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Pancreas
Illu pancrease.svg
Illu pancreas duodenum.jpg
1: Head of pancreas
2: Uncinate process of pancreas
3: Pancreatic notch
4: Body of pancreas
5: Anterior surface of pancreas
6: Inferior surface of pancreas
7: Superior margin of pancreas
8: Anterior margin of pancreas
9: Inferior margin of pancreas
10: Omental tuber
11: Tail of pancreas
12: Duodenum
Gray's subject #251 1199
Artery inferior pancreaticoduodenal artery, superior pancreaticoduodenal artery, splenic artery
Vein pancreaticoduodenal veins, pancreatic veins
Nerve pancreatic plexus, celiac ganglia, vagus[1]
Precursor pancreatic buds
MeSH Pancreas
Dorlands/Elsevier Pancreas

The pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as an exocrine gland, secreting pancreatic juice containing digestive enzymes that pass to the small intestine. These enzymes help in the further breakdown of the carbohydrates, protein, and fat in the chyme.

Contents

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.

Structure Appearance Function
Islets of Langerhans Lightly staining, large, spherical clusters Hormone production and secretion (endocrine pancreas)
Pancreatic acini Darker staining, small, berry-like clusters Digestive enzyme production and secretion (exocrine pancreas)

Function

The pancreas is a dual-function gland, having features of both endocrine and exocrine glands.

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Endocrine

The part of the pancreas with endocrine function is made up of approximately a million[3] cell clusters called islets of Langerhans. Four main cell types exist 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(increase Glucose in blood), β cells secrete insulin (decrease Glucose in blood), δ cells secrete somatostatin (regulates/stops α and β cells), 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

In contrast to the endocrine pancreas, which secretes hormones into the blood, the exocrine pancreas produces digestive enzymes and an alkaline fluid (referred to as pancreatic juice), and secretes them into the small intestine through a system of exocrine ducts in response to the small intestine hormones secretin and cholecystokinin. 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.

Sympathetic (adrenergic) Parasympathetic (muscarinic)
α2: decreases secretion from beta cells, increases secretion from alpha cells M3[7] increases stimulation of alpha cells and beta cells

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.

Pancreatic cancer, particularly cancers of the Exocrine Pancreas remain one of the most deadly cancers, and the mortality rate is very high.

History

The pancreas was first identified for western civilization 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 πᾶν ("all", "whole"), and κρέας ("flesh").[8] – presumably because of its fleshy consistency.

Embryological development

Schematic illustrating the development of the pancreas from a dorsal and a ventral bud. During maturation the ventral bud flips to the other side of the gut tube (arrow) where it typically fuses with the dorsal lobe. An additional ventral lobe which usually regress during development is omitted.

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. The ventral pancreatic bud becomes the head and uncinate process, and comes from the hepatic diverticulum.

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-0, forms α- and γ- cells, which produce the peptides glucagon and pancreatic polypeptide, respectively. The second line, influenced by Pax-6, produces β- and δ-cells, which secrete insulin and somatostatin, respectively.

Insulin and glucagon can be detected in the fetal circulation by the fourth or fifth month of fetal development.[9]

In non-human animals

Pancreatic tissue is present in all vertebrate species, but its precise form and arrangement varies widely. There may be up to three separate pancreases, two of which arise from ventral buds, and the other dorsally. In most species (including humans), these fuse in the adult, but there are several exceptions. Even when a single pancreas is present, two or three pancreatic ducts may persist, each draining separately into the duodenum (or equivalent part of the hindgut). Birds, for example, typically have three such ducts.[10]

In teleosts, and a few other species (such as rabbits), there is no discrete pancreas at all, with pancreatic tissue being distributed diffusely across the mesentery and even within other nearby organs, such as the liver or spleen. In a few teleost species, the endocrine tissue has fused to form a distinct gland within the abdominal cavity, but otherwise it is distributed amongst the exocrine components. The most primitive arrangement, however, appears to be that of lampreys and lungfish, in which pancreatic tissue is found as a number of discrete nodules within the wall of the gut itself, with the exocrine portions being little different from other glandular structures of the intestine.[10]

Additional images

References

  1. ^ Physiology at MCG 6/6ch2/s6ch2_30
  2. ^ Histology at BU 10404loa
  3. ^ Hellman B, Gylfe E, Grapengiesser E, Dansk H, Salehi A (2007). "[Insulin oscillations--clinically important rhythm. Anti-diabetics should increase the pulsative component of the insulin release]" (in Swedish). Lakartidningen 104 (32-33): 2236–9. PMID 17822201. 
  4. ^ BRS physiology 4th edition ,page 255-256, Linda S. Constanzo, Lippincott publishing
  5. ^ The Body, by Alan E. Nourse, in the Time-Life Science Library Series (op. cit., p. 171.)
  6. ^ 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. OCLC 32308337. 
  7. ^ 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. 
  8. ^ Harper, Douglas. "Pancreas". Online Etymology Dictionary. http://www.etymonline.com/index.php?term=pancreas. Retrieved 2007-04-04. 
  9. ^ a b c d Carlson, Bruce M. (2004). Human embryology and developmental biology. St. Louis: Mosby. pp. 372–4. ISBN 0-323-01487-9. 
  10. ^ a b Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 357-359. ISBN 0-03-910284-X. 

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