thyroid gland, endocrine gland, situated in the neck, that secretes hormones necessary for growth and proper metabolism. It consists of two lobes connected by a narrow segment called the isthmus. The lobes lie on either side of the trachea, the isthmus in front of it. Thyroid tissue is composed of millions of tiny saclike follicles, which store thyroid hormone in the form of thyroglobulin, a glycoprotein. Blood capillaries attached to the gland yield a constant supply of plasma. The protein thyroglobulin is the chief component of the jellylike substance, called colloid, that is secreted by the follicles. It attaches to the thyroid hormone for storage purposes; when the hormone is ready to be released, the protein detaches itself. Before it is released into the bloodstream, the thyroid hormone is converted into thyroxine and small quantities of the other closely related thyroid hormones. The amount of thyroxine production (and therefore the metabolic rate) is dependent on a sufficient intake of iodine and on stimulation by thyroid-stimulating hormone (TSH) from the pituitary gland. Metabolic disorders result when the thyroid secretes too little or too much thyroxine. Deficiencies in thyroid secretion (hypothyroidism) occur when there is insufficient iodine in the diet. A disease known as goiter results from the deficiency, although it has been virtually eliminated by the use of iodized salt. Hypothyroidism that results from glandular malfunction is known as myxedema in the adult and cretinism in infancy and childhood. Treatment is by administration of thyroxine. Excessive secretion of thyroxine, or hyperthyroidism, causes an increased metabolic rate, loss of weight despite good appetite, protrusion of the eyeballs, rapid pulse, and irritability. The condition, also known as Graves' disease, may be accompanied by enlargement of the thyroid. The thyroid gland also produces the hormone calcitonin, which is involved in the regulation of serum calcium in the body. See also endocrine system.

thymus gland, mass of glandular tissue located in the neck or chest of most vertebrate animals. In humans, the thymus is a soft, flattened, pinkish-gray organ located in the upper chest under the breastbone. It is relatively large in the newborn infant (about the size of the baby's fist), and continues to grow throughout childhood up to the age of puberty when it weighs about 1.2 oz (35 grams). Then it gradually decreases in size until it blends in with the surrounding tissue. The functions of the thymus were not well understood until the early 1960s, when its role in the development of the body's system of immunity was discovered. Beginning during fetal development, the thymus processes many of the body's lymphocytes, which migrate throughout the body via the bloodstream, seeding lymph nodes and other lymphatic tissue. The main cells undergoing this processing are the T cells, a heterogeneous groups of cells essential in protecting the body against invasions by foreign organisms (see immunity). If the thymus fails to develop or is removed early in fetal life, the immune system cannot develop completely. Normally, by the time the infant is a few months old, the immune system has sufficiently formed so as to function throughout life. However, further growth and development of lymphoid tissue still depends on intervention by the thymic cells. After the initial seeding process, the thymus releases a hormonal substance that stimulates further growth of lymphoidal tissue, although such a substance has not yet been isolated.

suprarenal gland: see adrenal gland.

sebaceous gland, gland in the skin of mammals that secretes an oily substance called sebum. In humans, sebaceous glands are primarily found in association with hair follicles but also occur in hairless areas of the skin, except for the palms of the hand and soles of the feet. Sebum is a mixture of fat and the debris of dead fat-producing cells. These cells are constantly replaced by new growth at the base of the glands. Generally the sebum is deposited on the hairs inside the follicles and is brought up to the surface of the skin along the hair shaft. In hairless areas, the sebum surfaces through ducts. Sebum lubricates and protects the hair and skin and prevents drying and irritation of membranes. Sebum may collect excessively as a result of poor hygiene, a diet rich in fats, or accelerated glandular activity, especially during adolescence. Excessive secretions of sebum may be related to acne, certain forms of baldness, and other skin disorders.

prostate gland, gland that is part of the male reproductive system. It is an organ about the size of a chestnut and consists of glandular and muscular tissue. It is situated below the neck of the bladder, encircling the urethra. The prostate produces a thin, milky, alkaline fluid that is secreted into the urethra at the time of emission of semen, providing an added medium for the life and motility of sperm. It is probable that prostatic fluid enhances fertility since the fluid flowing from the testes and seminal vesicles is acidic and sperm are not optimally mobile unless their medium is relatively alkaline.

