INTRODUCTION

Hormone, chemical that transfers information and instructions between cells in animals and plants. Often described as the body’s chemical messengers, hormones regulate growth and development, control the function of various tissues, support reproductive functions, and regulate metabolism (the process used to break down food to create energy). Unlike information sent by the nervous system, which is transmitted via electronic impulses that travel quickly and have an almost immediate and short-term effect, hormones act more slowly, and their effects typically are maintained over a longer period of time.

Hormones were first identified in 1902 by British physiologists William Bayliss and Ernest Starling. These researchers showed that a substance taken from the lining of the intestine could be injected into a dog to stimulate the pancreas to secrete fluid. They called the substance secretin and coined the term hormone from the Greek word hormo, which means “to set in motion.” Today more than 100 hormones have been identified.

Hormones are made by specialized glands or tissues that manufacture and secrete these chemicals as the body needs them. The majority of hormones are produced by the glands of the endocrine system, such as the pituitary, thyroid, adrenal glands, and the ovaries or testes. These endocrine glands produce and secrete hormones directly into the bloodstream. However, not all hormones are produced by endocrine glands. The mucous membranes of the small intestine secrete hormones that stimulate secretion of digestive juices from the pancreas. Other hormones are produced in the placenta, an organ formed during pregnancy, to regulate some aspects of fetal development.

Hormones are classified into two basic types based on their chemical makeup. The majority of hormones are peptides, or amino acid derivatives that include the hormones produced by the anterior pituitary, thyroid, parathyroid, placenta, and pancreas. Peptide hormones are typically produced as larger proteins. When they are called into action, these peptides are broken down into biologically active hormones and secreted into the blood to be circulated throughout the body. The second type of hormones is steroid hormones, which include those hormones secreted by the adrenal glands and ovaries or testes. Steroid hormones are synthesized from cholesterol (a fatty substance produced by the body) and modified by a series of chemical reactions to form a hormone ready for immediate action.

II
HOW HORMONES WORK

Plasma Membrane

The plasma membrane that surrounds eukaryotic cells is a dynamic structure composed of two layers of phospholipid molecules interspersed with cholesterol and proteins. Phospholipids are composed of a hydrophilic, or water-loving, head and two tails, which are hydrophobic, or water-hating. The two phospholipid layers face each other in the membrane, with the heads directed outward and the tails pointing inward. The water-attracting heads anchor the membrane to the cytoplasm, the watery fluid inside the cell, and also to the water surrounding the cell. The water-hating tails block large water-soluble molecules from passing through the membrane while permitting fat-soluble molecules, including some hormones, to freely cross the membrane and activate changes within the cell. Proteins embedded in the plasma membrane carry out a variety of functions. When hormones bind with receptor proteins embedded in the cell membrane, both molecules undergo structural changes that activate chemical changes within the cell.

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Most hormones are released directly into the bloodstream, where they circulate throughout the body in very low concentrations. Some hormones travel intact in the bloodstream. Others require a carrier substance, such as a protein molecule, to keep them dissolved in the blood. These carriers also serve as a hormone reservoir, keeping hormone concentrations constant and protecting the bound hormone from chemical breakdown over time.

Hormones travel in the bloodstream until they reach their target tissue, where they activate a series of chemical changes. To achieve its intended result, a hormone must be recognized by a specialized protein in the cells of the target tissue called a receptor. Typically, hormones that are water-soluble use a receptor located on the cell membrane surface of the target tissues. A series of special molecules within the cell, known as second messengers, transport the hormone’s information into the cell. Fat-soluble hormones, such as steroid hormones, pass through the cell membrane and bind to receptors found in the cytoplasm. When a receptor and a hormone bind together, both the receptor and hormone molecules undergo structural changes that activate mechanisms within the cell. These mechanisms produce the special effects induced by the hormone.

Receptors on the cell membrane surface are in constant turnover. New receptors are produced by the cell and inserted into the cell wall, and receptors that have reacted with hormones are broken down or recycled. The cell can respond, if necessary, to irregular hormone concentrations in the blood by decreasing or increasing the number of receptors on its surface. If the concentration of a hormone in the blood increases, the number of receptors in the cell wall may go down to maintain the same level of hormonal interaction in the cell. This is known as downregulation. If concentrations of hormones in the blood decrease, upregulation increases the number of receptors in the cell wall.

Some hormones are delivered directly to the target tissues instead of circulating throughout the entire bloodstream. For example, hormones from the hypothalamus, a portion of the brain that controls the endocrine system, are delivered directly to the adjacent pituitary gland, where their concentrations are several hundred times higher than in the circulatory system.

III
HORMONAL EFFECTS

Hormonal effects are complex, but their functions can be divided into three broad categories. Some hormones change the permeability of the cell membrane. Other hormones can alter enzyme activity, and some hormones stimulate the release of other hormones.

Recent studies have shown that the more lasting effects of hormones ultimately result in the activation of specific genes. For example, when a steroid hormone enters a cell, it binds to a receptor in the cell’s cytoplasm. The receptor becomes activated and enters the cell’s nucleus, where it binds to specific sites in the deoxyribonucleic acid (DNA), the long molecules that contain individual genes. This activates some genes and inactivates others, altering the cell’s activity. Hormones have also been shown to regulate ribonucleic acids (RNA) in protein synthesis.

A single hormone may affect one tissue in a different way than it affects another tissue, because tissue cells are programmed to respond differently to the same hormone. A single hormone may also have different effects on the same tissue at different times in life. To add to this complexity, some hormone-induced effects require the action of more than one hormone. This complex control system provides safety controls so that if one hormone is deficient, others will compensate.

