endocrine system

(physiology)

The chemical integrating system in animals that lack a vertebral (spinal) column. An endocrine system consists of those glandular cells, tissues, and organs whose products (hormones) supplement the rapid, short-term coordinating functions of the nervous system. Almost all of the information about invertebrates pertains to the more highly evolved groups that will be discussed below, the annelids, echinoderms, mollusks, and most particularly two classes of arthropods, the insects and crustaceans. Several of the hormones in invertebrates are neurohormones, that is, they are produced by nerve cells. See also Neurosecretion.

Insects

Increase in linear dimensions of an insect can only occur at periodic intervals when the restricting exoskeleton is shed during a process known as molting. Once an insect becomes an adult, it ceases to molt. The orderly sequence of molts that leads from the newly hatched insect to the adult is controlled by three hormones. The brain produces a neurohormone which stimulates a pair of glands in the prothorax, the prothoracic glands, causing release of the molting hormone, ecdysone. A third hormone, the juvenile hormone, produced by a pair of glands near the brain, functions during the juvenile molts to suppress the differentiation of adult tissues. Juvenile hormone permits growth but prevents maturation. See also Ecdysone.

Two neurohormones with antagonistic actions are involved in regulating the water content of insects. One, the diuretic hormone, promotes water loss by increasing the volume of fluid secreted into the Malpighian tubules, the excretory organs. The second, the antidiuretic hormone, acts to conserve water by causing the wall of the rectum to increase the volume of water resorbed from its lumen while lowering the excretion rate from the Malpighian tubules.

Bursicon, a protein neurohormone, is responsible for the tanning and hardening of the newly formed cuticle. During the development of some insects, a period of arrested development occurs, termed the diapause. The mechanisms controlling the onset and the termination of diapause are largely unknown. However, in some insects there is evidence for a hormone, proctodone, that reinitiates development. A very few species of insects, most notably the stick insect (Carausius morosus), have the ability to change color. This insect becomes darker at night and lighter by day as a result of the rearrangement of pigment granules within the epidermal cells. Darkening is due to a hormone produced in the brain.

Crustaceans

Higher crustaceans have a structure, the sinus gland, which in most stalk-eyed species lies in the eyestalk and is the storage and release site of a molt-inhibiting hormone. The Y-organs, a pair of structures found in the anterior portion of the body near the excretory organs, are the source of the crustacean molting hormone, crustecdysone. Chemically, crustecdysone is very similar to the ecdysone from insects; both substances can cause molting in crustaceans and insects. The sinus gland is a neuroendocrine structure, but the Y-organs are nonneural.

Crustacean pigment cells (chromatophores) are under hormonal control, as in insects. The active substances are released from the sinus glands and the postcommissural organs, which lie near the esophagus. Substances have been found that cause dispersion of the pigment within the chromatophore, as well as substances with the opposite action. See also Chromatophore.

The compound eye of crustaceans has three retinal pigments. The movements of these pigments with illumination level control the amount of light impinging on the photosensitive cells. A substance has been found that causes the migration of these pigments toward the light-adapted positions and another substance that causes migration toward dark-adapted positions.

The pericardial organs, which are found near the heart, are neuroendocrine organs that cause an increase in the amplitude of the heart beat. The sinus gland contains the hyperglycemic hormone, which causes a rise in blood glucose.

Annelids

Strong evidence for hormones in annelids has been obtained from studies of the reproductive system. One substance that has been found in some marine annelids inhibits maturation of the gametes. This substance was thought to have been produced by the brain, but studies show that another structure, the infracerebral gland, which lies ventral to the brain, may be involved.

Echinoderms

The radial nerves of starfishes contain two substances that are required for the maintenance of a normal reproductive cycle. One, the shedding substance, induces spawning. The second, shedhibin, inhibits the former.

Mollusks

The best-established endocrine organs in mollusks, the optic glands, occur in the octopus and squid. They are a pair of small structures, found near the brain, that produce a substance which causes gonadal maturation. The optic glands in turn are regulated by inhibitory nerves from the brain. There is also some evidence that a portion of the nervous system (the pleural ganglia) may secrete a hormone that affects water balance. Removal of the pleural ganglia from a freshwater snail results in swelling of the animal due to the influx of water.

