Definitions

gland

gland

[gland]
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.

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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A gland is an organ in an animal's body that synthesizes a substance for release such as hormones or breast milk, often into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland).

Types

Glands can be divided into two groups:

  • Endocrine glands — are glands that secrete their product directly onto a surface rather than through a duct.
  • Exocrine glands — secrete their products via a duct, the glands in this group can be divided into three groups:
    • Apocrine glands — a portion of the secreting cell's body is lost during secretion. Apocrine gland is often used to refer to the apocrine sweat glands, however it is thought that apocrine sweat glands may not be true apocrine glands as they may not use the apocrine method of secretion.
    • Holocrine glands — the entire cell disintegrates to secrete its substances (e.g., sebaceous glands)
    • Merocrine glands — cells secrete their substances by exocytosis (e.g., mucous and serous glands). Also called "eccrine."

The type of secretory product of an Exocrine gland may also be one of three categories:

  • Serous glands — secrete a watery, often protein-rich product.
  • Mucous glands — secrete a viscous product, rich in carbohydrates (e.g., glycoproteins).
  • Sebaceous glands — secrete a lipid product.

Formation

Every gland is formed by an ingrowth from an [epithelium|epithelial] surface. This ingrowth may from the beginning possess be a tubular structure, but in other instances glands may start as a solid column of cells which subsequently becomes tubulated.

As growth proceeds, the column of cells may divide or give off offshoots, in which case a compound gland is formed. In many glands the number of branches is limited, in others (salivary, pancreas) a very large structure is finally formed by repeated growth and sub-division. As a rule, the branches do not unite with one another, but in one instance, the liver, this does occur when a reticulated compound gland is produced. In compound glands the more typical or secretory epithelium is found forming the terminal portion of each branch, and the uniting portions form ducts and are lined with a less modified type of epithelial cell.

Glands are classified according to their shape.

  • If the gland retains its shape as a tube throughout it is termed a tubular gland.
  • In the second main variety of gland the secretory portion is enlarged and the lumen variously increased in size. These are termed alveolar or saccular glands.

Specific glands

A list of human exocrine glands is available here.

A list of human endocrine glands is available here.

Crypts of Lieberkuhn

The Crypts of Lieberkuhn in the lower gastrointestinal tract are also referred to as "glands."

Additional images

References

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