In men over 50 enlargement of the prostate (benign prostatic hypertrophy) is common. Sometimes the result is pressure on the urethra and bladder, which interferes with urination, precipitating urinary retention and kidney disease. Balloon dilatation of the urethra and medication with alpha blockers, finasteride (Proscar), and extracts of saw palmetto have joined traditional surgical removal of the prostate (prostatectomy) as therapies. See also prostate cancer, prostatitis.

pituitary gland, small oval endocrine gland that lies at the base of the brain. It is sometimes called the master gland of the body because all the other endocrine glands depend on its secretions for stimulation (see endocrine system).

Anatomy and Function

Physiologically, the pituitary is divided into two distinct lobes that arise from different embryological sources. The anterior lobe, or adenohypophysis, grows upward from the pharyngeal tissue at the roof of the mouth. An intermediate lobe also originates in the pharynx, but in humans it is greatly reduced in structure and function. The posterior lobe, or neurohypophysis, grows downward from neural tissue. It is structurally continuous with the hypothalamus of the brain, to which it remains attached by the hypophyseal, or pituitary, stalk. The hypothalamus controls almost all secretions of the pituitary. The posterior lobe is controlled by nerve fibers that originate in hypothalamic neurons and the anterior lobe by substances that are transported from the hypothalamus by tiny blood vessels.

Pituitary Hormones

The tissues in the anterior lobe consist of extensive vascular areas interspersed among glandular cells that secrete at least six different hormones. It was formerly believed that a master molecule was stimulated by various enzymes to produce these hormones, but present evidence indicates that each is individually synthesized, probably by a specific type of glandular cell. Three such types of cells exist in the anterior pituitary gland: acidophils, basophils, and chromophobes. The growth hormone, thought to be synthesized by certain acidophils, stimulates all the tissues in the body to grow by effecting protein formation.

The remaining five important hormones influence body functions by stimulating target organs. Adrenocorticotropic hormone (ACTH) controls the secretion of steroid hormones by the adrenal cortex, which affects glucose, protein, and fat metabolism; thyrotropin controls the rate of thyroxine synthesis by the thyroid gland, which is the principal regulator of body metabolic rate; prolactin, which regulates the formation of milk after the birth of an infant; and three separate gonadotropic hormones (follicle-stimulating hormone, luteinizing hormone, and luteotropic hormone) control the growth and reproductive activity of the gonads.

The release of each of the hormones from the anterior lobe is controlled by a specific substance secreted by nerve cells in the hypothalamus. These substances, called releasing factors, are transmitted by nerve fibers to tiny capillaries in the hypophyseal stalk. They move through blood vessels to the anterior lobe, where each releasing factor is responsible for the release of a specific pituitary hormone.

The two hormones that are produced by the posterior lobe are synthesized by nerve cells in the hypothalamus. They are transported by nerve fibers to nerve endings in the posterior lobe, where they are released. The hormones are antidiuretic hormone (ADH or vasopressin), which alters the permeability of the kidney tubules, permitting more water to be retained by the body; and oxytocin, which aids in the release of milk from mammary glands and causes uterine contractions. The only hormone that is synthesized by the intermediate lobe is the melanocyte-stimulating hormone, which appears to control skin pigmentation.

