Difference between revisions of "Gland" - New World Encyclopedia
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− | {{Copyedited}}{{Paid}}{{Approved}}{{Images OK}}{{Submitted}}{{Claimed}}{{Contracted}} | + | {{Edboard}}{{Copyedited}}{{Paid}}{{Approved}}{{Images OK}}{{Submitted}}{{Claimed}}{{Contracted}} |
[[Image:Gray1026.png|thumb|Human submaxillary gland. At the right is a group of mucous alveoli, at the left a group of serous alveoli.]] | [[Image:Gray1026.png|thumb|Human submaxillary gland. At the right is a group of mucous alveoli, at the left a group of serous alveoli.]] | ||
− | A '''gland''' is a collection of [[cell (biology)|cells]] ([[tissue]]) or an [[Organ (anatomy)|organ]] in an [[animal]]'s body that | + | A '''gland''' is a collection of [[cell (biology)|cells]] ([[tissue]]) or an [[Organ (anatomy)|organ]] in an [[animal]]'s body that utilizes substances delivered by the [[blood]] in producing a newly synthesized or concentrated chemical substance for secretion, either for use by another part of the body or for elimination from the body. For example, it may synthesize a [[hormone]] for release into the bloodstream ([[endocrine system|endocrine gland]]) or secrete a substance directly into cavities inside the body or its outer surface ([[exocrine gland]]). More broadly, the term gland is sometimes used to encompass any collection of cells that secretes a chemical substance, such as [[plant]] glands that produce nectar in [[flower]]s. |
− | Glands are essentially collections of cells | + | In the human body, the major system of glands, the endocrine system, produces as variety of hormones that Glands are one of the hundreds of essentially collections of cells working together in harmony, accomplishing a particular function of benefit to the entire body. Each part of a living [[organism]] (cells, tissues, glands, etc.), depends upon other parts to do their particular role and keep the organism functioning. This involves [[Biology#Interactions: Harmony and dual purposes|dual purposes]]: While each gland (cell, tissue) is performing actions necessary for its own maintenance, self-preservation, and self-strengthening, it also is performing specific actions that contribute to the health of the body. The body, on the other hand, supports each gland and its individual purpose by providing nutrients, [[oxygen]], and other necessary materials, and by transporting away the toxic waste materials. |
Glands can be divided into two groups: | Glands can be divided into two groups: | ||
− | * ''Endocrine glands'' | + | * ''Endocrine glands'' secrete their product directly into the blood rather than through a duct (ductless glands) and are know collectively as the [[endocrine system]]. Representatives of this group include the [[hypothalamus]], [[pituitary gland]], [[thyroid gland]], [[adrenal gland]], and [[pancreas]]. |
− | * ''Exocrine glands'' | + | * ''Exocrine glands'' secrete their products into [[duct (anatomy)|duct]]s (duct glands) or directly onto the exterior surface (not through the bloodstream). Typical exocrine glands include [[sweat gland]]s, [[salivary gland]]s, [[mammary gland]]s, and many glands of the [[digestive system]]. |
− | Some glands | + | Some glands would qualify for both groups because of their dual functions. The pancreas, for example, has both an endocrine function, in releasing [[hormone]]s into the bloodstream, and an exocrine function, in releasing digestive [[enzyme]]s into the digestive tract. Other glands serving similar dual functions include the liver, ovaries, and testes. |
==Formation== | ==Formation== | ||
− | + | During an organism's embryonic development, its glands are formed by an ingrowth from an [[epithelium|epithelial]] surface. This ingrowth may from the beginning possess a tubular structure, or, in other instances, it may start as a solid column of [[cell (biology)|cells]] that 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 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. In compound glands, the secretory epithelium forms 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. | Glands are classified according to their shape. | ||
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==Exocrine glands== | ==Exocrine glands== | ||
− | Exocrine glands release their secretions through ducts into the external environment (Silverthorn 2004), or directly onto the exterior surface or an area contiguous with the exterior surface, not utilizing the blood to deliver the product. They include such glands as the salivary glands, sweat glands, and glands within the [[gastrointestinal tract]]. Most utilize ducts, but some glands classified as exocrine glands, such as the digestive glands in the intestine and the sebaceous glands of the [[skin]], actually open directly | + | Exocrine glands release their secretions through ducts into the external environment (Silverthorn 2004), or directly onto the exterior surface or an area contiguous with the exterior surface, not utilizing the blood to deliver the product. They include such glands as the salivary glands, sweat glands, and glands within the [[gastrointestinal tract]]. Most utilize ducts, but some glands classified as exocrine glands, such as the digestive glands in the intestine and the sebaceous glands of the [[skin]], actually open directly onto the body surface. |
− | The chemical | + | The chemical substances released by exocrine glands include sweat, digestive enzymes, tears (through tear ducts), and even [[silk]] from [[spider]]s or silkworm larvae. |
There are multiple ways of classifying exocrine glands: | There are multiple ways of classifying exocrine glands: | ||
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* The glandular portion may be tubular, [[acinus|acinar]], or may be a mix of the two (called tubuloacinar). If the glandular portion branches, then the gland is called a branched gland. | * The glandular portion may be tubular, [[acinus|acinar]], or may be a mix of the two (called tubuloacinar). If the glandular portion branches, then the gland is called a branched gland. | ||
