Notes
Exocrine Pancreas
Sections
Pancreatic secretions
Two major components:
- Digestive enzymes
– Acts on fats, carbohydrates, proteins, and lipids. - Bicarbonate
– Neutralizes acidic chyme in the duodenum.
Pancreas and its associated structures
- Pancreas primarily consists of acini: clusters of secretory exocrine epithelial cells.
- Main pancreatic duct: carries pancreatic juice from secretory acini to the duodenum, formed from smaller ducts.
- Common bile duct: forms where the hepatic and cystic ducts merge.
- Hepatopancreatic duct: forms when main pancreatic duct and common bile duct merges, empties into the duodenum via its sphincter; it transports both pancreatic juice from the pancreas and bile from the liver and gallbladder to the duodenum.
- Accessory pancreatic duct: drains directly into the duodenum proximal to the hepatopancreatic duct.
Acinus
Exocrine secretory unit of the pancreas
- Acinar region = site of secretion, secrete digestive enzymes
- Duct region = modify initial secretion, secrete bicarbonate.
Digestive enzymes and bicarbonate merge to form pancreatic juice.
- Both neural and hormonal stimuli regulate pancreatic juice secretion for its subsequent release and action in the duodenum.
- Hormonal regulation occurs via: secretin and cholecysotkinin (CCK).
CCK
- Triglycerides, fatty acids, and amino acids triggers CCK release from the duodenum.
- CCK triggers acinar section of pancreatic acinar digestive enzymes.
(Digestive enzymes include pancreatic amylase, pancreatic lipases, nucleases, and proteases.)
Secretin
- Acidic chyme triggers secretin release from the duodenum.
- Acts on pancreatic ductal cells, which triggers bicarbonate release.
Potentiation
- Amplification of one hormone's effects by another hormone, which generates a larger signal than if either hormone acted alone.
- CCK and secretin act together via potentiation to amplify each other's signal.
(Pancreatic duct cells can also by stimulated by CCK, while acinar cells similarly respond to secretin stimulation) - CCK and secretin stimulation of both ductal and acinar cells promotes additional bicarbonate & digestive enzyme secretion, respectively.
Protease Activation
Like in the stomach, pancreatic proteases are stored and secreted as inactive precursor enzymes (zymogens). Their storage as zymogens protects acinar cells from autodigestion.
- Duodenum = the location of enzyme activation and subsequent action.
- Inactive proteases (eg, chymotrypsinogen) and, specifically, the inactive trypsinogen, (which are secreted by the pancreas) in the duodenal lumen.
- Proteolytic enzyme enterokinase (bound to the duodenal epithelium plasma membrane).
– Converts trypsinogen to its active form trypsin. - Trypsin, which is itself another proteolytic enzyme, activates the precursor zymogens (such as chymotrypsinogen) to their active protease forms (chymotrypsin).
– Trypsin activates pancreatic zymogens by cleaving them into their active proteases. These proteases can now themselves break down proteins.
Note that pancreatic amylase, lipase, and nucleases are released in their active form.
Full-Length Text
- Here we will learn about the pancreas – an accessory organ of the digestive system, which aids with digestion but is not itself part of the digestive tract.
- To begin, let's start a table.
- Denote that the pancreas is both an:
- Exocrine organ, which secretes pancreatic juice that aids digestion, as well as an
- Endocrine organ, which secretes hormones involved in metabolism.
We will only focus on its exocrine function in this tutorial, so let's specifically address physiological function of pancreatic juice, now.
- Denote that the pancreatic secretions have 2 major components:
- Digestive enzymes, which act on fats, carbohydrates, proteins, and lipids.
- Bicarbonate, which neutralizes acidic chyme in the duodenum.
For context, let's draw the pancreas and its associated structures.
- First, draw the pancreas and show that it primarily consists of acini, which are clusters of secretory exocrine epithelial cells.
- Indicate that the duodenum wraps around the head of the pancreas.
- Next, draw the main pancreatic duct, which carries pancreatic juice from secretory acini to the duodenum.
- Show that it's formed from smaller ducts.
- Now, draw the common bile duct, which forms where the hepatic and cystic ducts merge.
- And show that it joins that main pancreatic duct to form the hepatopancreatic duct, which empties into the duodenum via its sphincter
- It transports both pancreatic juice from the pancreas and bile from the liver and gallbladder to the duodenum.
- Also show that the main pancreatic duct splits into a smaller accessory pancreatic duct, which drains directly into the duodenum proximal to the hepatopancreatic duct.
Next, let's zoom in to the exocrine secretory unit of the pancreas: the acinus.
- Label the acinus.
- Label the duct.
- Show that acinar cells are arranged in a bulb-like structure: they are the primary site of secretion.
- Indicate their lumen.
- Now, show the ductal cells and label the duct, itself.
- Next, show that the acinar cells secrete digestive enzymes, which pass from the lumen into the duct.
- And then, that the epithelial cells secrete bicarbonate.
- Indicate that the digestive enzymes and bicarbonate merge to form pancreatic juice.
- Next, denote that both neural and hormonal stimuli regulate pancreatic juice secretion for its subsequent release and action in the duodenum.
- Denote that hormonal regulation occurs via: secretin and cholecysotkinin (CCK).
- Recall that triglycerides, fatty acids, and amino acids triggers CCK release from the duodenum.
- Show that CCK is released from the duodenum and triggers acinar section of pancreatic acinar enzymes.
- Further elaborate that digestive enzymes include pancreatic amylase, pancreatic lipases, nucleases, and proteases.
- Recall that acidic chyme triggers secretin release from the duodenum.
- Show that secretin, which is initially is released from the duodenum and travels in circulation, acts on pancreatic ductal cells, which triggers bicarbonate release.
- Write that potentiation is the amplification of one hormone's effects by another hormone, which generates a larger signal than if either hormone acted alone.
- Denote that via potentiation, CCK and secretin act together to amplify each other's signal.
- Pancreatic duct cells can also by stimulated by CCK, while acinar cells similarly respond to secretin stimulation.
Write that CCK and secretin stimulation of both ductal and acinar cells promotes additional bicarbonate & digestive enzyme secretion, respectively.
Let's now address protease activation.
- Recall, that, like in the stomach, pancreatic proteases are stored and secreted as inactive precursor enzymes (zymogens).
- Their storage as zymogens protects acinar cells from autodigestion.
- Draw a longitudinal cross section of the duodenum – the location of enzyme activation and subsequent action.
- Indicate the mucosal layer, whose epithelial cells interface the duodenal lumen.
- We omit the other layers for clarity.
- Show the inactive proteases (eg, chymotrypsinogen) and, specifically, the inactive trypsinogen, (which are secreted by the pancreas) in the duodenal lumen.
- Show proteolytic enzyme enterokinase, and indicate it is bound to the duodenal epithelium plasma membrane.
- Indicate that it converts trypsinogen to its active form trypsin.
- Next, show that trypsin, which is itself another proteolytic enzyme, activates the precursor zymogens (such as chymotrypsinogen) to their active protease forms (chymotrypsin).
- Trypsin activates pancreatic zymogens by cleaving them into their active proteases.
- These proteases can now themselves break down proteins.
- Note that pancreatic amylase, lipase, and nucleases are released in their active form.