Disorders of Pituitary Hormone Secretion

Oversecretion of the pituitary hormone human growth hormone can cause gigantism if it occurs before growth of the long bones is complete, or acromegaly if it begins during adulthood. Undersecretion of human growth hormone can lead to dwarfism if experienced during childhood, and decreased endocrine function accompanied by lethargy and loss of sexual capacity in the adult.

pineal gland, small organ (about the size of a pea) situated in the brain. Long considered vestigial in humans, the structure, which is also called the pineal body or the epiphysis, is present in most vertebrates. It is sensitive to different levels of light and is essential to the functioning of an animal's biological clock. In many animals, including humans, the pineal gland synthesizes a hormone called melatonin in periods of darkness. Melatonin synthesis is halted when light hits the retina of the eye, sending impulses to the gland via the optic nerve. Besides influencing daily, or circadian, rhythms such those of as sleep and temperature, the pineal gland and melatonin appear to direct annual rhythms and seasonal changes in animals. The pineal gland and melatonin are now being studied for their roles in sleep, reproduction, aging, and seasonal affective disorder. In humans the pineal gland begins to produce melatonin at age 3 months; production falls steadily from puberty on.

mammary gland, organ of the female mammal that produces and secretes milk for the nourishment of the young. A mammal may have from 1 to 11 pairs of mammary glands, depending on the species. Generally, those mammals that bear larger litters have more glands. The mammary gland of the cow and of some other mammals is known as the udder.

In humans, there is one pair of mammary glands, also known as mammae, or breasts. They are rudimentary in both sexes until the age of puberty when, in response to ovarian hormones, they begin to develop in the female. During pregnancy, they distend still further in preparation for nursing the infant. Pregnant women are prevented from lactating (producing milk) by the presence in the blood of high levels of estrogen and progesterone, secreted by the placenta until birth occurs.

After birth, response to prolactin, the milk-stimulating hormone, is no longer inhibited by placental hormones, and lactation begins. Mammary tissue contains between 15 and 20 compartments called lobes, each of which is divided into smaller compartments called lobules. The lobes and lobules are connected by a network of tubes whose cells manufacture the liquid and fatty substances that form milk. The tubes of each lobe connect with a duct, and all ducts lead to the nipple, where the milk is secreted when the nipple is sucked by the young. The letdown of milk during the nursing process is aided by oxytocin, a hormone secreted by the pituitary. The physical force of an infant's sucking on the breast is a major stimulus to milk production. Disorders of the mammary gland include mastitis and breast cancer.

lacrimal gland: see tears.

gland, organ that manufactures chemical substances. A gland may vary from a single cell to a complex system of tubes that unite and open onto a surface through a duct. The endocrine glands, e.g., the thyroid, adrenals, and pituitary, produce hormones that are secreted directly into the bloodstream (see endocrine system). Exocrine glands secrete their substances onto an external or internal body surface. Most exocrine glands, e.g., the salivary and lacrimal glands, release their secretions through ducts. However, some open directly onto a body surface, as in the sebaceous glands of the skin and the digestive glands of the intestinal mucosa. A simple exocrine gland may consist only of a tube lined with secretory cells. In more complex types, clumps of cells produce the secretion and a duct or system of ducts discharges the secreted material. Some glands have dual functions, e.g., the liver, pancreas, ovary, and testis produce both a secretion that is emitted through a duct and a hormone that is taken up by the blood. Such structures are called mixed glands. Among the substances produced by exocrine glands in humans are sweat, lubricants like mucus and tears, and digestive juices. There are specialized exocrine glands in the animal world that produce such substances as the shells of bird eggs, spiderwebs, and the cocoons of the silkworm larvae. Simple glands are also common in the plant kingdom. The sweet nectar of flowers and the resinous pitch of pine trees are substances produced by plant glands.

exocrine gland: see gland.

eccrine gland: see sweat.

ductless gland: see endocrine system; gland.

apocrine gland: see sweat.

