The pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both exocrine (secreting pancreatic juice containing digestive enzymes) and endocrine (producing several important hormones, including insulin, glucagon, and somatostatin).
Histology
Under a microscope, stained sections of the pancreas reveal two different types of parenchymal tissue.[2] Lightly staining clusters of cells are called islets of Langerhans, which produce hormones that underlie the endocrine functions of the pancreas. Darker staining cells form acini connected to ducts. Acinar cells belong to the exocrine pancreas and secrete digestive enzymes into the gut via a system of ducts.
Function
The pancreas is a dual-function gland, having features of both endocrine and exocrine glands.
Endocrine
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The part of the pancreas with endocrine function is made up of a million[3] cell clusters called islets of Langerhans. There are four main cell types in the islets. They are relatively difficult to distinguish using standard staining techniques, but they can be classified by their secretion: α cells secrete glucagon, β cells secrete insulin, δ cells secrete somatostatin and gastrin, and PP cells secrete pancreatic polypeptide.[4]
The islets are a compact collection of endocrine cells arranged in clusters and cords and are crisscrossed by a dense network of capillaries. The capillaries of the islets are lined by layers of endocrine cells in direct contact with vessels, and most endocrine cells are in direct contact with blood vessels, by either cytoplasmic processes or by direct apposition. According to the volume The Body, by Alan E. Nourse,[5] the islets are "busily manufacturing their hormone and generally disregarding the pancreatic cells all around them, as though they were located in some completely different part of the body."
Exocrine
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In contrast to the endocrine pancreas, which secretes hormones into the blood, the exocrine pancreas produces digestive enzymes and an alkaline fluid, and secretes them into the small intestine through a system of exocrine ducts. Digestive enzymes include trypsin, chymotrypsin, pancreatic lipase, and pancreatic amylase, and are produced and secreted by acinar cells of the exocrine pancreas. Specific cells that line the pancreatic ducts, called centroacinar cells, secrete a bicarbonate- and salt-rich solution into the small intestine.[6]
Regulation
The pancreas receives regulatory innervation via hormones in the blood and through the autonomic nervous system. These two inputs regulate the secretory activity of the pancreas.
Diseases of the pancreas
Because the pancreas is a storage depot for digestive enzymes, injury to the pancreas is potentially very dangerous. A puncture of the pancreas generally requires prompt and experienced medical intervention.
Diseases associated with the pancreas include:
Pancreatitis
Pancreatitis is inflammation of the pancreas. There are two forms of pancreatitis, which are different in causes and symptoms, and require different treatment:
Diabetes mellitus
The pancreas is central in the pathophysiology of both major types of diabetes mellitus. In type 1 diabetes mellitus, there is direct damage to the endocrine pancreas that results in insufficient insulin synthesis and secretion. Type 2 diabetes mellitus, which begins with insulin resistance, is characterized by the ultimate failure of pancreatic β cells to match insulin production with insulin demand.
Exocrine pancreatic insufficiency
Exocrine pancreatic insufficiency (EPI) is the inability to properly digest food due to a lack of digestive enzymes made by the pancreas. EPI is found in humans afflicted with cystic fibrosis and Shwachman-Diamond Syndrome. It is caused by a progressive loss of the pancreatic cells that make digestive enzymes. Chronic pancreatitis is the most common cause of EPI in humans. Loss of digestive enzymes leads to maldigestion and malabsorption of nutrients.
Cystic fibrosis
Cystic fibrosis, also known as mucoviscidosis, is a hereditary disease that affects the entire body, causing progressive disability and early death. There is no cure for cystic fibrosis, and most affected individuals die young from lung failure. Cystic fibrosis is caused by a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The product of this gene helps create sweat, digestive juices, and mucus. Although most people without CF have two working copies of the CFTR gene, only one is needed to prevent cystic fibrosis. Cystic fibrosis develops when neither gene works normally. Therefore, it is considered an autosomal recessive disease. The name cystic fibrosis refers to the characteristic 'fibrosis' (tissue scarring) and cyst formation within the pancreas. Cystic fibrosis causes irreversible damage to the pancreas, which often results in painful inflammation. (pancreatitis).
Pseudocysts
A pancreatic pseudocyst is a circumscribed collection of fluid rich in amylase and other pancreatic enzymes, blood and necrotic tissue, typically located in the lesser sac.
Congenital malformations
Pancreas divisum
Pancreas divisum is a malformation in which the pancreas fails to fuse together. It is a rare condition that affects only 6% of the world's population and of these few only 1% ever have symptoms that require surgery.
Annular pancreas
Annular pancreas is characterized by a pancreas that encircles the duodenum. It results from an embryological malformation in which the early pancreatic buds undergo inappropriate rotation and fusion, which can lead to small bowel obstruction.
Neoplasms
See pancreatic cancer.
Zollinger-Ellison syndrome
Zollinger-Ellison syndrome is a collection of findings in individuals with gastrinoma, a tumor of the gastrin-producing cells of the pancreas. Unbridled gastrin secretion results in elevated levels of the hormone, and increased hydrochloric acid secretion from parietal cells of the stomach. It can lead to ulceration and scarring of the stomach and intestinal mucosa.
