molecule's structural arrangement) that ultimately leads to a change in cellular function.

Different cells respond differently to the same ligand. In addition, different ligand-receptor complexes can induce the same biochemical response in some cell types. For example, the hormones glucagon and epinephrine both stimulate increased glucose breakdown in liver cells.

Some hormones bind to receptors embedded in the plasma membrane at the surface of the cell, while others are able to interact with receptors inside the cell (either in the nucleus or the cytoplasm). The former require the aid of molecules called second messengers, such as cyclic AMP, which convey the signal within the cell.

Major classes of vertebrate hormones and their function

Vertebrate hormones may be classified by their chemical make-up. Alternatively, they may be grouped by their solubility and mode of action (i.e., whether they bind to intracellular receptors or to receptors on the cell surface).

According to this latter schema, there are three categories of vertebrate hormones:

  • Small lipophilic (lipid-soluble) molecules that are able to diffuse across the plasma membrane of the target cell and interact with intracellular receptors of the cytoplasm or nucleus. The resulting complexes bind to transcription-control regions in DNA, affecting expression of specific genes. The steroid hormones and thyroxine are two examples of this type.
  • Lipophilic molecules that bind to cell-surface receptors, such as the eicosanoids.
  • Hydrophilic (water-soluble) molecules that bind to receptors on the cell surface because they cannot diffuse across cell membrane. There are two subgroups: (a) peptide hormones, such as insulin, growth hormone, and glucagon, which range in size from a few amino acids to protein-size compounds; and (b) small charged molecules, such as epinephrine and histamine, derived from amino acids, which function as both hormones and neurotransmitters.

Lipophilic molecules that diffuse across the plasma membrane

Cholesterol is an important precursor of the steroid hormones, which produce their physiological effects by binding to steroid hormone receptor proteins inside the cytoplasm of the cell. The combined hormone-receptor complex then moves into the nucleus of the cell, where it binds to specific DNA sequences, causing changes in gene transcription and cell function (Beato 1996). It has been shown, however, that some steroid receptors are membrane-associated rather than intracellular (Hamme, 2003).

Steroids, such as cholesterol and the steroid hormones, are characterized by a carbon skeleton with four fused rings. They are distinguished by the functional groups attached to the rings.

The five major classes of steroids are as follows:

Thyroxine, produced by thyroid cells, also binds to internal receptors. Thyroid hormones stimulate the breakdown of glucose, fats, and proteins by increasing the levels of many enzymes that catalyze these metabolic reactions.

Lipophilic molecules that bind to cell-surface receptors

Eicosanoids are 20-carbon fatty acids derived from arachidonic acid; the group includes prostaglandins, prostacyclins, thromboxanes, and leukotrienes. Eicosanoids are considered local hormones because they are short-lived; they alter the activities in cells where they are synthesized (autocrine signaling) and in nearby cells (paracrine signaling). Prostaglandins may stimulate inflammation, regulate blood flow, control transport, and induce sleep. Aspirin, for example, works as an anti-inflammatory agent by inhibiting the synthesis of prostaglandin.

Hydrophilic molecules that bind to cell-surface receptors

  • Peptide hormones are composed of amino-acid chains. Examples of small peptide hormones are TRH and vasopressin. Peptides composed of scores or hundreds of amino acids, such as insulin and growth hormone, are referred to as protein hormones. More complex protein hormones bear carbohydrate side chains and are called glycoprotein hormones. Luteinizing hormone, follicle-stimulating hormone, and thyroid-stimulating hormone are glycoprotein hormones.
The structure of epinephrine (adrenaline), a neurohormone involved in stimulating the "fight-or-flight" response.
  • Some water-soluble hormones are derived from a single amino acid. Histamine, a hormone and neurotransmitter derived from the amino acid histidine, is involved in the dilation of blood vessels. The catecholamines, chemical compounds derived from the amino acid tyrosine, may act as neurohormones. The most abundant catecholamines are epinephrine (adrenaline), norepinephrine (noradrenaline), and dopamine. Released by the adrenal glands in situations of stress, catecholamines cause general physiological changes that prepare the body for physical activity (the fight-or-flight response). Some typical effects are increases in heart rate, blood pressure, blood glucose levels, and a general reaction of the sympathetic nervous system.

