Hormone
Hormones are chemicals produced by glands, tissues, and organs to control the function of a target organ or regulate the production of another hormone. The body produces a specific amount of a hormone in response to stimuli (signals) from inside and outside. The hormone balance helps keep the body functioning in ordinary or stressful situations--a process called homeostasis.
Dozens of human hormones play important roles in growth, sex and reproduction, digestion, blood composition, and stress control. Other animals and plants produce hormones as well. Several hormones may work as a team on the same organ or tissue. Their combined effect may be greater than the sum of their single effects (synergism).
Many hormones are produced by the endocrine system, a large group of ductless glands. They include the pituitary and pineal, at the base of the brain; the thyroid and parathyroid in the throat; and the adrenal glands, sex glands (ovaries and testes), thymus, and pancreas in the trunk. The digestive system and other tissues and organs also produce hormones.
Hormones are either proteins or lipid-like steroids. Some protein hormones are long chains (polypeptides) ranging from three to over two hundred amino acids. Glycoprotein hormones have both carbohydrate and peptide structures. Protein hormones are manufactured and then stored or circulated until they are needed. Steroids, including all sex and adrenal gland hormones, are synthesized as needed from the steroid cholesterol.
The first modern European scientists to be interested in body glands and their secretions were sixteenth-and seventeenth-century anatomists such as Andreas Vesalius in Italy and Regnier de Graaf in the Netherlands. The concept of a hormone (from the Greek word meaning shock or impulse) was developed by the British biochemists Ernest Starling and William Bayliss after they isolated the digestive substance secretin in 1906. Scientists soon realized that the first hormone to actually be isolated and synthesized was the adrenal substance epinephrine, by the Japanese-American chemist Jokichi Takemine in 1901. His achievement was based on earlier work by the British physiologist Edward Sharpey-Schäfer and the American pharmacologist John Jacob Abel.
The next milestone was the isolation of the thyroid hormone thyroxine in 1914 by the American biochemist Edward Kendall. Too much or too little thyroxine can cause illness. One of the earliest known thyroid-gland disorders is called Graves's disease, characterized by the thyroid gland's increased size and activity. It was first defined in 1835 by Irish scholar and physician Robert James Graves. Its cause is not known, but Graves's disease is linked to stress and may be hereditary.
In 1921, the Canadian physicians Frederick Banting and Charles Best isolated the pancreatic hormone insulin. It was soon used to control diabetes.
Another area of important exploration was with adrenal gland cortex hormones. The cortex (outer covering) of the adrenal glands produces several hormones. In the 1930s, the American biochemist Edward Kendall and the Swiss chemist Tadeus Reichstein independently isolated the first in a series of these hormones, cortisone, and established that it was a steroid. Cortisone was the first hormone to be used medically, by the American researcher Philip Hench in the 1940s to reduce inflammation in rheumatoid arthritis and other connective tissue diseases. Other corticoids (adrenal cortex steroid hormones) are hydrocortisone (corticol), corticosterone, and aldosterone. Cushing's disease, described in 1912 by the American neurosurgeon and physiologist Harvey Cushing (1869-1939), results from excessive production of cortisone, hydrocortisone, and corticosterone. It causes fat redistribution from the lower body to the trunk, facial puffiness, and diabetes.
In the 1930s, many scientists, including Adolf Butenandt in Germany, Leopold Ruzicka in Switzerland, and Percy Julian in the United States, began studying hormones produced by the male testes and female ovaries.
Hormones produced by the pituitary gland are human growth hormone (hGH), or somatotropin (hST). Growth hormone is necessary for body growth and development. The American biologist Herbert Evans (1882-1971) experimented during the 1930s with pituitary extracts that increased growth in laboratory animals. In the 1940s, Evans and his colleague Choh Hao Li isolated it. In the 1960s and 1970s, Li and others independently synthesized it through genetic engineering.
Eventually, scientists learned to make some hormones in the laboratory. American biochemist Vincent Du Vigneaud synthesized the small (eight amino acid) pituitary hormone oxytocin, which generates milk production in the mammary glands and causes uterine contractions. This achievement led to the synthesis of many larger and more complex hormones for medical use.
No systems of the body operate completely independently of others. The Argentinean physiologist Bernardo Houssay (1887-1971) showed how the pituitary gland affected the secretion of insulin and other hormones. For this work, he shared the 1947 Nobel prize in physiology or medicine. Adrenocorticotropic hormone (ACTH) stimulates the adrenal gland cortex to secrete corticol (hydrocortisone), corticosterone, and aldosterone. ACTH's properties were first investigated in the 1930s by the Canadian James Collip, as well as Evans and Houssay. Li was one of several scientists to isolate and synthesize ACTH.
The body must have some way of regulating hormone levels and production. In the 1950s, the British anatomist Geoffrey Harris proposed that a part of the brain called the hypothalamus and the pituitary gland jointly control production of hormones. His theory was confirmed by the American endocrinologists Roger Guillemin and Andrew Victor Schally, as well as other scientists. Feedback or signals such as a hormone's blood level begins the process.
The increase in knowledge about hormones is reflected in studies and treatment of Addison's disease. In 1849, Thomas Addison, a British physician and pathologist, matched symptoms of progressive weakness, increased skin pigmentation, and weight loss to an atrophied condition of the adrenal gland. This was the first medical description of an adrenal gland disease. Addison 's disease symptoms are now known to be caused by low levels of two hormones, aldosterone and hydrocortisone (cortisol), and can be controlled with replacement hormones.
Today scientists are increasing their understanding of hormones' interaction with target tissue cells and how this information can be used in the treatment of illness. Researchers are also interested in the activity of hormone-like substances called growth factors, produced by individual cells throughout the body.
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