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:
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).
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:
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.
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).
Hormonal signaling typically involves the following six steps:
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.
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:
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 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.
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.
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).
|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|
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.
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.
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 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:
Non-traditional plant hormones include brassinolide, a plant-specific steroid hormone involved in developmental processes.
Many hormones and their analogues are used as medication:
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.
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.
|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||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||Follicle-stimulating hormone||FSH||anterior pituitary||gonadotrope||cAMP|
|peptide||Gastrin||GRP||stomach, duodenum||G cell|
|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||Insulin||INS||pancreas||beta cells||tyrosine kinase|
|peptide||Insulin-like growth factor (or somatomedin)||IGF||liver||tyrosine kinase|
|peptide||Luteinizing hormone||LH||anterior pituitary||gonadotropes||cAMP|
|peptide||Melanocyte stimulating hormone||MSH or α-MSH||anterior pituitary/pars intermedia||cAMP|
|peptide||Parathyroid hormone||PTH||parathyroid gland||parathyroid chief cell||cAMP|
|peptide||Somatostatin||SRIF||hypothalamus, islets of Langerhans||delta cells|
|peptide||Thyroid-stimulating hormone||TSH||anterior pituitary||thyrotropes||cAMP|
|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||Leukotrienes||LT||white blood cells|
|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
Adipose tissue: leptin, adiponectin
Target-derived NGF, BDNF, NT-3
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