Hormone << HAWR mohn >> is any of a number of chemical substances produced within an animal or a plant. A hormone is produced in one part of an organism, but it causes an effect in a different part. Thus, hormones serve as a means of communication among various parts of an organism. They act as “chemical messengers” that help these parts function in a coordinated way.
The word hormone comes from a Greek word that means to set in motion. In human beings and other animals, hormones control such body activities as growth, development, and reproduction. In plants, hormones regulate many aspects of growth. If an organism fails to produce the proper kind or amount of hormones, serious disturbances—or even death—may result. For example, giants and dwarfs—among both animals and plants—can result from faulty hormone production.
In 1902, scientists found the first definite evidence of the existence of hormones. That year, British researchers discovered that a chemical substance controlled certain activities involved in digestion. Since then, scientists have identified more than 30 hormones produced by the human body. They also have developed ways of extracting hormones from living tissue and techniques for manufacturing them in the laboratory.
Human hormones
Most hormones in the human body are produced by organs called endocrine, or ductless, glands. The major endocrine glands include the two adrenal glands, the pituitary gland, the four parathyroid glands, the sex glands, and the thyroid gland. A few hormones are produced by endocrine tissue present in organs that are not primarily endocrine glands. Such organs include the stomach and pancreas. See Gland.
The endocrine glands secrete hormones into the blood, which carries them throughout the body. After a hormone arrives at its target, the organ or tissue it affects, it causes certain actions to occur.
Hormones regulate a variety of body functions. They may be grouped according to the functions they control. These functions include the way the body uses food; growth; sex and reproduction; the regulation of the composition of the blood; the reaction of the body to emergencies; and the control of hormones themselves.
Metabolic hormones
regulate the various steps in metabolism, the process by which the body converts food into energy and living tissue. For example, the endocrine tissue of the stomach and small intestine secretes a number of digestive hormones. These hormones control the secretion of digestive juices, which break down food into simple substances that can be used by the body.
After molecules of digested food enter the bloodstream, other hormones control their use by the cells of the body. For example, the hormones insulin and glucagon, both secreted by the pancreas, regulate the amount of sugar available to the cells. Insulin enables cells to use sugar from the blood. If the pancreas secretes too little insulin, a serious disease called diabetes mellitus results. Glucagon causes the liver to release additional sugar into the blood.
Two hormones produced by the thyroid gland—thyroxine and triiodothyronine—control the rate at which the cells use food to release energy. Overproduction of these hormones results in many physical and emotional disturbances, including excitability, muscular weakness, rapid pulse and respiration, and weight loss. Underproduction causes such symptoms as low body temperature, mental and physical sluggishness, and weight gain. By controlling the production of energy, these hormones regulate the way in which the body uses food in building new tissue. Thus, they play a major role in the creation of new proteins by the body cells.
Thyroid hormones also play a role in regulating growth and development. A deficiency of these hormones in infants is called congenital hypothyroidism. If left untreated, this condition can cause physical and mental disability.
Other hormones also control the way cells use food to build new tissue. The glucocorticoids are a group of hormones that function primarily in regulating the metabolism of carbohydrates (sugars and starches), fats, and proteins. They control the processes by which the body converts digested proteins into carbohydrates and fats. These hormones include corticosterone, cortisol, and cortisone. The glucocorticoids are secreted by the cortex (outer part) of each adrenal gland. Insulin and growth hormone (GH), a hormone secreted by the anterior lobe (front part) of the pituitary gland, also regulate the creation of new tissue.
GH also controls the use of food in other ways. For example, it stimulates cells to use fat, rather than sugar, as an energy source, and so helps maintain a fairly high level of sugar in the blood. Such a level is necessary for the brain to function properly.
Growth and sex hormones.
The body’s development from infancy to adulthood involves a complex process of physical changes. Hormones play a key role in regulating these changes.
GH controls overall growth during childhood. Faulty production of this hormone during childhood can cause a person to become a dwarf or a giant. In adults, GH enables certain tissues to maintain their proper size and structure. Insulin, glucocorticoids, and thyroxine also play major roles in tissue growth and maintenance.
Beginning at the age of about 11 to 15, young people go through a period of rapid growth and physical change. Hormones control the development that occurs during this period, called puberty. At the start of puberty, the hypothalamus, the portion of the brain nearest the pituitary gland, greatly increases its secretion of gonadotropin-releasing hormone. This hormone acts on the anterior lobe of the pituitary. It stimulates the gland to secrete the gonadotropic hormones—follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These hormones, in turn, act on the gonads (sex glands)—the testicles in males and the ovaries in females.
