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A hormone (from Greek ὁρμή - "impetus") is a chemical messenger that carries a signal from one cell (or group of cells) to another via the blood. All multicellular organisms produce hormones (including plants - see phytohormone).
In general, hormones regulate the function of their target cells, i.e., cells that express a receptor for the hormone. The action, or net effect of hormones is determined by a number of factors including its pattern of secretion and the response of the receiving tissue - the signal transduction response.
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.
Hormonal 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.
Hormonal signalling across this hierarchy involves the following:
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 receptors
Most hormones initiate a cellular response by initially combining with either a specific intracellular or cell membrane associatedreceptor 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 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.
Physiology of hormones
Most 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 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:
Other hormones (stimulating- or releasing-hormones)
Plasma concentrations of ions or nutrients, as well as binding globulins
Neurons and mental activity
Environmental changes, e.g., of light or temperature
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 effects vary widely, but can include:
stimulation or inhibition of growth,
In puberty hormones can effect mood and mind
induction or suppression of apoptosis (programmed cell death)
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 hormones
Spelling 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 the unvoiced aspirant h in gonadotrophin.
Boosts the supply of oxygen and glucose to the brain and muscles (by increasing heart rate and stroke volume, vasodilation, increasing catalysis of glycogen in liver, breakdown of lipids in fat cells.
dilate the pupils
Suppress non-emergency bodily processes (e.g. digestion)
Suppress immune system
Boosts the supply of oxygen and glucose to the brain and muscles (by increasing heart rate and stroke volume, vasoconstriction and increased blood pressure, breakdown of lipids in fat cells.
Increase skeletal muscle readiness.
hypothalamus, islets of Langerhans, gastrointestinal system
delta cells in islets Neuroendocrince cells of the Periventricular nucleus in hypothalamus
Inhibit release of GH and TRH from anterior pituitary Suppress release of gastrin, cholecystokinin (CCK), secretin, motilin, vasoactive intestinal peptide (VIP), gastric inhibitory polypeptide (GIP), enteroglucagon in gastrointestinal system Lowers rate of gastric emptying
Reduces smooth muscle contractions and blood flow within the intestine  Inhibit release of insulin from beta cells  Inhibit release of glucagon from beta cells  Suppress the exocrine secretory action of pancreas.
Convert endometrium to secretory stage
Make cervical mucus permeable to sperm.
Inhibit immune response, e.g. towards the human embryo.
Decrease uterine smooth muscle contractility
Inhibit onset of labor.
Support fetal production of adrenal mineralo- and glucosteroids.
Raise epidermal growth factor-1 levels
Increase core temperature during ovulation
Reduce spasm and relax smooth muscle (widen bronchi and regulate mucus)
Reduce gall-bladder activity
Normalize blood clotting and vascular tone, zinc and copper levels, cell oxygen levels, and use of fat stores for energy.
Assist in thyroid function and bone growth by osteoblasts
Relsilience in bone, teeth, gums, joint, tendon, ligament and skin Healing by regulating collagen
Nerve function and healing by regulating myelin
Prevent endometrial cancer by regulating effects of estrogen.
^ Massaro D, Massaro GD (2004). "Estrogen regulates pulmonary alveolar formation, loss, and regeneration in mice". American Journal of Physiology. Lung Cellular and Molecular Physiology287 (6): L1154-9. PMID 15298854 url=http://ajplung.physiology.org/cgi/content/full/287/6/L1154.
^ Pentikäinen V, Erkkilä K, Suomalainen L, Parvinen M, Dunkel L. Estradiol Acts as a Germ Cell Survival Factor in the Human Testis in vitro.
The Journal of Clinical Endocrinology & Metabolism 2006;85:2057-67 PMID 10843196
^ ab http://www.vivo.colostate.edu/hbooks/pathphys/reprod/placenta/endocrine.html