Endocrine system (vertebrate)

A system of chemical communication among cells. The classical vertebrate endocrine system consists of a group of discrete glands that secrete unique products (hormones) into the bloodstream. These products travel in the blood to distant sites or targets where they cause specific physiological responses. Thus endocrine glands differ from exocrine glands, in that they lack ducts and deliver their secretions in the bloodstream. The classical definition of an endocrine system has become harder to apply with the discovery of scattered cells rather than discrete glands that act as endocrine organs, of endocrine cells that affect themselves (autocrine effect) or nearby targets (paracrine effect) by diffusion through extracellular fluids rather than the bloodstream, and of neurons that secrete hormones (neurosecretion). All of these mechanisms, however, allow for chemical intercellular communication and can be considered part of the endocrine system. See also Neurosecretion.

One important function of the endocrine system, along with the nervous system, is to maintain homeostasis, that is, a constancy of the internal environment of an organism. Thus an organism reacts and adjusts physiologically to changes in its external environment. The roles of the endocrine and nervous systems in maintaining homeostasis are many, complementary, and overlapping. See also Homeostasis; Nervous system (vertebrate).

Nature of hormones

Hormones are the products of endocrine cells. They are either proteinlike or steroidal. Peptide hormones are produced by protein synthetic mechanisms directed by the genes of the endocrine cells. They are stored in endocrine cells in secretory granules that bud off the endoplasmic reticulum and Golgi membranes, where protein synthesis occurs. The granules leave the cell by endocytosis and enter the bloodstream. Steroid hormones, on the other hand, are produced from cholesterol by a number of well-characterized, enzyme-catalyzed steps. Cholesterol is thus converted stepwise to various steroid families: the hormones of the adrenal cortex (cortisol and aldosterone), the estrogens (estradiol) from the ovary, and the androgens (testosterone) from the testis. Steroid hormones diffuse across the endocrine cell plasma membrane to enter the circulation. See also Hormone; Protein; Steroid.

Since in most cases the blood carries hormones throughout the body, there must be a system by which only certain tissues respond to each hormone. This is accomplished by receptors, which are binding sites either on the surface of the target cell or within its nucleus. Receptors are high-molecular-weight proteins; the structure of some, such as the insulin receptor, is known. In general, peptide hormones cannot cross the plasma membrane, so their receptors are located there, whereas steroid hormones do pass through the plasma membrane of their targets and bind to nuclear receptors, probably located in the deoxyribonucleic acid (DNA).

In order for peptide hormones to stimulate physiological changes within the target cell, the “message” must be passed from the hormone-receptor complex of the plasma membrane to the interior of the cell. This process of signaling across the plasma membrane is accomplished by so-called second messengers of which the best known is adenosine 3′,5′-cyclic monophosphate (cyclic AMP). Steroid hormones, of course, have no need for second messengers since they are lipid-soluble and pass readily through the plasma membrane and into the cell. See also Second messengers.

Once hormones are bound with their receptors and have stimulated their target cells, physiological responses occur. This may involve such biochemical processes as conversion of an inactive form of an enzyme into an active one, stimulation of critical enzymatic pathways, increase in transport of glucose or amino acids into cells, or synthesis of new proteins. These events may result in overall changes in cell or organ function, metabolism, growth, or even the behavior of the organism.

The endocrine system is regulated by control mechanisms, the means by which homeostasis is achieved. The most common relationship between the hormone and its target is one of negative feedback, whereby the response to the hormonal stimulus turns off the original stimulus. For example, the endocrine pancreatic beta cells produce insulin in response to high blood sugar levels. Insulin is released into the blood, where it causes its target cells to take up glucose, thus reducing blood sugar. When blood glucose concentration falls, the secretion of insulin is turned off. The system is turned back on when the blood glucose content gradually increases again. See also Carbohydrate metabolism; Insulin.