Under the influence of FSH and LH, the gonads grow and begin to secrete large amounts of sex hormones. The male sex hormones, including testosterone and androsterone, are called androgens. The female sex hormones include progesterone and the estrogens. The most important estrogens are estradiol, estriol, and estrone. The cortex of adrenal glands in both men and women also secretes some sex hormones, especially androgens.
The sex hormones regulate the remarkable changes that occur during puberty. They help trigger a person’s rapid growth in height and weight and, at the end of puberty, they stop this growth. Androgens cause the male sex organs to mature, and they stimulate male sexual behavior. Androgens also stimulate the development of such secondary male characteristics as a deep voice and a beard. Estrogens cause the female sex organs to develop fully, and they establish female sexual behavior. They also stimulate the development of secondary female characteristics, such as full breasts and wide hips. In a woman’s body, FSH, LH, estrogens, and progesterone work together to control the menstrual cycle (see Menstruation). Progesterone also regulates processes necessary for pregnancy.
Blood composition hormones.
Healthy blood contains fairly exact levels of several chemical substances. If the level of these chemicals becomes too high or too low, the body can be harmed.
A number of hormones work together to ensure that the composition of the blood remains within normal ranges. Parathormone, secreted by the parathyroid glands, and calcitonin, from the thyroid, regulate the level of calcium in the blood. Parathormone also controls the amount of phosphate. The mineralocorticoids, a group of hormones secreted by the adrenal cortex, control the balance between salts and water in the blood. Aldosterone is the most important mineralocorticoid. Vasopressin, also called antidiuretic hormone, regulates the water level of the blood. It is produced by the hypothalamus, but it is stored in and released by the posterior lobe (rear part) of the pituitary.
Stress hormones
are secreted in case of anger, fright, or injury. The medulla (inner portion) of the adrenal glands secretes epinephrine and norepinephrine, also known as adrenalin and noradrenalin. These substances prepare the body for stress. For example, epinephrine increases the pulse and speeds the conversion of food to energy in the muscles. The glucocorticoids also help the body adjust to stress.
Endocrine control hormones
affect the production of other hormones. They include FSH and LH, the anterior pituitary hormones that regulate the secretions of the gonads. The anterior pituitary also secretes thyroid-stimulating hormone (TSH) and adrenocorticotropic hormone (ACTH). TSH stimulates the thyroid to secrete thyroxine. ACTH stimulates the adrenal cortex, causing increased secretions of glucocorticoids, mineralocorticoids, and adrenal sex hormones.
The anterior pituitary itself is regulated by hormones released by the hypothalamus. These substances are called releasing hormones. The gonadotropin-releasing hormone triggers the secretion of FSH and LH. Other releasing hormones stimulate the pituitary’s production of ACTH, GH, and TSH. Prolactin, another hormone of the anterior pituitary, is also controlled by a hypothalamic hormone. One of the effects of prolactin is the stimulation of milk production in nursing mothers.
The hypothalamus, which forms part of the brain, consists of nerve tissue. Thus, the releasing hormones link the body’s nervous and endocrine systems into one coordinated control unit. The sense organs gather information on changes in the environment, which they relay to the brain through the nervous system. If these changes call for a hormonal response, the hypothalamus triggers the appropriate pituitary secretions.
Other hormones
in human beings include oxytocin and relaxin, both of which affect the process of birth. Oxytocin, like vasopressin, is produced by the hypothalamus and is stored in and secreted by the posterior pituitary. The ovaries produce relaxin. Relaxin widens the birth canal, the passageway through which a baby leaves its mother’s body. Oxytocin causes the muscles of the uterus to contract during labor. Oxytocin also stimulates the release of milk from the mother’s breasts when the infant nurses.
Melanocyte-stimulating hormone (MSH) is a hormone secreted by the anterior lobe of the pituitary. Among certain amphibians, fish, and reptiles, MSH regulates the amount of pigment (coloring matter) in the skin. Its function in the human body is not yet understood.
Hormones of other animals
Other vertebrates, especially other mammals, have most of the same hormones people have. Many of these chemicals are nearly identical in structure and effect to human hormones. This similarity enables scientists to learn about human hormones by studying those of other vertebrates. In some cases, doctors can use hormones secreted by animals to treat patients whose bodies do not produce sufficient amounts of certain hormones.
Invertebrates also produce hormones, some of which play an important role in growth and development. For example, hormones control the changes through which such insects as bees and butterflies pass while growing.
Plant hormones
Plant hormones are produced mainly in actively growing parts, such as the tips of roots and stems. These hormones influence growth and are often called growth regulators. There are three main types of plant hormones: (1) auxins, (2) cytokinins, and (3) gibberellins.
