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A hormone (from Greek ὁρμή, "impetus") is a class of regulatory biochemicals produced in particular parts of organisms by specific cells, glands, and/or tissues and then transported by the bloodstream to other parts of the body, with the intent of influencing a variety of physiological and behavioral activities, such as the processes of digestion, metabolism, growth, reproduction, and mood control. Generally, only a small amount of hormone is required to alter cell metabolism. In essence, it is a 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, typically into fenestrated capillaries. Hormones with paracrine function diffuse through the interstitial spaces to nearby target tissues.
A variety of exogenous chemical compounds, both natural and synthetic, have hormone-like effects on both humans and wildlife. Their interference with the synthesis, secretion, transport, binding, action, or elimination of natural hormones in the body can change the homeostasis, reproduction, development, and/or behavior, just as endogenously produced hormones do.
See also Signal transduction.
Hormonal signaling involves the following:
Hormone cells are typically of a specialized cell type, residing within a particular endocrine gland, such as thyroid gland, ovaries, and testes. Hormones exit their cell of origin via exocytosis or another means of membrane transport. 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.
Most 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 a cell may have several different receptors that recognize different hormones and activate 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. To bind their receptors, these hormones must cross the cell membrane. They can do so because they are lipid-soluble. 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 associated with the plasma membrane.
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 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 affinity between the hormone and its receptor. Estrogen metabolites generally have much lower receptor affinity than the parent hormone.
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 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 that influence the metabolism and excretion of hormones. Thus, higher hormone concentration alone cannot 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.
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.
Eicosanoids are considered to act as local hormones.
Hormones have the following effects on the body:
A hormone may also regulate the production and release of other hormones. Hormone signals control the internal environment of the body through homeostasis.
As hormones are defined functionally, not structurally, they may have diverse chemical structures. Hormones occur in multicellular organisms (plants, animals, fungi, brown algae and red algae). These compounds occur also in unicellular organisms, and may act as signaling molecules, but there is no consensus if, in this case, they can be called hormones.
Those classes of hormones are found too in other groups of animals. In insects and crustaceans, there is a hormone with an unusual chemical structure, compared with other animal hormones, the juvenile hormone, a sesquiterpenoid.
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 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" or "supraphysiological 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, though not without potentially adverse side effects. An example is the ability of pharmacologic doses of glucocorticoids to suppress inflammation.