The pancreas // is a glandular organ in the digestive system and endocrine system of vertebrates. It is an endocrine gland producing several important hormones, including insulin, glucagon, somatostatin, and pancreatic polypeptide which circulate in the blood. The pancreas is also a digestive organ, secreting pancreatic juice containing digestive enzymes that assist digestion and absorption of nutrients in the small intestine. These enzymes help to further break down the carbohydrates, proteins, and lipids in the chyme.
The pancreas is an endocrine organ that lies in the abdomen, specifically the upper, left abdomen. It is found below the stomach, surrounded by the duodenum. The pancreas is typically 5.75-9.5 cm long.
The pancreas is divided into a head, which rests against the second and first part of the duodenum, a body, which passes underneath the base of the stomach, and a tail, which ends adjacent to the spleen. The head of the pancreas surrounds the superior mesenteric artery, which emerges from it.
- The head lies within the concavity of the duodenum.
- The uncinate process emerges from the lower part of head, and lies deep to the superior mesenteric artery and vein.
- The neck is the constricted part between the head and the body.
- The body lies behind the stomach.
- The tail is on the left end of the pancreas. It lies in contact with the spleen and runs in the lienorenal ligament.
The superior pancreaticoduodenal artery from gastroduodenal artery and the inferior pancreaticoduodenal artery from superior mesenteric artery run in the groove between the pancreas and the duodenum and supply the head of pancreas. The pancreatic branches of splenic artery also supply the neck, body and tail of the pancreas. The largest of those branches is called the arteria pancreatica magna; its occlusion, although rare, is fatal.
Under a microscope, stained sections of the pancreas reveal two different types of parenchymal tissue. 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.
|Islets of Langerhans||Lightly staining, large, spherical cluster||Hormone production and secretion (endocrine pancreas)|
|Pancreatic acini||Darker-staining, small, berry-like clusters||Digestive enzyme production and secretion (exocrine pancreas)|
The size of the pancreas varies considerably;. A pancreas may exist as two separate lobes on either side of the duodenum. This can result from the two lobes not fusing in development and is a congenital abnormality known as pancreas divisum, also called a divided pancreas.
The pancreas forms from the embryonic foregut and is therefore of endodermal origin. Pancreatic development begins [with] 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. 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. 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. 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 glucagon and pancreatic polypeptides, respectively. The second line, influenced by Pax-6, produces β- and δ-cells, which secrete insulin and somatostatin, respectively.
The part of the pancreas with endocrine function is made up of approximately a million 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 (alpha) cells secrete glucagon (increase glucose in blood), β (beta) cells secrete insulin (decrease glucose in blood), Δ (delta) cells secrete somatostatin (regulates/stops α and β cells) and PP cells, or gamma cells, secrete pancreatic polypeptide.
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, either by cytoplasmic processes or by direct apposition. According to the volume The Body, by Alan E. Nourse, 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." The islets of Langerhans play an imperative role in glucose metabolism and regulation of blood glucose concentration.
The pancreas as an exocrine gland helps out the digestive system. It secretes pancreatic fluid that contains digestive enzymes that pass to the small intestine. These enzymes help to further break down the carbohydrates, proteins and lipids (fats) in the chyme.
In humans, the secretory activity of the pancreas is regulated directly via the effect of hormones in the blood on the islets of Langerhans and indirectly through the effect of the autonomic nervous system on the blood flow.
The exocrine component of the pancreas, often called simply the exocrine pancreas, is the portion of the pancreas that performs exocrine functions. It has ducts that are arranged in clusters called acini (singular acinus). Pancreatic secretions are secreted into the lumen of the acinus, and then accumulate in intralobular ducts that drain to the main pancreatic duct, which drains directly into the duodenum.
Control of the exocrine function of the pancreas is via the hormones gastrin, cholecystokinin and secretin, which are hormones secreted by cells in the stomach and duodenum, in response to distension and/or food and which cause secretion of pancreatic juices.
There are two main classes of exocrine pancreatic secretions:
|Secretion||Cell producing it||Primary signal|
|bicarbonate ions||Centroacinar cells||Secretin|
|digestive enzymes||Basophilic cells||CCK|
The pancreas is also the main source of enzymes for digesting fats (lipids) and proteins. (The enzymes that digest polysaccharides, by contrast, are primarily produced by the walls of the intestines.)
The cells are filled with secretory granules containing the precursor digestive enzymes. The major proteases which the pancreas secretes are trypsinogen and chymotrypsinogen. Secreted to a lesser degree are pancreatic lipase and pancreatic amylase. The pancreas also secretes phospholipase A2, lysophospholipase, and cholesterol esterase.
The precursor enzymes (termed zymogens or proenzymes) are inactive variants of the enzymes; thus autodegradation, which can lead to pancreatitis, is avoided. Once released in the intestine, the enzyme enteropeptidase (formerly, and incorrectly, called enterokinase) present in the intestinal mucosa activates trypsinogen by cleaving it to form trypsin. The free trypsin then cleaves the rest of the trypsinogen, as well as chymotrypsinogen to its active form chymotrypsin.
- Sympathetic (adrenergic)
- α2: decreases secretion from beta cells, increases secretion from alpha cells, β2: increases secretion from beta cells
- Parasympathetic (muscarinic)
- M3: increases stimulation of alpha cells and beta cells
A puncture of the pancreas, which may lead to the secretion of digestive enzymes such as lipase and amylase into the abdominal cavity as well as subsequent pancreatic self-digestion and digestion and damage to organs within the abdomen, generally requires prompt and experienced medical intervention.
Pancreatitis, inflammation of the pancreas. A variety of factors cause a high pressure within pancreatic ducts. Pancreatic duct rupture, and pancreatic juice leakage causes a pancreatic self-digestion. Therefore, pancreatitis occurs. Gallstones and alcohol are the two most common causes for pancreatitis.
Pancreatic Cancers, particularly cancer of the exocrine pancreas, remain one of the most deadly cancers, and the mortality rate is very high. Pancreatic endocrine tumors are rare. Representative: insulinoma (95% benign, 5% malignant), gastrinomas (malignant).
Diabetes mellitus type 1 (Also known as Juvenile Diabetes) is a chronic autoimmune disorder in which the immune system attacks the insulin-secreting cells in the pancreas. This causes the patient's blood sugar levels to rise to a dangerous level. To correct this, the patient must take 3+ insulin shots per day. There may be also some correlations between diabetes, chronic pancreatitis and pancreatic cancer.
Diabetes mellitus type 2 is more common among overweight adults, but has been seen in children also. Unlike Type 1, it can be permanently corrected with weight loss and medicine.
It is possible for one to live without a pancreas, provided that the patient takes insulin for proper regulation of blood glucose concentration and pancreatic enzyme supplements to aid digestion.
The pancreas was first identified for western civilization by Herophilus (335–280 BC), a Greek anatomist and surgeon. Only a few hundred years later, Rufus of Ephesus, another Greek anatomist, gave the pancreas its name. The term "pancreas" is derived from the Greek πᾶν ("all", "whole"), and κρέας ("flesh") – it is presumed because of its fleshy consistency.
Pancreatic tissue is present in all vertebrate species, but its precise form and arrangement vary 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 foregut). Birds, for example, typically have three such ducts.
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 among 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.
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This article uses anatomical terminology; for an overview, see anatomical terminology.
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