Pituitary gland and hypothalamus

The pituitary gland, or hypophysis, is located near the base of the brain. It secretes many hormones and controls the function of other endocrine glands. The production and release of the various pituitary hormones are regulated in turn by small peptide-releasing hormones from the hypothalamus of the brain. These factors are produced by neurosecretory neurons and travel to the adenohypophysis (anterior lobe of the pituitary) by way of a portal blood system. The releasing hormones stimulate specific cells of the adenohypophysis to produce and release their hormones. Generally speaking, each of the adenohypophysial hormones is affected by a separate releasing hormone. Thus, the hypothalamic thyrotropin-releasing hormone stimulates the synthesis and release of thyroid-stimulating hormone (thyrotropin) by the adenohypophysis. Other adenohypophysial hormones include adrenocorticotropic hormone, which stimulates the production of steroid hormones by the adrenal cortex; growth hormone, which stimulates protein synthesis and growth in many cells; prolactin, which stimulates the production of milk by the mammary glands and is important in many other functions; follicle-stimulating hormone, which induces growth of the follicles of the ovary prior to ovulation; and luteinizing hormone, which induces ovulation. The release of both follicle-stimulating and luteinizing hormones is governed by gonadotropin-releasing hormone.

Other hypothalamic hormones do not reach the pituitary by way of the bloodstream; instead they travel down the long axons of the neurosecretory cells into the neurohypophysis (posterior lobe of the pituitary). Oxytocin acts upon the mammary glands of female mammals to cause milk release in response to suckling by the young, and stimulates the uterus to contract at the end of pregnancy to aid in expulsion of the offspring. Vasopressin (or antidiuretic hormone) is important in water conservation by the kidney tubules and also produces an increase in blood pressure. See also Pituitary gland.

Thyroid gland

The thyroid gland lies in the neck region of mammals. It produces two closely related hormones, triiodothyronine and thyroxine. These both increase the metabolic rate of an organism, and increase enzyme activity and protein synthesis. The thyroid hormones act along with growth hormone to promote cell growth and development. Thyroid hormones are peptides but their three-dimensional structures may be similar to those of steroid hormones. Thus they are unusual in their ability to pass through the plasma membrane of their target cells and bind to nuclear receptors.

The control of hormone secretion by the thyroid (as well as by the adrenal cortex and gonads) involves more complex feedback relationships. These endocrine glands are affected by the levels of hormones from the adenohypophysis. In the case of the thyroid gland, thyrotropin-releasing factor from the hypothalamus stimulates the release of thyroid-stimulating hormone by the adenohypophysis. In response, the thyroid secretes thyroxine and triiodothyronine. High blood levels of the thyroid hormones inhibit the secretion of both thyrotropin-releasing factor (long-loop feedback) and thyroid-stimulating hormone (short-loop feedback).

Calcium regulation

The parathyroid glands derive their name from the fact that in mammals they are embedded within the thyroids. These small glands are essential for life, as they regulate the concentration of calcium ion in blood and other extracellular fluids. If calcium is too low, the animal goes into tetanic convulsions and dies, whereas if calcium is too high, abnormal calcification and stone formation can occur. Parathyroid hormone is a protein hormone that raises the blood calcium levels (hypercalcemia). The hormone acts upon bone to cause the release of calcium and phosphate, and upon the kidney to increase the reabsorption (conservation) of calcium and excretion of phosphate.

Vitamin D is now recognized as a steroidlike hormone, although it does not originate from an endocrine gland. It is synthesized from precursors present in the diet or produced after exposure of skin lipids to ultraviolet light. Vitamin D plays roles in calcium conservation by the kidney and in bone mineralization, but its most important function is to enhance calcium transport across intestinal cells and thus conserve dietary calcium. See also Vitamin D.

Calcitonin is a newly recognized peptide hormone produced by thyroid cells in mammals (different cells from those that produce thyroid hormones) and from the ultimobranchial glands of nonmammalian vertebrates. Calcitonin is hypocalcemic and acts by inhibiting calcium loss from bone. Of the three calcium-regulating hormones, it appears to be the least important. See also Parathyroid gland.

Carbohydrate regulation

Insulin and glucagon are peptide hormones produced by endocrine cells of the pancreatic islets. Insulin is produced by the pancreatic beta cells and is the only hormone that decreases blood sugar (glucose) levels. It acts on its target cells (skeletal muscle, fat cells) to increase the uptake of glucose, amino acids, and fatty acids. Once taken into cells, glucose is used in metabolic reactions or stored as glycogen, a large carbohydrate. Insulin also causes the conversion of amino acids to proteins and fatty acids to fats in the target cells. In the absence of insulin, as in diabetes mellitus, the target cells cannot take up glucose, and thus the body must utilize amino acids and fats as energy sources. These processes result in the accumulation of toxic metabolic products which eventually disrupt the acid-base balance, leading to coma and death. See also Diabetes.

