Pancreas - Anatomy & Physiology
Introduction
The pancreas has exocrine and endocrine tissue. The exocrine part secretes pancreatic juice; a solution containing enzymes for carbohydrate, protein and triacylglycerol digestion. Pancreatic juice drains into the small intestine where it is functional. The exocrine part is the larger of the two parts of the pancreas. The endocrine part secretes hormones for the regulation of blood glucose concentration, including insulin, glucagon and somatostatin. The functional units of the exocrine part are the islets of Langerhans.
Development
The pancreas develops from endoderm, except for the connective tissue which develops from splanchnic mesoderm. Development begins with evaginations of the digestive tube caudal to the stomach. Two pancreatic buds form, one in the dorsal mesogastrium and one in the ventral mesogastrium. Some epithelial cells lose their connections to the developing duct system of the exocrine pancreas and develop into the islets of Langerhans of the endocrine pancreas. As the stomach rotates, the ventral bud moves to become more dorsal. The two buds then fuse; the left lobe is derived from the dorsal bud and the right lobe from the ventral bud. The duct of the ventral lobe (pancreatic duct) joins with the bile duct to form the common bile duct which opens into the duodenum at the major duodenal papilla. The duct of the dorsal lobe (accessory duct) enters the duodenum at the minor duodenal papilla. There is species variation in the persistance of each duct.
Structure
- Located in the craniodorsal part of the abdomen in close association with the duodenum.
- Divided into three parts; a body and left and right lobes.
- The lobes are losely united by interlobular tissue.
- Generally, the portal vein runs between the left and right lobes. (see species differences).
- Rougly "V" shaped in all species.
- As mentioned in the development section, there are two ducts present in the pancreas. Their presence reflects the convergent development pattern of the pancreas. However in some species one or other of the ducts may atrophy.
- The pancreatic duct is the biggest of the two opens into the duodenum with the bile duct at the major duodenal papilla.
- The accessory duct opens on the opposite aspect of the duodenum at the minor duodenal papilla.
Function
Exocrine
Endocrine Function
- Functional units are the islets of Langerhans, which are embedded throughout the exocrine tissue.
- Cells of the islets produce hormones that maintain normoglycaemia.
- There are four different cell types within each islet that each produce different hormones.
- α cells produce glucagon
- β cells produce insulin
- δ cells produce somatostatin
- F cells produce pancreatic polypeptide
- Glucagon is released when blood glucose levels are low
- Insulin
- Somatostatin acts in a paracrine fashion to inhibit both glucagon and insulin secretion.
Vasculature
Innervation
Lymphatics
Histology
Species Differences
Carnivore
- Portal vein runs between the left and right lobes.
- In dogs, both pancreatic and accessory ducts persist throughout development.
- In cats, the distal part of the accessory duct atrophies during development, so only the pancreatic duct persists.
Ruminant
- Portal vein runs between the left and right lobes.
- In the ox, the distal part of the pancreatic duct atrophies during development, so only the accessory duct persists.
- In sheep and goats the distal part of the accessory duct atrophies during development, so only the pancreatic duct persists.
Horse
- Portal vein perforates the pancreas.
- Both pancreatic and accessory ducts persist throughout development.
Pig
- Portal vein perforated the pancrea.
- The distal part of the pancreatic duct atrophies during development, so only the accessory duct persists.