Understanding the Cause

Pancreas - The Production House For Insulin

The pancreas is an elongated organ, light tan or pinkish in color, that lies below the stomach, in a bend of the duodenum (the beginning portion of the small intestine). It is covered with a very thin connective tissue capsule that extends inward as septa (walls), partitioning the gland into lobules (small lobes). Functionally the pancreas is divided into two portions:

1. The Exocrine Pancreas
2. The Endocrine Pancreas

What constitutes the Exocrine Pancreas?

The bulk of the pancreas is composed of pancreatic exocrine cells and their associated ducts. The exocrine cells secrete the digestive juices. Pancreatic exocrine cells are arranged in grape-like clusters called acini (singular acinus). The exocrine cells themselves are packed with secretory granules which contain digestive enzymes that are extruded into the lumen of the acinus.

The exocrine portion of the pancreas accounts for about 80% of the total glandular volume. It consists of at least two functional units:

1. acinar cells, which secrete primarily digestive enzymes;
2. centroacinar or ductal cells, which secrete fluids and electrolytes.

From there these secretions flow into larger and larger ducts, which eventually coalesce into the main pancreatic duct which drains directly into the duodenum. The exocrine functions are concerned with digestion.

Pancreatic secretion is regulated by several peptides that are released from the gastrointestinal tract. Some of these peptides, such as secretin and cholecystokinin (CCK), stimulate pancreatic secretions, whereas somatostatin and pancreatic polypeptide inhibit their release.

The pancreas secretes about 20 digestive enzymes and cofactors. Some enzymes are activated in the duodenum. These enzymes account for most of the intraluminal digestion of dietary proteins, triglycerides and carbohydrates. They are also important in the cleavage of certain vitamins (such as A and B12) from carrier molecules, thereby allowing them to be absorbed efficiently. Because pancreatic enzymes are secreted in great excess, maldigestion and serious nutritional deficiencies occur only when over 90% of the gland has been destroyed.

What constitutes the Endocrine Pancreas?

Embedded within this exocrine tissue are roughly one million small clusters of cells called the Islets of Langerhans or simply the ‘islets’. The endocrine pancreas refers to these cells within the pancreas that synthesize and secrete hormones. The endocrine function consists primarily of the secretion of the two major hormones, insulin and glucagons. Humans have roughly one million islets.

Pancreatic islets house three major cell types, each of which produces a different endocrine product - hormone:

• Alpha cells (A cells) secrete the hormone glucagon. Glucagon is a catabolic hormone, that is, it mobilizes glucose, fatty acids and amino acids from stores into the blood.
• Beta cells (B cells) produce insulin and are the most abundant of the islet cells. Insulin is an anabolic hormone, that is, it increases the storage of glucose, fatty acids and amino acids in cells and tissues.
• Delta cells (D cells) secrete the hormone somatostatin, which is also produced by a number of other endocrine cells in the body. Somatostatin may regulate, locally, the secretion of the other pancreatic hormones; in brain (hypothalamus) and spinal cord it may act as a neurohormone and neurotransmitter.

Interestingly, the different cell types within an islet are not randomly distributed - beta cells occupy the central portion of the islet and are surrounded by a "rind" of alpha and delta cells. Aside from the insulin, glucagon and somatostatin, a number of other "minor" hormones have been identified as products of pancreatic islets cells. The function and origin of these pancreatic polypeptides are still uncertain although the hormone may influence gastrointestinal function and promote intra-islet homeostasis.

Islets are richly vascularized, allowing their secreted hormones ready access to the circulation. Although islets comprise only 1-2% of the mass of the pancreas, they receive about 10 to 15% of the pancreatic blood flow. Additionally, they are innervated by parasympathetic and sympathetic neurons, and nervous signals clearly modulate secretion of insulin and glucagon. Both insulin and glucagon are important in the regulation of carbohydrate, protein and lipid metabolism.