IV
TYPES OF HORMONES

Hormones exist in mammals, including humans, as well as in invertebrates and plants. The hormones of humans, mammals, and other vertebrates are nearly identical in chemical structure and function in the body. They are generally characterized by their effect on specific tissues.

A
Human Hormones

Menstrual Cycle

A typical menstrual cycle lasts 28 days. It begins with three to five days of menstruation, the shedding of the uterine lining, during which hormone levels are low. At the end of menstruation, a pituitary hormone stimulates new follicles to develop in the ovary. These secrete estrogen as they mature, causing cells in the lining of the uterus to proliferate. Mid-cycle, one mature follicle releases an egg. The empty follicle forms the corpus luteum, an endocrine body that secretes progesterone. Under the added influence of progesterone, the uterine lining thickens further and swells in preparation for the implantation of a fertilized egg. If fertilization does not take place, the corpus luteum dies and hormone levels fall. Without hormonal support, the uterine lining disintegrates and discharges, beginning a new menstrual period and cycle.

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Human hormones significantly affect the activity of every cell in the body. They influence mental acuity, physical agility, and body build and stature. Growth hormone is a hormone produced by the pituitary gland. It regulates growth by stimulating the formation of bone and the uptake of amino acids, molecules vital to building muscle and other tissue.

Sex hormones regulate the development of sexual organs, sexual behavior, reproduction, and pregnancy. For example, gonadotropins, also secreted by the pituitary gland, are sex hormones that stimulate egg and sperm production. The gonadotropin that stimulates production of sperm in men and formation of ovary follicles in women is called a follicle-stimulating hormone. When a follicle-stimulating hormone binds to an ovary cell, it stimulates the enzymes needed for the synthesis of estradiol, a female sex hormone. Another gonadotropin called luteinizing hormone regulates the production of eggs in women and the production of the male sex hormone testosterone. Produced in the male gonads, or testes, testosterone regulates changes to the male body during puberty, influences sexual behavior, and plays a role in growth. The female sex hormones, called estrogens, regulate female sexual development and behavior as well as some aspects of pregnancy. Progesterone, a female hormone secreted in the ovaries, regulates menstruation and stimulates lactation in humans and other mammals.

Other hormones regulate metabolism. For example, thyroxine, a hormone secreted by the thyroid gland, regulates rates of body metabolism. Glucagon and insulin, secreted in the pancreas, control levels of glucose in the blood and the availability of energy for the muscles. A number of hormones, including insulin, glucagon, cortisol, growth hormone, epinephrine, and norepinephrine, maintain glucose levels in the blood. While insulin lowers the blood glucose, all the other hormones raise it. In addition, several other hormones participate indirectly in the regulation. A protein called somatostatin blocks the release of insulin, glucagon, and growth hormone, while another hormone, gastric inhibitory polypeptide, enhances insulin release in response to glucose absorption. This complex system permits blood glucose concentration to remain within a very narrow range, despite external conditions that may vary to extremes.

Hormones also regulate blood pressure and other involuntary body functions. Epinephrine, also called adrenaline, is a hormone secreted in the adrenal gland. During periods of stress, epinephrine prepares the body for physical exertion by increasing the heart rate, raising the blood pressure, and releasing sugar stored in the liver for quick energy.

Insulin Secretion

This light micrograph of a section of the human pancreas shows one of the islets of Langerhans, center, a group of modified glandular cells. These cells secrete insulin, a hormone that helps the body metabolize sugars, fats, and starches. The blue and white lines in the islets of Langerhans are blood vessels that carry the insulin to the rest of the body. Insulin deficiency causes diabetes mellitus, a disease that affects at least 10 million people in the United States.

Astrid and Hanns-Frieder Michler/Photo Researchers, Inc.

Hormones are sometimes used to treat medical problems, particularly diseases of the endocrine system. In people with diabetes mellitus type 1, for example, the pancreas secretes little or no insulin. Regular injections of insulin help maintain normal blood glucose levels. Sometimes, an illness or injury not directly related to the endocrine system can be helped by a dose of a particular hormone. Steroid hormones are often used as anti-inflammatory agents to treat the symptoms of various diseases, including cancer, asthma, or rheumatoid arthritis. Oral contraceptives, or birth control pills, use small, regular doses of female sex hormones to prevent pregnancy.

Initially, hormones used in medicine were collected from extracts of glands taken from humans or animals. For example, pituitary growth hormone was collected from the pituitary glands of dead human bodies, or cadavers, and insulin was extracted from cattle and hogs. As technology advanced, insulin molecules collected from animals were altered to produce the human form of insulin.

With improvements in biochemical technology, many hormones are now made in laboratories from basic chemical compounds. This eliminates the risk of transferring contaminating agents sometimes found in the human and animal sources. Advances in genetic engineering even enable scientists to introduce a gene of a specific protein hormone into a living cell, such as a bacterium, which causes the cell to secrete excess amounts of a desired hormone. This technique, known as recombinant DNA technology, has vastly improved the availability of hormones.

Recombinant DNA has been especially useful in producing growth hormone, once only available in limited supply from the pituitary glands of human cadavers. Treatments using the hormone were far from ideal because the cadaver hormone was often in short supply. Moreover, some of the pituitary glands used to make growth hormone were contaminated with particles called prions, which could cause diseases such as Creutzfeldt-Jakob disease, a fatal brain disorder. The advent of recombinant technology made growth hormone widely available for safe and effective therapy.

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