hormone n : the secretion of an endocrine gland that is transmitted by the blood to the tissue on which it has a specific effect [syn: endocrine, internal secretion]
EtymologyFrom ορμή (orme) "rapid motion forwards, onrush, onset, assault, impulse to do a thing, effort", from ορμάω, ορμώ (ormao, ormo) "to set in motion, to urge on, to cheer on, to make a start, to hasten on".
A hormones (from Greek ὁρμή - "impetus") are chemicals released by cells that affect cells in other parts of the body. Only a small amount of hormone is required to alter cell metabolism.It is also chemical messenger that transports a signal from one cell to another. All multicellular organisms produce hormones; plant hormones are also called phytohormones. Hormones in animals are often transported in the blood. Cells respond to a hormone when they express a specific receptor for that hormone. The hormone binds to the receptor protein, resulting in the activation of a signal transduction mechanism that ultimately leads to cell type-specific responses.
Endocrine hormone molecules are secreted (released) directly into the bloodstream, while 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.
Hierarchical nature of hormonal controlHormonal regulation of some physiological activities involves a hierarchy of cell types acting on each other either to stimulate or to modulate the release and action of a particular hormone. The secretion of hormones from successive levels of endocrine cells is stimulated by chemical signals originating from cells higher up the hierarchical system. The master coordinator of hormonal activity in mammals is the hypothalamus, which acts on input that it receives from the central nervous system.
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.
Hormone signalingHormonal signalling across this hierarchy involves the following:
- Biosynthesis of a particular hormone in a particular tissue
- Storage and secretion of the hormone
- Transport of the hormone to the target cell(s)
- Recognition of the hormone by an associated cell membrane or intracellular 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 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 gland (e.g., the thyroid gland, the ovaries, or the testes). Hormones may exit their cell of origin via exocytosis or another means of membrane transport. However, the hierarchical model is an oversimplification of the hormonal signaling process. 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.
Interactions with receptorsMost hormones initiate a cellular response by initially combining with either a specific intracellular or cell membrane associated 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. For many hormones, including most protein hormones, 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 secondary messengers (e.g. cyclic AMP). Some protein hormones also interact with intracellular receptors located in the cytoplasm or nucleus by an intracrine mechanism.
For hormones such as steroid or thyroid hormones, their receptors are located 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 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. However, it has been shown that not all steroid receptors are located intracellularly, some are plasma membrane associated.
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:
- The number of hormone molecules available for complex formation
- The number of receptor molecules available for complex formation and
- The binding affinity between hormone and receptor.
Physiology of hormonesMost cells are capable of producing one or more molecules, which act as signalling molecules to other cells, altering their growth, function, or metabolism. The classical hormones produced by cells in the endocrine glands 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.
The rate of hormone biosynthesis and secretion is often regulated by a homeostatic negative feedback control mechanism. Such a mechanism depends on factors which influence the metabolism and excretion of hormones. Thus, higher hormome concentration alone can not trigger the negative feedback mechanism. Negative feedback must be triggered by overproduction of an "effect" of the hormone.
Hormone secretion can be stimulated and inhibited by:
One special group of hormones is the tropic hormones that stimulate the hormone production of other endocrine glands. For example, thyroid-stimulating hormone (TSH) causes growth and increased activity of another endocrine gland, the thyroid, which increases output of thyroid hormones.
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.
In order to release active hormones quickly into the circulation, hormone biosynthetic cells may produce and store biologically inactive hormones in the form of pre- or prohormones. These can then be quickly converted into their active hormone form in response to a particular stimulus.
Hormone effectsHormone effects vary widely, but can include:
- stimulation or inhibition of growth,
- In puberty hormones can affect mood and mind
- induction or suppression of apoptosis (programmed cell death)
- activation or inhibition of the immune system
- 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
Many of the responses to hormone signals can be described as serving to regulate metabolic activity of an organ or tissue.
Chemical classes of hormonesVertebrate hormones fall into three chemical classes:
- Amine-derived hormones are derivatives of the amino acids tyrosine and tryptophan. Examples are catecholamines and thyroxine.
- Peptide hormones consist of chains of amino acids. Examples of small peptide hormones are TRH and vasopressin. Peptides composed of scores or hundreds of amino acids are referred to as proteins. 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 eicosanoids. Examples of steroid hormones are testosterone and cortisol. Sterol hormones such as calcitriol are a homologous system. The adrenal cortex and the gonads are primary sources of steroid hormones. Examples of eicosanoids are the widely studied prostaglandins.
PharmacologyMany hormones and their analogues are used as medication. The most commonly-prescribed hormones are estrogens and progestagens (as methods of hormonal contraception and as HRT), thyroxine (as levothyroxine, for hypothyroidism) and steroids (for autoimmune diseases and several respiratory disorders). Insulin is used by many diabetics. Local preparations for use in otolaryngology often contain pharmacologic equivalents of adrenaline, while steroid and vitamin D creams are used extensively in 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.
Important human hormonesSpelling is not uniform for many hormones. Current North American and international usage is estrogen, gonadotropin, while British usage retains the Greek diphthong in oestrogen and favors the earlier spelling gonadotrophin (from trophē ‘nourishment, sustenance’ rather than tropē ‘turning, change’.
hormone in Arabic: هرمون
hormone in Bosnian: Hormon
hormone in Bulgarian: Хормон
hormone in Catalan: Hormona
hormone in Czech: Hormon
hormone in Danish: Hormon
hormone in German: Hormon
hormone in Modern Greek (1453-): Ορμόνη
hormone in Spanish: Hormona
hormone in Esperanto: Hormono
hormone in Persian: هورمون
hormone in French: Hormone
hormone in Korean: 호르몬
hormone in Croatian: Hormoni
hormone in Ido: Hormono
hormone in Indonesian: Hormon
hormone in Icelandic: Hormón
hormone in Italian: Ormone
hormone in Hebrew: הורמון
hormone in Pampanga: Hormone
hormone in Georgian: ჰორმონი
hormone in Kurdish: Hormon
hormone in Latin: Hormon
hormone in Latvian: Hormoni
hormone in Lithuanian: Hormonas
hormone in Hungarian: Hormon
hormone in Macedonian: Хормони
hormone in Malay (macrolanguage): Hormon
hormone in Dutch: Hormoon
hormone in Japanese: ホルモン
hormone in Norwegian: Hormon
hormone in Norwegian Nynorsk: Hormon
hormone in Occitan (post 1500): Ormona
hormone in Polish: Hormon
hormone in Portuguese: Hormona
hormone in Romanian: Hormon
hormone in Russian: Гормоны
hormone in Simple English: Hormone
hormone in Slovak: Hormón
hormone in Slovenian: Hormon
hormone in Serbian: Хормони
hormone in Sundanese: Hormon
hormone in Finnish: Hormoni
hormone in Swedish: Hormon
hormone in Thai: ฮอร์โมน
hormone in Tajik: Ҳормон
hormone in Turkish: Hormon
hormone in Ukrainian: Гормон
hormone in Urdu: Hormone
hormone in Yiddish: הארמאן
hormone in Chinese: 激素
Allen-Doisy hormone, adjuvant, adrenosterone, androgen, androsterone, autacoid, bile, carminative, chalone, cholecystokinin, chondrotrophic hormone, corticosterone, counterirritant, dehydrocorticosterone, digestive secretion, emmenagogue, endocrine, expectorant, gall, gastric juice, insulin, intestinal juice, maturative, mucus, pancreatic juice, progesterone, prostatic fluid, rheum, salivary secretion, semen, sperm, tears, thyroxin, vasodilator, vitamin