Jump to content

Hormone

Edit links
From Wikipedia, the free encyclopedia
This is an old revision of this page, as edited by Jfdwolff (talk | contribs) at 23:13, 18 May 2004. The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

A hormone (from Greek horman - "to set in motion") is a chemical messenger from one cell (or group of cells) to another. All multicellular organisms produce hormones. The best known hormones are those produced by endocrine glands of vertebrate animals, but hormones are produced by nearly every organ system and tissue type in a human or animal body. Hormone molecules are secreted (released) directly into the bloodstream, other body fluids, or into adjacent tissues. They move by circulation or diffusion to 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 transduction of the receiving tissue.

Hormone actions vary widely, but can include stimulation or inhibition of growth, induction of apoptosis (cell death), activation or inhibition of the immune system, regulating metabolism and preparation for a new activity (e.g. fighting) or phase of life (e.g., caring for offspring). Many of the responses to hormone signals can be described as serving to regulate metabolic activity of an organ or tissue. Hormones also control the reproductive cycle of virtually all multicellular organisms.

Physiology of hormones

Homeostatic regulation

The rate of production of a given hormone is most commonly regulated by a homeostatic control system.

Every cell is capable of producing a vast number of regulatory molecules. The classical endocrine glands and their hormone products are specialized to serve regulation on the overall organism level, but can in many instances be used in other ways or only on the tissue level.

Homeostatic regulation of hormones depends on the metabolism and excretion of hormones.

Hormone secretion can be controlled by:

  • Plasma concentrations of ions or nutrients
  • Neurons and mental activity
  • Other hormones (stimulating or releasing-hormones)
  • Environmental changes, e.g. of light or temperature

Structure

Vertebrate hormones fall into four chemical classes:

  1. Amine-derived hormones are derivatives of the amino acids tyrosine and tryptophan. Examples are catecholamines and thyroxine.
  2. Peptide hormones are comprised of chains of amino acids. Examples of small peptide hormones are TRH and vasopressin. Peptides comprised of scores or hundreds of amino acids are referred to as proteins. Examples of protein hormones include insulin and growth hormone.
  3. Steroid hormones are derived from cholesterol. The adrenal cortex and the gonads are primary sources. Examples of steroid hormones are testosterone and cortisol.
  4. Lipid and phospholipid hormones are derived from lipids such as linoleic acid and phospholipids such as arachidonic acid. The main example is the hormone class of the eicosanoids, which includes the widely studies prostaglandins.

Pharmacological effects

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 than 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

The most important hormones in humans include:

One special group of hormones are trophic hormones that act as stimulants of 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 and studied class of hormones is that of the "Hunger Hormones" - ghrelin, orexin and PYY 3-36 - and their antagonists - e.g. leptin.

Spelling note

Current North American and international usage is estrogen, gonadotropin. British usage retains the Greek diphthong in oestrogen and the unvoiced aspirant h in gonadotrophin.

See also

Retrieved from "https://en.wikipedia.org/w/index.php?title=Hormone&oldid=3642608"