The Pancreas - Structure and Function

Question 1

Describe the embryology of the pancreas.

Answer 1

The pancreas develops as a ventral bud (part of the hepatobiliary bud) and the dorsal bud. During development, the duodenum will rotate to form a C shape, the ventral bud will swing around and both buds fuse.

Question 2

What are the five parts of the pancreas?

Answer 2

Uncinate process, head, neck, body and tail

Question 3

Where is the pancreas positioned? What are its posterior relations?

Answer 3

Within the C of the duodenum

Posterior to the pancreas is the inferior vena cava, the abdominal aorta and the left kidney

Question 4

Describe the blood vessels around the pancreas.

Answer 4

The coeliac trunk is just superior to the pancreas

The superior mesenteric arteries arise from in between the uncinate process and the body of the pancreas

Question 5

What are the main endocrine products of the pancreas?

Answer 5

Glucagon, Insulin, Somatostatin, Pancreatic Polypeptide

Question 6

What proportion of the pancreas is endocrine and what proportion is exocrine?

Answer 6

2% endocrine

98% exocrine

Question 7

Why is somatostatin described as endocrine cyanide?

Answer 7

It inhibits most other endocrine processes

Question 8

Describe the development of the endocrine and exocrine parts of the pancreas from ductal budding.

Answer 8

The endocrine and exocrine parts of the pancreas develop from the ductal budding but the endocrine components lose their connection with the duct whereas the exocrine parts retain their connection.

Question 9

Where are most of the endocrine cells of the pancreas found?

Answer 9

Tail of the pancreas

Question 10

What percentage of the islets of Langerhans are a) alpha cells, b) beta cells and c) delta cells?

Answer 10

Alpha cells - 15-20%
Beta cells - 60-70%
Delta cells - 5-10%

Question 11

How does the staining of the islets of Langerhans differ from the rest of the pancreas?

Answer 11

Islets of Langerhans stain lighter than the other cells

Question 12

Describe the structure of pancreatic acini. Mention the two components of pancreatic juice.

Answer 12

Acini consist of a blind ended tubule surrounded by acinar cells and a duct lined by duct cells. Between the duct cells and the acinar cells you find centroacinar cells.
The acinar cells secrete the viscous, low volume, enzyme rich component of pancreatic juice
The duct cells secrete the high volume, watery, bicarbonate rich components of pancreatic juice

Question 13

What is the role of the bicarbonate produced by the duct cells?

Answer 13

Bicarbonate neutralises the acid chyme and hence protects the mucosa
Bicarbonate also raises the pH to optimum range for pancreatic enzymes

Question 14

How does bicarbonate secretion change with duodenal pH?

Answer 14

As duodenal pH decreases, bicarbonate secretion increases to a maximum at around pH 3.

Question 15

Why does bicarbonate secretion stop at pH 5 (i.e. when it is still acidic)?

Answer 15

Neutralisation of the acid chyme is also affected by the bicarbonate in bile and the Brunner’s glands secreting alkaline fluid

Question 16

Describe the movement of sodium in the pancreas.

Answer 16

Sodium moves down its gradient via the paracellular pathway (tight junctions) to the lumen

Question 17

Describe the movement of bicarbonate and protons in the duct cells of the pancreas.

Answer 17

Water and carbon dioxide react under the influence of carbonic anhydrase to produce bicarbonate and protons. The bicarbonate is pumped into the lumen via exchange with chloride ions. The protons are pumped into the blood via exchange with sodium. These are both examples of secondary active transport. The sodium gradient is maintained by a sodium/potassium pump (primary active transport).
There are potassium and chloride channels allowing these ions to leave the duct cell.

Question 18

What is the role of CFTR in this mechanism of bicarbonate secretion?

Answer 18

CFTR allows the outwards movement of chloride ions (active process using ATP) so the gradient can be used to pump out bicarbonate ions. A defective CFTR doesn’t allow chloride ions out so you get thick secretions.

Question 19

How are proteases secreted by the acinar cells and how are they activated?

Answer 19

Proteases are secreted as zymogens. The acinar cells also produce trypsin inhibitors to prevent activation of proteases in the pancreas.
Enterokinase is required to convert trypsinogen to trypsin, which then converts other zymogens into active proteases. Enterokinase is only found in the duodenum.

Question 20

What happens if there is a blockage in the pancreatic duct?

Answer 20

The pancreatic juices accumulate and overload the protection and result in auto-digestion.

Question 21

Describe the requirement for lipase to work properly.

Answer 21

Lipases need co-lipases (which are produced in an inactive form) and they need bile salts to emulsify the lipids to be able to digest the lipids well.

Question 22

What does Orlistat do?

Answer 22

Pancreatic lipase inhibitor - reduced digestion and absorption of fats leading to steatorrhoea

Question 23

What is the main nerve controlling pancreatic secretion?

Answer 23

VAGUS NERVE

Question 24

What happens in the pancreas during the gastric phase of secretion?

Answer 24

Food arriving in the stomach stimulates pancreatic secretion via the vagus nerve.

Question 25

What mediates the pancreatic secretion during the intestinal phase?

Answer 25

Hormones released by enteroendocrine cells.

Question 26

Which hormones stimulate the release of the bicarbonate component and the enzyme component of pancreatic juice?

Answer 26

Bicarbonate component - secretin

Enzyme component - cholecystokinin + vagus nerve

Question 27

Describe the control of bicarbonate secretion.

Answer 27

H+ concentration is detected by the S cells in the duodenum.
The influx of protons causes release of secretin into the systemic circulation.
The secretin goes to the pancreas and stimulates the duct cells to release bicarbonate.
Secretin binds to receptors on duct cells activating chloride channel. This is done using cAMP as a secondary messenger. Chloride efflux. Increased activity of anion exchanger.

Question 28

Describe the control of enzyme secretion.

Answer 28

Peptides and fats are detected by the I cells in the duodenum and cholecystokinin is released, which acts on acinar cells and makes them release pro-enzymes and trypsin inhibitor. CCK binds to CCK 1 receptors on acinar cells. This activates PLC IP3 messenger.
Vagus can also stimulate the release of proenzymes and trypsin inhibitor from acinar cells.

Question 29

How do you switch off CCK?

.

Answer 29

The end of the cephalic phase (once the meal has been eaten) shuts down the vagal stimulation.
Absorption of fats and peptides removes the local stimulus for CCK release from enteroendocrine cells

Question 30

How does CCK affect bicarbonate secretion? Does secretin affect enzyme secretion in the same way?

Answer 30

CCK potentiates the effects of secretin.
CCK alone will not trigger the production of any bicarbonate but CCK and secretin together will cause a massive increase in bicarbonate secretion.
Vagus nerve has a similar effect to CCK.
Secretin has NO effect on enzyme secretion.