The pancreas is the main enzyme producing accessory gland of the digestive system. It has both exocrine and endocrine functions.

Exocrine Pancreas

The exocrine part of the pancreas has closely packed serous acini, similar to those of the digestive glands. It secretes an enzyme rich alkaline fluid into the duodenum via the pancreatic duct. The alkaline pH is due to the presence of bicarbonate ions, and helps to neutralise the acid chyme from the stomach, as it enters the duodenum. The enzymes digest proteins, carbohydrates, lipids and nucleic acids. These enzymes include: trypsin and chymotrypsin (secreted as inactive precursors, and activated by the action of enterokinase, an enzyme secreted by the duodenal mucosa). An enzyme called CCK stimulates the release of these enzymes, from stored granules in the secretory cells of the acini. Secretin (from neuroendocrine cells in the small intestine) stimulate the release of watery alkaline secretions.

The endocrine part of the pancreas, consists of isolated islands of lighter staining cells called islets of Langerhans. The secretions of the acini empty into ducts lined with a simple low cuboidal epithelium, which becomes stratified cuboidal in the larger ducts.

Endocrine Pancreas

The islets of Langerhans are clumps of secretory cells (up to around 3000) supported by reticulin fibres, and containing numerous fenestrated capillaries. There is a delicate capsule around each islet. They are paler than the surrounding exocrine cells, as they have less rER. These islets do not have an acinar organisation.

The islet cells are indistinguishable from each other in sections, but in fact three secretory cells types are present:

1. alpha - secrete glucagon,
2. beta - secrete insulin
3. delta - secrete somatostatin

The islets are supplied by up to three arterioles, which form a branching network of fenestrated capillaries, into which the hormones are secreted. The islet is drained by about six venules, which pass between the exocrine acini to the interlobular veins.