Pancreas

Pancreas

PKGhatak, MD

The pancreas is an important organ but not essential to life since the isolation and purification of Insulin in 1921 by Dr. Banting and a medical student Mr. Best. The pancreas is a mixed gland contains both exocrine (secretion by duct) and endocrine (secretion directly enters blood) glands.

Development of Pancreas:

The pancreas has two sources of origin. Two independent buds develop from the foregut endoderm - one dorsal bud and one ventral bud, at the junction of the foregut and midgut, next to the origin of the biliary bud. The dorsal bud produces the major part of the gland and the main duct, the ventral bud produces a part of the head, uncinate process and the accessory duct.

Exocrine development:

In the beginning, the primitive pancreatic cells differentiate into the acinar cell line (gland with ducts) and endocrine cell line. Each acinar duct joins with the adjoining acini and ultimately forms two major ducts as mentioned above. Each acinar duct joins with the adjoining acini and ultimately forms two major ducts as mentioned above. The main pancreatic duct opens into the 2nd part of the duodenum and the duct is a bit dilated called the ampulla of Vater and is surrounded by smooth muscle sphincter- sphincter of Oddi. The common bile duct joins the pancreatic duct in the ampulla and the opening in the pancreatic duct is guarded by the sphincter of Boyden. The accessory pancreatic duct opens just proximal to the bile duct+ pancreatic duct opening. In adults, many congenital anatomical variations of pancreatic ducts and the occasional head of the pancreas are encountered during surgery.

Endocrine Development:

The developing endocrine cells are grouped together in several clusters and are widely dispersed all through the gland. These clusters are called Langerhans Islands. Endocrine cell populations are further divided into alpha, beta, delta and C cells. All these cells are present on every island. Initial tiny ducts are attached to each islet but soon the ducts disappear.

Location of Pancreas:

The pancreas is located deep inside the abdomen on the posterior abdominal wall, behind the peritoneum. It is firmly anchored on the wall; the head of the pancreas fills the c-shaped space of the duodenal curvature, the main body lies across the abdominal wall, the tail part almost touching the spleen. In front of the pancreas is the Lesser Sac of the Omentum, the stomach slides over it easily.

Exocrine Function of Pancreas:

Pancreatic enzymes are secreted by acini cells and are rich in digestive enzymes -

Pancreatic Amylase, Trypsin, Chymotrypsin, Lipase, Phospholipase, Cholesterol esterase. In addition, the ductal cells secrete Chloride and Bicarbonate.

The total volume of pancreatic secretion is 2 to 3 liters. a day.

The pancreatic juice is very alkaline and acts with bile neutralizes highly acidic gastric discharge in the duodenum.

The action of Pancreatic enzymes on Food:

As the names imply, the amylase digests complex carbohydrates into glucose, trypsin and chymotrypsin digest proteins into simple amino acids and Lipase digest fat into fatty acid and glycerol, cholesterol ester breaks down cholesterol. In the digestive process, duodenal enzymes play a significant part also.

Endocrine Function of Pancreas:

Alpha cells secrete Glucagon.

Beta cells secrete Insulin and Amylin.

Delta cells secrete Somatotropin, Ghrelin and Pancreatic polypeptides. Gastrin is secreted in an early stage of development and later the stomach secretes gastrin exclusively.

 The C-cell function is not known.

The action of Glucagon. Glucagon breaks down glycogen stores in the liver and skeletal muscles. It stimulates amino acid conversion to glucose (neogluconogenesis). Glycogen turns fatty acids into fat molecules in the liver. Glucagon delays the release of Insulin from the pancreas.

The action of Insulin.  Insulin binds with insulin receptors present on the surface of every cell. A protein molecule GLUT4(glucose transporter 4) comes up to the surface of the cells and an Insulin molecule bound to the receptor fuses with the membrane. This opens up channels for glucose molecules to enter the inside of the cell. The glucose molecule is immediately converted to glucose phosphate by the enzyme phosphatase. This keeps the concentration gradient in favor of glucose crossing inside the cells. Insulin favors glycogen synthesis, the conversion of fatty acids to fat molecules and amino acids to protein. Insulin promotes body growth and increases growth hormone secretion from the pituitary gland. Insulin delays glucagon secretion and thereby reduces glycogen breakdown.

The action of Somatostatin. Somatostatin is a gastrointestinal motility inhibitor. Somatostatin decreases both exocrine and endocrine secretion of the pancreas, decreases secretions from the duodenal and small intestinal glands. It reduces growth hormone release from the pituitary gland. It reduces gastric motility, gastric acid and gastrin secretion.

The action of pancreatic polypeptides. These small molecules suppress pancreatic exocrine secretion, gall bladder contraction and gastric motility.

The action of Ghrelin. Ghrelin promotes growth hormone release, muscle growth, increases appetite. 

Blood Supply of Pancreas: Pancreas is richly supplied by arterial circulation. Celiac artery and Superior mesenteric artery supply arterial blood. The venous drainage goes to the liver by the Portal vein.

Insulin sensor: The alpha and beta cells act as the sensor of blood glucose levels. Alfa cells modify glucagon release by a negative feedback loop; whereas the beta cells release more insulin when sugar levels are high and shunt down Insulin release when the sugar level is low.

Nerve Supply of Pancreas. Like every abdominal organ, the pancreas has two nervous systems innervation - namely Somatic and Autonomic nervous systems. Nerve fibers going in and out of the pancreas pass through the celiac ganglion but many fibers just pass through.

Somatic sensory. The nerve cells are located in the Dorsal Root Ganglia (DRG) of the spinal nerves T 6 to L 2. The nerve fibers carrying the pain and other sensations enter the spinal cord and travel along the intermediate lateral tract of the spinal cord to the nuclei of the Thalamus. The 2nd order neurons from the thalamus cross the midline to reach the cerebral cortex. The DRG is sensitive to Capsaicin, CGRP (calcitonin gene related peptide). The neurons generate substance P.

Motor division.  The main center for secretary and contractile functions are located in the Nodosa Ganglion of the Vegas nerve. The axions from these neurons directly innervate all the cells of the pancreas.

Autonomic Nervous System:

Sympathetic supply. The nerve cells are located in the Celiac ganglion, Mesenteric ganglia, Paravertebral ganglia. The alpha fibers of sympathetic nerves produce vasoconstriction, delays secretion and decreases endocrine function. Beta fibers stimulate Insulin and glucagon production and release.

Parasympathetic Nervous System:

The nerve cells are located in the Dorsal ganglia of the Vegas nerve. The axion of these nerves makes connections with the neuron present in the pancreas itself. Parasympathetic stimulation increases both exocrine and endocrine secretion and production.

Neurotransmitters:

Somatic nerves are Cholinergic. Sympathetic nerves are noradrenergic, glycinergic, and respond by releasing Neuropeptide gamma. Parasympathetic nerves are nicotinic-cholinergic at the ganglia and muscarine choline at the postganglionic terminal fibers.

The pancreas is fully developed at birth but only the exocrine function is present at birth. At about 15 weeks of life, the endocrine function begins.

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