The table below provides some examples of water-soluble hormones that bind to cell-surface receptors. The size of the hormone is given in amino acids (note that some hormones have two polypeptide chains of varying lengths, which are designated either A and B or alpha and beta).

Type Name Size Origin Major effects
Peptide Follicle-stimulating hormone (FSH) alpha: 92, beta: 118 anterior pituitary Stimulates growth of oocytes and ovarian follices
Peptide Glucagon 29 pancreas alpha cells Stimulates glucose synthesis
Peptide Insulin A: 21, B: 30 pancreas beta cells Regulates glucose uptake; stimulates cell proliferation
Peptide Luteinizing hormone (LH) 10, beta chain 115 anterior pituitary maturation of oocyte; stimulates estrogen and progesterone secretion by ovarian follices
Growth factor nerve growth factor (NGF) 118 all tissues innervated by sympathetic neurons growth and differentiation of sympathetic neurons
Growth factor Epidermal growth factor (EGF) 53 salivary and other glands? Growth of epidermal and other body cells
Growth factor Platelet-derived growth factor A: 125, B: 109 platelets and cells in many other tissues Proliferation of fibroblasts and other cell types; wound healing
Neurohormone Oxytocin 9 posterior pituitary gland Stimulation of smooth muscle contraction
Neurohormone Vasopressin 9 posterior pituitary gland Stimulation of water reabsorption in the kidney

Regulation

By rate of synthesis

Organisms must be able to respond instantly to many changes in their internal or external environment; such rapid responses are mediated primarily by peptide hormones and catecholamines. Signaling cells that produce them store these hormones in secretory vesicles just under the plasma membrane. All peptide hormones, including [[insulin], are synthesized as part of a longer propolypeptide, which is cleaved (split) by specific enzymes to generate the active molecule just after it is transported to a secretory vesicle. Because of their hydrophilic (water-loving) nature, peptide hormones travel freely in the blood as they dissolve. Peptide hormones mediate short responses that are terminated by their own breakdown.

In contrast, steroid-producing cells, like those in the adrenal cortex, store only a small supply of hormone precursor; when stimulated, they are converted to active hormone, which then diffuses across the cell membrane into the blood. Because cells store little of the active hormone, release takes from hours to days. Steroids are hydrophobic (water-fearing), so they are transported by carrier proteins, and are not rapidly degraded. Thus, responses to thyroxine and steroid hormones take awhile to occur but effects last much longer than those triggered by peptide hormones.

By feedback control

The rate of hormone biosynthesis and secretion is often regulated by feedback circuits, in which changes in the level of one hormone affect the levels of other hormones. This type of control is especially important in coordinating the complex processes of cell growth and differentiation.

By other hormones

The trophic hormones are a special class of hormones that stimulate the hormonal production of endocrine glands. For example, thyroid-stimulating hormone (TSH) causes growth and increased activity of the thyroid, which in turn increases output of thyroid hormones.

Plant hormones

A member of the auxin family, the first group of plant hormones discovered.

Plant hormones are internally secreted molecules that typically coordinate the responses of plant tissues to environmental signals, such as light or infection.

Plant hormones are traditionally divided into five major groups, although several additional plant hormones have recently been discovered:

  • Auxin was the first plant hormone to be identified; early experiments leading to its discovery were conducted by Charles Darwin in the 1880s. Auxins regulate various aspects of plant development, including cell division and differentiation.
  • Abscisic acid (ABA) was originally thought to play a major role in the abscission (shedding) of fruits and in bud dormancy. Some of its confirmed effects (which are mostly inhibitory) include stimulating the closure of stomata in dry conditions, inhibiting shoot growth, and inducing seeds to synthesize proteins.
  • Cytokinins (CKs) are active in promoting cell division, and are also involved in cell growth and differentiation.
  • Ethylene acts at trace levels throughout the life of the plant by stimulating or regulating the ripening of fruit, the opening of flowers, and the shedding of leaves.
  • Gibberellins (GA) are involved in promoting stem elongation and mobilizing food reserves in seeds. The absence of gibberellins results in the dwarfism of some plant varieties.