Auxins
cause various effects on different parts of a plant. In stems and roots, auxins regulate the elongation (lengthening) of cells. By stimulating cell elongation, auxins affect the manner in which stems bend toward light and away from gravity. Auxins also control the process by which roots bend toward gravity, but they do so by preventing the elongation of cells.
In many plants, auxins secreted by the bud at the tip of a stem prevent lower buds on the stem from growing. Thus, they slow the growth of side branches. Such branches could use up energy a plant needs to grow tall and sturdy. Auxins also stimulate the growth of fruit and prevent fruit and leaves from falling off a plant.
Cytokinins
control cell division in plants. They apparently work together with other growth regulators, especially auxins. Cytokinins play an important role in determining which cells of a young plant will become root cells, which cells will become leaf cells, and so on.
Gibberellins
stimulate many plants to grow larger. When used in experiments, they have made the stems of dwarf plants lengthen rapidly. Gibberellins also help regulate blossoming in certain plants. They cause the seeds and buds of many species to begin growing after dormancy (long periods of inactivity).
Other growth regulators
include abscisic acid and ethylene. Abscisic acid blocks plant growth, thus stimulating dormancy. Ethylene regulates, among other things, the ripening of fruit.
How hormones work
Most human hormones can be divided into two groups according to their chemical structure. One group, called steroids, consists of the sex hormones and the hormones of the adrenal cortex. Most other human hormones contain some form of amino acids, the building blocks of proteins.
Both steroids and amino acid hormones work by combining with specialized molecules called receptors in the cells of target tissues. Union of a hormone with its receptor triggers chemical changes that affect many cell processes and activities. Scientists think that steroids attach to receptors inside cells, then move into the cell’s nucleus. Within the nucleus, steroids affect the activity of genes, tiny chemical structures that carry hereditary information. Amino acid hormones appear to attach to receptors on the cell’s membrane (outer surface). In some cases, attachment of a hormone to the membrane affects which substances can enter or leave the cell. In other cases, binding of a hormone activates additional chemicals called enzymes that affect the cell’s activities.
Synthetic hormones
Since the 1940’s, biochemists have learned to synthesize (create artificially) many hormones in the laboratory. Until that time, almost all hormones had to be extracted from animals or plants. Only tiny quantities of hormones occur in living tissue. Synthesis has greatly increased the availability of hormones for human use.
Some synthetic hormones, including synthetic human hormones, are exact duplicates of the natural secretions. But scientists have altered the chemical structure of many synthetic hormones to provide more powerful action. The most common synthetic hormones include auxins; glucocorticoids, particularly cortisone; sex hormones; growth hormone; and insulin. Not all hormones can be synthesized. Scientists do not know the exact chemical structure of some. The structure of other hormones is too complex for practical large-scale synthesis.
Uses of hormones
Medical uses.
Physicians use hormones to treat people with hormone deficiencies. The body of a patient with such a condition cannot produce an adequate supply of one or more hormones. Hormone therapy enables a person to overcome many of the symptoms of various diseases. Such therapy cannot cure these diseases. It merely controls them. Hormone deficiency diseases include Addison’s disease, diabetes mellitus, diabetes insipidus, and myxedema. See Addison’s disease; Diabetes; Thyroid gland (Underactive thyroid).
Certain other conditions, which are not directly related to hormone deficiencies, may also be treated with hormones. These conditions include arthritis and asthma, for which many physicians prescribe cortisone.
In addition, hormones may be given to alter a function of the body in some way. Birth control pills, for example, contain synthetic female sex hormones. By taking these hormones, a woman alters the endocrine balance that controls the menstrual cycle. This alteration blocks ovulation (the release of eggs), thus making it almost impossible for pregnancy to occur.
Agricultural uses.
Farmers use synthetic plant hormones, especially auxins, to achieve a variety of results in crop production. When treated with auxins, certain plants—including cucumbers and tomatoes-—produce seedless fruit. Farmers also use auxin sprays on apples to prevent the fruit from falling off the tree before harvest. In addition, auxins serve as weedkillers. When sprayed in large concentrations, auxins cause broadleaf weeds to grow in an uncontrolled manner so they die.
Much research has been done on the use of hormones in livestock production. In the United States, certain synthetic estrogens are used to stimulate the fattening of cattle. In 1979, the Food and Drug Administration (FDA), an agency of the federal government, took steps to ban the use of one such growth stimulant. This hormone, known as diethylstilbestrol or DES, had been linked to the development of cancer in human beings. Traces of DES had been found in the meat of slaughtered animals (see DES). In 1993, the FDA approved the commercial use of synthetic bovine somatotropin (BST). This hormone has been shown to increase the amount of milk produced by dairy cows.