adrenal gland or suprarenal gland, endocrine gland (see endocrine system) about 2 in. (5.1 cm) long situated atop each kidney. The outer yellowish layer (cortex) of the adrenal gland secretes about 30 steroid hormones, the most important of which are aldosterone and cortisol. Cortisol regulates carbohydrate, protein, and fat metabolism, and its secretion is controlled by the output of adrenocorticotropic hormone (ACTH) from the pituitary gland. Aldosterone regulates water and salt balance in the body; its secretion is only slightly influenced by the pituitary. Steroid hormones also counteract inflammation and allergies and influence the secondary sex characteristics to a limited degree. The adrenal cortex controls metabolic processes that are essential to life and if it ceases to function death ensues within a few days. Artificial synthesis of the steroid hormones has made it possible to treat many conditions related to underactivity of the adrenal cortex, e.g., Addison's disease. The inner reddish portion (medulla) of the adrenal gland, which is not functionally related to the adrenal cortex, secretes epinephrine (adrenaline) and norepinephrine. The release of these hormones is stimulated when an animal is excited or frightened, causing increased heart rate, increased blood flow to the muscles, elevated blood sugar, dilation of the pupils of the eyes, and other changes that increase the body's ability to meet sudden emergencies.

Cowper's gland: see reproductive system.

Endocrine gland in the throat that secretes hormones vital to metabolism and growth. Secretion of thyroid hormones—mostly thyroxine (T₄)—is controlled by thyroid-stimulating hormone (TSH), released by the pituitary gland when the level of thyroid hormones in the blood drops below a certain threshold (see endocrine system). These hormones' primary action in adults is to regulate cellular oxygen consumption (metabolic rate). They also lower blood cholesterol and are necessary for normal growth and development in children. The thyroid also produces calcitonin, a hormone that stimulates deposition of calcium from the blood into the bones, balancing the action of parathyroid hormone. Seealso goitre; Graves disease; iodine deficiency.

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or lachrymal duct and gland

Structures that produce, distribute, and carry away tears. An almond-shaped gland above the outer corner of each eye secretes tears between the upper eyelid and the eyeball. Tears moisten and lubricate the conjunctiva (the membrane that lines the eyelid and covers the white of the eye) and then flow into the barely visible openings (near the inner corners of the eyelids) of the tear ducts, which lead to the nasal cavity. Oil (from sebaceous glands on the edge of the eyelid) keeps tears from spilling out unless secretion increases because of crying or a reflex triggered by stimuli such as eye irritation, bright lights, or spicy foods.

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Either of two types of perspiration glands in the skin. Eccrine sweat glands, controlled by the sympathetic nervous system, use evaporation to cool the skin by secreting water when body temperature rises. Apocrine sweat glands, usually associated with hair follicles, are concentrated in the underarms and genital region. Starting at puberty, hormones stimulate them to continuously secrete a fatty sweat. Certain specialized glands, such as mammary glands and wax-secreting glands of the ear canal, probably developed from this type of gland.

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or suprarenal gland

Either of two small triangular endocrine glands located on top of the kidneys. In humans, each gland weighs about 0.18 oz (5 g) and consists of an inner medulla, which produces the catecholamine hormones epinephrine and norepinephrine, and an outer cortex (about 90percnt of the gland), which secretes the steroid hormones aldosterone, cortisol, and androgens (the last two in response to ACTH from the pituitary gland). Diseases of the adrenal glands include pheochromocytoma (a tumour of the medulla) and the cortical disorders Addison disease, adrenal hypertrophy, Cushing syndrome, and primary aldosteronism.

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Small oil-producing gland in the skin, usually connected to a hair follicle by a duct into which it releases sebum, a component of the slightly greasy film on the skin that helps keep it flexible and prevents too much water loss or absorption. The glands are distributed over the entire body except the palms and soles, most abundantly on the scalp and face. Large and well developed at birth, they shrink during childhood but enlarge again and increase their sebum output at puberty (apparently in response to male hormones), often leading to acne.

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Any of the organs that secrete saliva. Three pairs of major glands secrete saliva into the mouth through distinct ducts: the parotid glands (the largest), between the ear and the back of the lower jaw; the submaxillary glands, along the side of the lower jaw; and the sublingual glands, in the floor of the mouth near the chin. There are also numerous small glands in the tongue, palate, lips, and cheeks. The presence, smell, or thought of food normally increases secretion.

