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Hormone
A hormone (from Greekhorman - "to set in motion") is a chemicalmessenger from one cell(or group of cells) to another. All multicellular organismsproduce hormones (including plants- see article phytohormone). The best-known animal (and human) hormones are those produced by endocrine glandsof vertebrateanimals, but hormones are produced by nearly every organsystem and tissuetype in a human or animal body. Hormone moleculesare secreted (released) directly into the bloodstream; however, some hormones, called ectohormones, are secreted to the outside environment. They move by circulation or diffusionto their target cells, which may be nearby cells (paracrine action) in the same tissue or cells of a distant organ of the body. 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 transductionof the receiving tissue. Hormone actions vary widely, but can include stimulation or inhibition of growth, induction or suppression of apoptosis(programmed cell death), activation or inhibition of the immune system, regulating metabolismand preparation for a new activity (e.g., fighting, fleeing, mating) or phase of life (e.g., puberty, caring for offspring, menopause). 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 regulatemetabolic activity of an organ or tissue. Hormones also control the reproductive cycleof virtually all multicellular organisms.
HistoryThe concept of internal secretion developed in the 19th century; Claude Bernarddescribed it in 1855, but did not specifically address the possibility of secretions of one organ acting as messengers to others. Still, various endocrine conditions were recognised and even treated adequately (e.g., hypothyroidismwith extract of thyroid glands). The major breakthrough was the identification of secretin, the hormone secreted by the duodenumthat stimulates pancreaticsecretions, by Ernest Starlingand William Baylissin 1902. Previously, the process had been considered (e.g., by Ivan Pavlov) to be regulated by the nervous system. Starling and Bayliss demonstrated that injecting duodenal extract into dogs rapidly increased pancreatic secretions, raising the possibility of a chemical messenger. Starling is also credited with introducing the term hormone, having coined it in a 1905lecture. Later reports indicate it was suggested to him by the Cambridge physiologist William B. Hardy(Henderson 2005). The remainder of the 20th centurysaw all the major hormones discovered, as well as the cloning of the relevant genesand the identification of the many interlocking feedback mechanisms that characterise the endocrine system. Physiology of hormonesMost cells are capable of producing one or more, sometimes many, molecules which signal other cells to alter their growth, function, or metabolism. The classical endocrine glandsand their hormone products are specialized to serve regulation on the overall organism level, but can often be used in other ways or only on the tissue level. The rate of production of a hormone is often regulated by a homeostaticcontrol system, generally by negative feedback. Homeostatic regulation of hormones depends, apart from production, on the metabolismand excretionof hormones. Hormone secretion can be stimulated and inhibited by:
One special group of hormones is trophic hormonesthat 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- hence increasing output of thyroid hormones. A recently-identified class of hormones is that of the "Hunger Hormones" - ghrelin, orexinand PYY 3-36- and "Satiety hormones" - e.g., leptin, obestatin. Types of hormonesVertebrate hormones fall into four chemical classes:
PharmacologyMany hormones are used as medication. The most commonly-prescribed hormones are estrogensand progestagens(in the contraceptive pilland as HRT), thyroxine(as levothyroxine, for hypothyroidism) and steroids(for autoimmune diseasesand several respiratory disorders). Insulinis used by many diabetics. Local preparations for use in otolaryngologyoften contain pharmacologicequivalents of adrenalin, while steroidand vitamin Dcreams are used extensively in dermatologicalpractice. 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 glucocorticoidto 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 diphthongin oestrogen and the unvoiced aspirant h in gonadotrophin. Amine hormonesAmine-derived hormones:
Peptide hormonesPeptide hormones:
Steroid and sterol hormonesSteroid hormones:
Sterol hormones:
Lipid hormonesLipidand phospholipidhormones (eicosanoids):
See also
Reference
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