This is an extract of our Endocrine Pancreas document, which we sell as part of our Organisation of the Body Notes collection written by the top tier of Oxford students.
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Endocrine pancreas Development
-pancreas develops from the endoderm of pancreatic buds- embryonic epithelium of pancreatic ducts contain both potential exocrine and endocrine cells. In development endocrine cells migrate from duct system and aggregated around capillaries to form Islets of langerhans- scattered throughout exocrine tissue has two types of glands: exocrine glands (secrete digestive enzymes into small intestine) and endocrine glands (islets of langerhans)
-islets contain 4 types of secretory cells (alpha, beta, sigma, F Light microscopy: each islet has 3000 secretory cells surrounded by fine collagenous network which has numerous fenestrated capillaries. A capsule surrounds each islet. Endocrine cells are small, pale stained, granular cytoplasm
-immunoperoxidase technique: identify different secretory cells based on their specific products
-B cells - 60% of cells in the islets, make up the central region, secrete insulin, proinsulin, C peptide
-alpha cells- 15% , found in the peripheral area, secrete glucagon
-sigma cells - 10% , scattered, secrete somatostain
-F cells -15% , found in the ventral region, secrete pancreatic polypeptide Innervation
-level of islet secretion occurs through innervations from both sympathetic and parasympathetic divisions of the ANS
-Parasympathetic innervations via the 'vagus nerve' increases insulin secretion
-sympathetic innervations-can be stimulatory/inhibitory depending on wheter B adrenergic (stimulatory) or a-adrenergic (inhibitory) Blood supply of the pancreas
-Rich blood supply- shown in specimens stained with red dye before fixationeach islet is supplied by three arterioles which ramify into a network of highly fenestrated capillaries
-Supply from branches of superior mesenteric and celiac arteries
-pancreatic veins drain into the hepatic portal vein- the liver is directly below the pancreas
Insulin Function: replenish fuel reserves in muscle, liver, adipose tissue
-during fasting B cells secrete less insulin- insulin decreases lipids are mobilised from adipose tissue, amino acids are mobilised from body proteins
-during feeding: insulin secretion increases-this decreases mobilisation of glycogen, triglycerides and stimulates uptake of carbohydrate, lipid and amino acid uptake by insulin sensitive target tissues
-insulin maintains the concentration of glucose in the plasma within narrow limits, allows the brain to have a constant supply of glucose very important for its function as it relies on glucose as its main energy supply Synthesis of insulin
-protein: made in the B cells of the islets of langerhans and expression of the gene forms preproinsulin
-the preproinsulin enters the rough endoplasmic reticulum where that starting sequence of 24 amino acids is cleaved to form proinsulin (has A,B,C domains)
-the trans-golgi packages the proinsulin and creates secretory granules. Proteases cleave the proinsulin at two spots and forms the C peptide, A and B chains
-the A and B chains are linked by two disulfide linkages and forms mature insulin. 21 amino acids on the A chain and 30 amino acids on the B chain.
-the secretory vesicle contains, proinsluin, insulin, C peptide - all three are released into the portal blood when glucose stimulates the B cell.
-C peptide has no biological action- but released in 1:1 ratio with insulinconcentration measured in urine determines the concentration of insulin in the blood
-proinsulin has some action, has 1/20th the potency as insulin
-60% secreted is insulin Control of insulin release a) increase in plasma glucose increases insulin secretion
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