Non-traditional plant hormones include brassinolide, a plant-specific steroid hormone involved in developmental processes.

The role of hormones in pharmacology

Many hormones and their analogues are used as medication:

  • The most commonly prescribed hormones are estrogens and progestagens (as methods of hormonal contraception and as hormone replacement therapy); other steroids (for autoimmune diseases and several respiratory disorders); and thyroxine (as levothyroxine, for hypothyroidism).
  • Insulin is used by many diabetics.
  • Local preparations for use in otolaryngology (the diagnosis and treatment of ear, nose, and throat disorders) often contain pharmacological equivalents of adrenaline.
  • Steroid and vitamin D creams are used extensively in dermatological practice.

A "pharmacological dose" of a hormone is a medical usage referring to an amount of a hormone far greater than its natural occurrence in a healthy body. The effects of pharmacological doses of hormones may be different from responses to naturally occurring amounts and may be therapeutically useful. An example is the ability of pharmacological doses of glucocorticoid to suppress inflammation.

Table of important human hormones

Spelling is not uniform for many hormones. For example, current North American and international usage is estrogen and gonadotropin, while British usage retains the Greek diphthong in oestrogen and the unvoiced aspirant h in gonadotrophin.

Structure Name Abbreviation Tissue Cells Mechanism
amine - tryptophan Melatonin (N-acetyl-5-methoxytryptamine) pineal gland pinealocyte
amine - tryptophan Serotonin 5-HT CNS, GI tract enterochromaffin cell
amine - tyrosine Thyroxine (thyroid hormone) T4 thyroid gland thyroid epithelial cell direct
amine - tyrosine Triiodothyronine (thyroid hormone) T3 thyroid gland thyroid epithelial cell direct
amine - tyrosine (cat) Epinephrine (or adrenaline) EPI adrenal medulla chromaffin cell
amine - tyrosine (cat) Norepinephrine (or noradrenaline) NRE adrenal medulla chromaffin cell
amine - tyrosine (cat) Dopamine DPM hypothalamus
peptide Antimullerian hormone (or mullerian inhibiting factor or hormone) AMH testes Sertoli cell
peptide Adiponectin Acrp30 adipose tissue
peptide Adrenocorticotropic hormone (or corticotropin) ACTH anterior pituitary corticotrope cAMP
peptide Angiotensinogen and angiotensin AGT liver IP3
peptide Antidiuretic hormone (or vasopressin, arginine vasopressin) ADH posterior pituitary varies
peptide Atrial-natriuretic peptide (or atriopeptin) ANP heart cGMP
peptide Calcitonin CT thyroid gland parafollicular cell cAMP
peptide Cholecystokinin CCK duodenum
peptide Corticotropin-releasing hormone CRH hypothalamus cAMP
peptide Erythropoietin EPO kidney
peptide Follicle-stimulating hormone FSH anterior pituitary gonadotrope cAMP
peptide Gastrin GRP stomach, duodenum G cell
peptide Ghrelin stomach P/D1 cell
peptide Glucagon GCG pancreas alpha cells cAMP
peptide Gonadotropin-releasing hormone GnRH hypothalamus IP3
peptide Growth hormone-releasing hormone GHRH hypothalamus IP3
peptide Human chorionic gonadotropin hCG placenta syncytiotrophoblast cells cAMP
peptide Human placental lactogen HPL placenta
peptide Growth hormone GH or hGH anterior pituitary somatotropes
peptide Inhibin testes Sertoli cells
peptide Insulin INS pancreas beta cells tyrosine kinase
peptide Insulin-like growth factor (or somatomedin) IGF liver tyrosine kinase
peptide Leptin LEP adipose tissue
peptide Luteinizing hormone LH anterior pituitary gonadotropes cAMP
peptide Melanocyte stimulating hormone MSH or α-MSH anterior pituitary/pars intermedia cAMP
peptide Oxytocin OXT posterior pituitary IP3
peptide Parathyroid hormone PTH parathyroid gland parathyroid chief cell cAMP
peptide Prolactin PRL anterior pituitary lactotrophs
peptide Relaxin RLN varies
peptide Secretin SCT duodenum S cell
peptide Somatostatin SRIF hypothalamus, islets of Langerhans delta cells
peptide Thrombopoietin TPO liver, kidney
peptide Thyroid-stimulating hormone TSH anterior pituitary thyrotropes cAMP
peptide Thyrotropin-releasing hormone TRH hypothalamus IP3
steroid - glu. Cortisol adrenal cortex (zona fasciculata) direct
steroid - min. Aldosterone adrenal cortex (zona glomerulosa) direct
steroid - sex (and) Testosterone testes Leydig cells direct
steroid - sex (and) Dehydroepiandrosterone DHEA multiple direct
steroid - sex (and) Androstenedione adrenal glands, gonads direct
steroid - sex (and) Dihydrotestosterone DHT multiple direct
steroid - sex (est) Estradiol E2 ovary granulosa cells direct
steroid - sex (est) Estrone ovary granulosa cells direct
steroid - sex (est) Estriol placenta syncytiotrophoblast direct
steroid - sex (pro) Progesterone ovary, adrenal glands, placenta granulosa cells direct
sterol Calcitriol (Vitamin D3) skin/proximal tubule of kidneys direct
eicosanoid Prostaglandins PG seminal vesicle
eicosanoid Leukotrienes LT white blood cells
eicosanoid Prostacyclin PGI2 endothelium
eicosanoid Thromboxane TXA2 platelets