Glucagon, in contrast, is a hyperglycemic hormone. It is a small peptide from the pancreatic islet alpha cells that acts upon liver cells to cause the conversion of glycogen to glucose by activation of key enzymes in a complex metabolic pathway.

Many other hormones elevate blood sugar levels. For example, epinephrine (adrenalin), an amino acid derivative from the adrenal medulla, acts by the same pathway as glucagon to convert glycogen to glucose, except that the targets of epinephrine are skeletal and heart muscle. Epinephrine is secreted in times of stress and serves to prepare the body for an emergency by increasing the availability of energy in the form of glucose and by increasing the heart rate and blood pressure.

Growth hormone, a large protein hormone from the adenohypophysis, is secreted in response to low blood sugar levels. This hormone elevates blood sugar by blocking the uptake of glucose by cells and by favoring the utilization of fats rather than glucose as an energy source.

Many of the adrenal cortical hormones, such as cortisol, are known collectively as glucocorticoids, because they also elevate blood glucose levels. These steroid hormones favor the production of glucose from proteins and fats rather than glycogen (gluconeogenesis). Glucocorticoids also exert an anti-inflammatory action, which makes them useful for treatment of arthritis and other diseases. See also Adrenal gland.

Salt and water regulation

Several hormones affect the ability of the kidney to conserve or excrete salts and water. The antidiuretic hormone (vasopressin) promotes water reabsorption by the kidney tubules, so that the organism excretes less water. The secretion of vasopressin is regulated by hypothalamic neuro-secretory neurons that are sensitive to the concentration of salts in the extracellular fluids. In the absence of vasopressin, an individual excretes great volumes of dilute urine, leading to severe dehydration (diabetes insipidus).

Salt excretion is regulated mainly by two hormones that act in opposition. Aldosterone is an adrenal cortical steroid that promotes the reabsorption of sodium by the kidney tubules and thus decreases its excretion in the urine. In contrast, atriopeptin (atrial natriuretic factor), a peptide that originates in heart muscle, acts upon the kidney to increase the excretion of sodium in the urine. See also Osmoregulatory mechanisms.

Reproductive hormones

Probably the best-studied endocrine glands are the gonads, the testes of the male and the ovaries of the female. The gonads are regulated by the follicle-stimulating hormone and luteinizing hormone from the adenohypophysis. In the male, follicle-stimulating hormone stimulates the initiation of sperm formation by the testis tubules, and luteinizing hormone acts on the nearby Leydig cells of the testis to produce testosterone, the principal male sex hormone. Testosterone acts by a paracrine mechanism to cause the final maturation of sperm, and by way of the blood to stimulate development of the male reproductive system and secondary sex characteristics.

In the female, follicle-stimulating hormone stimulates the growth of the ovarian follicles at the beginning of each reproductive cycle. As the follicles grow, they produce estradiol. Increasing levels of estradiol cause feedback inhibition of gonadotropin-releasing hormone. High levels of estradiol also have an unusual positive feedback effect upon the hypothalamus and adenohypophysis to cause a surge in the secretion of luteinizing hormone, which results in ovulation. The corpus luteum, a remnant of the ovulated follicle, produces both estradiol and the second major female sex hormone, progesterone. Progesterone is necessary for the maintenance of a quiescent uterus during pregnancy, and both estrogen and progesterone are important in the regulation of the female reproductive cycle. Estradiol is also essential for the growth and maturation of the female reproductive system and secondary sex characteristics. In both males and females, the sex hormones affect reproductive behavior. See also Ovary; Reproductive behavior.

Other hormones

There are many other factors that act in various ways to achieve homeostasis or intercellular communication, such as a large number of peptides found in both gastrointestinal cells and the brain. These were recognized for many years as gastrointestinal hormones which aid in secretion of digestive juices and motility of the gastrointestinal tract. Their function in the brain appears to be different, and there is evidence that they act in pain reception or analgesia, as factors that stimulate or curb appetite, or in memory or other functions. This field of neuropeptide hormones is in its infancy and serves to emphasize the close relationship between the endocrine and nervous systems in intercellular communication.

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