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Chestnut-shaped male reproductive organ, located under the bladder, which adds secretions to the sperm during ejaculation of semen. It surrounds the urethra (see urinary system) and is rounded at the top, narrowing to a blunt point. The prostate consists of 30–50 glands, supported by connective tissue, that discharge fluids into the urethra and two ejaculatory ducts. Those ducts, which also carry sperm and fluid discharged by the seminal vesicles, join the urethra inside the prostate. The prostate contributes 15–30percnt of the seminal fluid. It reaches its mature size at puberty. Around age 50, it commonly shrinks and decreases its secretions; an increase in size after midlife may be due to inflammation or malignancy. Seealso prostatic disorder.

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or hypophysis

Endocrine gland lying on the underside of the brain that plays a major role in regulating the endocrine system. The anterior pituitary lobe secretes six hormones that play specific roles in stimulating production of cortisol and androgens by the adrenal cortex (corticotropin), growth of eggs and sperm (follicle-stimulating hormone), production of progesterone and testosterone (luteinizing hormone), linear growth in children and bone maintenance in adults (growth hormone), milk production (prolactin), and production of thyroid hormone (thyrotropin). The posterior lobe stores and releases two hormones, oxytocin and vasopressin, from nerve cells in specialized regions of the hypothalamus that control pituitary function. These hormones stimulate uterine contraction and milk secretion (oxytocin) and blood pressure and fluid balance (vasopressin).

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or pineal body

Endocrine gland in the brain that produces melatonin. It is large in children and begins to shrink at puberty. The gland may play a significant role in sexual maturation, circadian rhythm and sleep induction, and seasonal affective disorder and depression. In animals it is known to play a major role in sexual development, hibernation, and seasonal breeding.

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Endocrine gland located close to and behind the thyroid gland. Humans typically have four parathyroid glands. Each gland secretes parathyroid hormone, which regulates blood calcium and phosphate levels. When serum calcium concentrations drop, increased hormone secretion releases calcium from bone into the bloodstream (see calcium deficiency). An increase in parathyroid hormone secretion also increases excretion of phosphate in the urine, thereby lowering serum phosphate concentrations. In addition, the hormone regulates magnesium metabolism by increasing its excretion. When thyroid removal is required, the parathyroid glands must be separated out and left in place to avoid hypoparathyroidism (parathyroid hormone deficiency). Seealso endocrine system.

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Milk-producing gland of female mammals, usually present but undeveloped and nonfunctional in males. Regulated by the endocrine system, it is derived from a modification of sweat glands. The mammary gland of a woman who has not borne children consists of a conical disk of glandular tissue, encased in fat that gives the breast its shape. The gland is made up of lobes drained by separate ducts that meet at the nipple. Pregnancy causes the cells lining the lobes to multiply, and lactation begins in response to hormones released starting at the time of birth. At the end of lactation, the glands return almost to their state before pregnancy. After menopause, they atrophy and are largely replaced by connective tissue and fat.

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Small, rounded mass of lymphoid tissue contained in connective tissue. They occur all along lymphatic vessels, with clusters in certain areas (e.g., neck, groin, armpits). They filter bacteria and other foreign materials out of lymph and expose them to lymphocytes and macrophages that can engulf them; these cells multiply in response to accumulation of such materials, which is why lymph nodes swell during infections. The nodes also produce lymphocytes and antibodies, to be carried by lymph throughout the lymphatic system. In Hodgkin disease and other lymphomas, malignant lymph cells proliferate, causing lymph node enlargement. Other cancers often invade lymphatic vessels, which can carry cells from the tumour to lymph nodes, where they are trapped and grow into secondary tumours. Lymph nodes are therefore removed in cancer surgery to detect or prevent tumour spread.

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Collection of cells or tissue that removes specific substances from the blood, alters or concentrates them, and then either releases them for further use by the body or eliminates them. Typically, the functional cells of a gland rest on a membrane and are surrounded by a meshwork of blood vessels. Endocrine, or ductless, glands (e.g., pituitary, thyroid, adrenal) discharge hormones into the bloodstream directly rather than through ducts (see endocrine system). Exocrine glands (e.g., digestive, mammary, salivary, sweat) discharge their products through ducts.

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