A hormone is a signaling molecule produced by living cells that changes the activity of target cells through specific receptors. In animals, hormones classically enter the bloodstream and act at distant sites, forming part of the endocrine system. They regulate growth, reproduction, metabolism, and homeostasis, often at very low concentrations. The term also includes plant hormones, which coordinate development and environmental responses without a blood-based circulatory system. Hormones are defined by their signaling role rather than by a single chemical structure. (openstax.org)
Sources and modes of communication
Animal endocrine glands release hormones into surrounding tissue fluid rather than through ducts. The hormones subsequently reach the circulation. Major sources include the pituitary, thyroid, parathyroid, adrenal, and pineal glands. Other organs combine endocrine functions with different activities: the pancreas, for example, produces digestive secretions as well as insulin and glucagon. The heart, kidneys, gastrointestinal tract, ovaries, and testes also contain hormone-producing cells. (openstax.org)
Endocrine signaling is distinguished from paracrine signaling, in which a messenger acts on nearby cells, and autocrine signaling, in which it acts on the secreting cell itself. A substance may participate in more than one signaling mode. Likewise, the distinction between a hormone and a neurotransmitter depends partly on how it is released and where it acts. Neural and endocrine communication are integrated, notably through the hypothalamus and pituitary gland. Their effects differ in timing, but hormonal responses are not invariably slow. (openstax.org)
Chemical classes
Three broad chemical groups account for most familiar animal hormones:
- Peptide and protein hormones consist of chains of amino acids. Examples include insulin, growth hormone, and oxytocin. Many are synthesized as larger precursors, processed, stored in secretory vesicles, and released when stimulated.
- Steroid hormones are synthesized from cholesterol. They include cortisol, aldosterone, testosterone, estradiol, and progesterone. Their lipid solubility influences both blood transport and receptor location.
- Amino-acid-derived hormones include catecholamines, such as epinephrine, and thyroid hormones. Despite their shared origin from amino acids, these groups have different solubility and signaling properties: catecholamines generally activate cell-surface receptors, whereas thyroid hormones act mainly through intracellular receptors. (openstax.org)
These categories do not encompass every biological messenger called a hormone. Plant hormones, in particular, include chemically diverse compounds, including the gaseous messenger ethylene. Chemical classification therefore helps explain mechanisms but does not determine whether a substance has a hormonal function. (openstax.org)
Receptors and cellular effects
A target cell must possess an appropriate receptor and the cellular machinery needed to respond. Hormones act as ligands that bind receptors and initiate signal transduction. The same hormone can produce different effects in different tissues because receptor subtypes and downstream pathways differ. Changes in receptor abundance can also increase or decrease cellular sensitivity. (openstax.org)
Water-soluble hormones generally bind receptors on the cell membrane. These receptors activate intracellular pathways that can alter enzyme activity, membrane transport, secretion, or protein synthesis. Some pathways use second messengers, such as cyclic AMP, to relay and amplify the signal. Amplification allows a relatively small extracellular hormone signal to generate a substantial cellular response. (openstax.org)
Steroid hormones act chiefly through receptors in the cytoplasm or nucleus, while thyroid hormones act principally through nuclear receptors. Hormone–receptor complexes regulate gene expression, influencing the production of particular proteins. Such effects can take longer to develop than changes to existing enzymes, although intracellular-receptor signaling is not the only mechanism available to lipid-soluble hormones. (ncbi.nlm.nih.gov)
Secretion, transport, and feedback
Hormone concentrations reflect secretion, transport, tissue uptake, and removal. Many peptide hormones circulate largely in free form, whereas steroid and thyroid hormones circulate substantially bound to carrier proteins. Binding provides a circulating reservoir and influences clearance. Consequently, total hormone concentration and free hormone concentration are related but not interchangeable measurements. (ncbi.nlm.nih.gov)
Secretion is commonly regulated by negative feedback. In the hypothalamic–pituitary–adrenal axis, hypothalamic corticotropin-releasing hormone promotes pituitary adrenocorticotropic hormone release, which stimulates cortisol production. Cortisol then inhibits upstream stimulation. Other controls respond directly to a regulated variable: pancreatic insulin secretion, for example, changes with blood glucose. Positive feedback also occurs, including the estrogen-associated luteinizing hormone surge preceding ovulation. (ncbi.nlm.nih.gov)
Hormone release may be pulsatile rather than continuous. Gonadotropin-releasing hormone is an important example: its pulse pattern contributes to regulation of pituitary gonadotropin secretion. A hormone measurement therefore represents a particular moment within a dynamic regulatory system, rather than necessarily a constant physiological level. (ncbi.nlm.nih.gov)
Hormones beyond human physiology
Hormonal regulation extends throughout the animal kingdom. In insects, ecdysteroids and juvenile hormone coordinate development and metamorphosis. Hormonal systems thus regulate transitions between life stages as well as the continuing functions of adult organisms. (ncbi.nlm.nih.gov)
In plants, hormone production is distributed among tissues rather than confined to animal-like endocrine glands. Auxins influence growth and directional responses; gibberellins participate in stem elongation and seed germination; cytokinins regulate growth and development; abscisic acid contributes to drought responses; and ethylene promotes ripening in many fruits. Additional groups include brassinosteroids, jasmonates, and strigolactones. Their effects depend on tissue, developmental stage, concentration, and interactions with other signals. For example, abscisic acid promotes closure of stomata, reducing water loss under water-limited conditions. (openstax.org)