Hemosuccus pancreaticus
Hemosuccus pancreaticus, also known as pseudohematobilia or Wirsungorrhage, is a rare cause of hemorrhage in the gastrointestinal tract. It is caused by a bleeding source in the pancreas, pancreatic duct, or structures adjacent to the pancreas, such as the splenic artery, that bleed into the pancreatic duct. Patients with hemosuccus may develop symptoms of gastrointestinal hemorrhage, such as blood in the stools, maroon stools, or melena. They may also develop abdominal pain. Hemosuccus pancreaticus is associated with pancreatitis, pancreatic cancer and aneurysms of the splenic artery. Angiography may be used to treat hemosuccus pancreaticus, where the celiac axis is injected to determine the blood vessel that is bleeding, because embolization of the end vessel may terminate the hemorrhage. Alternatively, a distal pancreatectomy may be required to stop the hemorrhage.
History
The pancreas was first identified by Herophilus (335-280 BC), a Greek anatomist and surgeon. Only a few hundred years later, Ruphos, another Greek anatomist, gave the pancreas its name. The term "pancreas" is derived from the Greek pan, "all", and kreas, "flesh", probably referring to the organ's homogeneous appearance.[8]
As food
Pancreases (specifically calf and lamb pancreases) are eaten in meals like sweetbread, often going by the name stomach.
Embryological development
The pancreas forms from the embryonic foregut and is therefore of endodermal origin. Pancreatic development begins the formation of a ventral and dorsal anlage (or buds). Each structure communicates with the foregut through a duct.
Differential rotation and fusion of the ventral and dorsal pancreatic buds results in the formation of the definitive pancreas.[9] As the duodenum rotates to the right, it carries with it the ventral pancreatic bud and common bile duct. Upon reaching its final destination, the ventral pancreatic bud fuses with the much larger dorsal pancreatic bud. At this point of fusion, the main ducts of the ventral and dorsal pancreatic buds fuse, forming the duct of Wirsung, the main pancreatic duct.
Differentiation of cells of the pancreas proceeds through two different pathways, corresponding to the dual endocrine and exocrine functions of the pancreas. In progenitor cells of the exocrine pancreas, important molecules that induce differentiation include follistatin, fibroblast growth factors, and activation of the Notch receptor system.[9] Development of the exocrine acini progresses through three successive stages. These include the predifferentiated, protodifferentiated, and differentiated stages, which correspond to undetectable, low, and high levels of digestive enzyme activity, respectively.
Progenitor cells of the endocrine pancreas arise from cells of the protodifferentiated stage of the exocrine pancreas.[9] Under the influence of neurogenin-3 and Isl-1, but in the absence of Notch receptor signaling, these cells differentiate to form two lines of committed endocrine precursor cells. The first line, under the direction of Pax-6, forms α- and γ- cells, which produce the peptides glucagon and pancreatic polypeptide, respectively. The second line, influenced by Pax-4, produces β- and δ-cells, which secrete insulin and somatostatin, respectively.
Insulin and glucagon can be detected in the fetal circulation by the fourth of fifth month of fetal development.[9]
Additional images
Accessory digestive system.
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The celiac artery and its branches; the stomach has been raised and the peritoneum removed.
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Lymphatics of stomach, etc. The stomach has been turned upward.
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Transverse section through the middle of the first lumbar vertebra, showing the relations of the pancreas.
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The duodenum and pancreas.
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Pancreas of a human embryo of five weeks.
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Pancreas of a human embryo at end of sixth week.
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Front of abdomen, showing surface markings for duodenum, pancreas, and kidneys.
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References
- ^ Physiology at MCG 6/6ch2/s6ch2_30
- ^ Histology at BU 10404loa
- ^ Hellman B, Gylfe E, Grapengiesser E, Dansk H, Salehi A (2007). "[Insulin oscillations--clinically important rhythm. Antidiabetics should increase the pulsative component of the insulin release]" (in Swedish). Lakartidningen 104 (32-33): 2236–9. PMID 17822201.
- ^ BRS physiology 4th edition ,page 255-256, Linda S. Constanzo, Lippincott publishing
- ^ The Body, by Alan E. Nourse, in the Time-Life Science Library Series (op. cit., p. 171.)
- ^ Maton, Anthea; Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. ISBN 0-13-981176-1.
- ^ Verspohl EJ, Tacke R, Mutschler E, Lambrecht G (1990). "Muscarinic receptor subtypes in rat pancreatic islets: binding and functional studies". Eur. J. Pharmacol. 178 (3): 303–11. doi:10.1016/0014-2999(90)90109-J . PMID 2187704.
- ^ Harper, Douglas. Pancreas. Online Etymology Dictionary. Retrieved on 2007-04-04.
- ^ a b c d Carlson, Bruce M. (2004). Human embryology and developmental biology. St. Louis: Mosby, pp372-4. ISBN 0-323-01487-9.
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