− | '''Method of secretion'''. Exocrine glands are named [[apocrine]], [[holocrine gland]], or [[merocrine gland]] based on how their product is secreted. | + | '''Method of secretion'''. Exocrine glands are named [[apocrine]], [[holocrine gland|holocrine]], or [[merocrine gland|merocrine]] glands based on how their product is secreted. |
*''Apocrine glands''. In [[apocrine]] glands, a portion of the [[plasma membrane]] buds off the cell, containing the secretion, such that a portion of the secreting [[cell (biology)|cell]]'s body is lost during secretion. ''Apocrine gland'' is often used to refer to the [[sweating|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. | *''Apocrine glands''. In [[apocrine]] glands, a portion of the [[plasma membrane]] buds off the cell, containing the secretion, such that a portion of the secreting [[cell (biology)|cell]]'s body is lost during secretion. ''Apocrine gland'' is often used to refer to the [[sweating|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''. In [[holocrine]] glands, the entire cell disintegrates to secrete its substance (e.g., sebaceous glands). | *''Holocrine glands''. In [[holocrine]] glands, the entire cell disintegrates to secrete its substance (e.g., sebaceous glands). |
Revision as of 17:35, 14 September 2007
A gland is a collection of cells (tissue) or an organ in an animal's body that utilizes substances delivered by the blood in producing a newly synthesized or concentrated chemical substance for secretion, either for use by another part of the body or for elimination from the body. For example, it may synthesize a hormone for release into the bloodstream (endocrine gland) or secrete a substance directly into cavities inside the body or its outer surface (exocrine gland). More broadly, the term gland is sometimes used to encompass any collection of cells that secretes a chemical substance, such as plant glands that produce nectar in flowers.
In the human body, the major system of glands, the endocrine system, produces as variety of hormones that Glands are one of the hundreds of essentially collections of cells working together in harmony, accomplishing a particular function of benefit to the entire body. Each part of a living organism (cells, tissues, glands, etc.), depends upon other parts to do their particular role and keep the organism functioning. This involves dual purposes: While each gland (cell, tissue) is performing actions necessary for its own maintenance, self-preservation, and self-strengthening, it also is performing specific actions that contribute to the health of the body. The body, on the other hand, supports each gland and its individual purpose by providing nutrients, oxygen, and other necessary materials, and by transporting away the toxic waste materials.
Glands can be divided into two groups:
- Endocrine glands secrete their product directly into the blood rather than through a duct (ductless glands) and are know collectively as the endocrine system. Representatives of this group include the hypothalamus, pituitary gland, thyroid gland, adrenal gland, and pancreas.
- Exocrine glands secrete their products into ducts (duct glands) or directly onto the exterior surface (not through the bloodstream). Typical exocrine glands include sweat glands, salivary glands, mammary glands, and many glands of the digestive system.
Some glands would qualify for both groups because of their dual functions. The pancreas, for example, has both an endocrine function, in releasing hormones into the bloodstream, and an exocrine function, in releasing digestive enzymes into the digestive tract. Other glands serving similar dual functions include the liver, ovaries, and testes.
Formation
During an organism's embryonic development, its glands are formed by an ingrowth from an epithelial surface. This ingrowth may from the beginning possess a tubular structure, or, in other instances, it may start as a solid column of cells that 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. In compound glands, the secretory epithelium forms 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.
Endocrine glands
Endocrine glands secrete their product directly onto a surface rather than through a duct. The human body contains several endocrine glands and cells that secrete hormones that carry out certain functions. The main glands in humans are the pineal gland, hypothalamus, pituitary gland, thyroid gland, parathyroid gland, adrenal gland, pancreas, and the reproductive glands (ovaries in women and testes in men).
Endocrine glands make up part of the endocrine system, which also includes hormones, and target cells as the three main elements.
Other organs of the body also produce and secrete hormones, but are generally not considered part of the endocrine system; these include the heart, kidney, liver, thymus, skin, and placenta. Sometimes the thymus gland and the kidney are included as part of the endocrine system, and the pineal gland sometimes is not included.
Greater detail on endocrine glands can be assessed here and a list of endocrine glands is available here.
Exocrine glands
Exocrine glands release their secretions through ducts into the external environment (Silverthorn 2004), or directly onto the exterior surface or an area contiguous with the exterior surface, not utilizing the blood to deliver the product. They include such glands as the salivary glands, sweat glands, and glands within the gastrointestinal tract. Most utilize ducts, but some glands classified as exocrine glands, such as the digestive glands in the intestine and the sebaceous glands of the skin, actually open directly onto the body surface.
The chemical substances released by exocrine glands include sweat, digestive enzymes, tears (through tear ducts), and even silk from spiders or silkworm larvae.