Code:

  • (and) = "Androgen is the generic term for any natural or synthetic compound, usually a steroid hormone, that stimulates or controls the development and maintenance of masculine characteristics in vertebrates by binding to androgen receptors."
  • (cat) = "Catecholamines are chemical compounds derived from the amino acid tyrosine containing catechol and amine groups."
  • (est) = "Estorgen, a group of steroid compounds, named for their importance in the estrous cycle, and functioning as the primary female sex hormone."
  • (pro) = "Progestagens (also spelled progestogens or gestagens) are hormones which produce effects similar to progesterone, the only natural progestagen."

References
ISBN links support NWE through referral fees

  • Beato, M., S. Chavez, and M. Truss. 1996. Transcriptional regulation by steroid hormones. Steroids 61(4): 240-251.
  • Bridgham, J. T., S. M. Carroll, and J. W. Thornton. 2006. Evolution of Hormone-Receptor Complexity by Molecular Exploitation. Science 312: 97-101.
  • Cooper, G. M., and R. E. Hausman. 2004. The Cell: A Molecular Approach. Washington, D.C.: ASM Press. ISBN 0878932143
  • Hammes, S. R. 2003. The further redefining of steroid-mediated signaling. Proc Natl Acad Sci 100(5): 2168-70.
  • Lodish, H., D. Baltimore, A. Berk, S. L. Zipursky, P. Matsudaira, and J. Darnell. 1995. Molecular Cell Biology. New York: Scientific American Books. ISBN 0716723808
  • Mathews, C. K. and K. E. van Holde. 1990. Biochemistry. San Francisco: Benjamin-Cummings. ISBN 0805350152
  • Stryer, L. 1995. Biochemistry, New York: W.H. Freeman. ISBN 0716720094


Hormones and endocrine glands - edit

Hypothalamus: GnRH - TRH - CRH - GHRH - somatostatin - dopamine | Posterior pituitary: vasopressin - oxytocin | Anterior pituitary: GH - ACTH - TSH - LH - FSH - prolactin - MSH - endorphins - lipotropin

Thyroid: T3 and T4 - calcitonin | Parathyroid: PTH | Adrenal medulla: epinephrine - norepinephrine | Adrenal cortex: aldosterone - cortisol - DHEA | Pancreas: glucagon- insulin - somatostatin | Ovary: estradiol - progesterone - inhibin - activin | Testis: testosterone - AMH - inhibin | Pineal gland: melatonin | Kidney: renin - EPO - calcitriol - prostaglandin | Heart atrium: ANP

Stomach: gastrin | Duodenum: CCK - GIP - secretin - motilin - VIP | Ileum: enteroglucagon | Liver: IGF-1

Placenta: hCG - HPL - estrogen - progesterone

Adipose tissue: leptin, adiponectin

Target-derived NGF, BDNF, NT-3

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