There are multiple ways of classifying exocrine glands:
Structure. Exocrine glands contain a glandular portion and a duct portion, the structures of which can be used to classify the gland.
- The duct portion may be branched (called compound) or unbranched (called simple).
- The glandular portion may be tubular, acinar, or may be a mix of the two (called tubuloacinar). If the glandular portion branches, then the gland is called a branched gland.
Method of secretion. Exocrine glands are named apocrine, holocrine, or merocrine glands based on how their product is secreted.
- Apocrine glands. In apocrine glands, a portion of the plasma membrane buds off the cell, containing the secretion, such that 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. In holocrine glands, the entire cell disintegrates to secrete its substance (e.g., sebaceous glands).
- Merocrine glands. In merocrine glands, cells secrete their substances by exocytosis (e.g., mucous and serous glands). Also called "eccrine."
Product secreted
- Serous glands. Serous glands secrete a watery, often protein-rich and often enzymes. Examples include chief cells and Paneth cells.
- Mucous glands. Mucous glands secrete a viscous product, rich in carbohydrates (eg. glycoproteins). Examples include Brunner's glands, esophageal glands, and pyloric glands.
- Mixed glands. Mixed glands secrete both protein and mucus. Examples include the salivary glands, although parotid gland is predominantly serous, and sublingual gland is predominantly mucous.
- Sebaceous glands. Sebaceous glands secrete a lipid product.
Types of exocrine glands
Glands typically may be referred to by two or more means, though some terms are rarely seen. The names of the anatomists who first described them are often employed.
name(s) | location | product | structure |
apocrine sweat glands | skin | - | coiled tubular |
Bartholin's glands, Tiedmann's glands, vulvovaginal glands | vulva, vagina | - | - |
Baughin's glands, anterior lingual glands | tongue, near tip | nonserious or mixed | - |
Brunner's glands, duodenal glands | duodenum | mucous | compound tubular |
bulbourethral glands, Cowper's glands, Mery's glands | penis, base | - | - |
Ciaccio's glands, accessory lacrimal glands | eye | - | - |
Cobelli's glands | esophagus, just above the cardia, in the mucosa | mucous | - |
Duverney's gland | vagina, on either side | - | - |
Ebner's glands | tongue | mucous | - |
eccrine sweat glands | skin | - | coiled tubular |
esophageal glands | esophagus | mucous | racemose |
exocrine pancreas | pancreas | serous | tubulo-acinar |
Fränkel's glands | vocal cords, below the edge | - | - |
gastric chief cell, Wasmann's glands | stomach | serous | - |
glomus coccygeum, coccygeal gland, Luschka's gland or ganglion | coccyx, near the tip | - | - |
goblet cells | digestive tract, respiratory tract | mucous | simple unicellular |
Henle's glands | eyelids, in the conjuctiva | - | tubular |
Huguier's glands | vagina | - | - |
Krause's glands | conjunctiva, middle portion | mucous | - |
Lieberkuhn's glands | intestines, surface of mucous membrane | - | simple tubular |
Littré's glands, Morgagni's glands | spongy portion of the urethra | - | racemose |
mammary gland | breast | - | compound tubulo-acinar |
Meibomian gland | eyelids | sebaceous | - |
Moll's glands | eyelids | - | - |
Montgomery's glands | mammary areola | sebaceous | - |
Naboth's glands | cervix and os uteri | mucous | - |
olfactory glands, Bowman's glands | nose, olfactory region | - | - |
Paneth cells | small intestine | serous | - |
parathyroid glands, Gley's glands, Sandstroem's glands | thyroid, on surface | - | - |
parotid gland | mouth | serous | tubulo-alveolar |
Peyer's patches (or glands) | ileum, lymphatic glands | - | - |
pyloric glands | stomach | mucous | simple branched tubular |
sebaceous gland | skin | sebum | acinar - branched |
Skene's glands, Guérin's glands | vagina | - | - |
sublingual gland, Rivini's gland | mouth | mucus (primarily) | tubulo-alveolar |
submandibular gland | mouth | mixed (M+S) | tubulo-alveolar |
sudoriparous glands, Boerhaave's glands | skin | - | - |
Sigmund's glands | epitrochlear lymph nodes | - | - |
Suzanne's gland | mouth, beneath the alveolingual groove | mucous | - |
Weber's glands | tongue | mucous | tubular |
Glands of Zeis | eyelids, free edges | sebaceous | - |
Additional images
Section of pancreas of dog. X 250.
ReferencesISBN links support NWE through referral fees
- This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.
- Howat, H. T., and H. Sarles. The Exocrine Pancreas. Philadelphia: Saunders, 1979. ISBN 0721647790.
- Silverthorn, D. Human Physiology, An Integrated Approach (3rd Edition). San Francisco, CA: Benjamin Cummings, 2004. ISBN 0131020153.
mechanism (Merocrine, Apocrine, Holocrine) shape (Tubular gland, Alveolar gland) secretion (Serous glands, Mucous glands, Serous demilune)
Ducts: Interlobar duct, Interlobular duct, Intralobular duct (Striated duct, Intercalated duct), Acinus
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