Microanatomy: esophagus, stomach, exocrine pancreas

Question 1

Organisation of the GI layers

Answer 1

• Mucosa consists of 3 layers: epithelium, lamina propria (loose CT), muscularis mucosa (SmM)
• Beneath mucosa is submucosa: dense CT
• Beneath sub mucosa is muscular is externa (upper 1/3rd of esophagus is SkM, lower 1/3rd is SmM)
• Outer-most layer is adventitia

Question 2

Stomach secretions and gastric glands

Answer 2

• Stomach secretes HCl, intrinsic factor, mucous, nzs, hormones
• Gastric gland has 4 parts (from top to bottom): pit (mucous cap cells), isthmus (stem cells), neck (parietal cells), and base (chief and endocrine-or EEC- cells)
• Mucous cap cells: lubrication and protection
• Chief cells: release inactive nz precursors
• Parietal cells: release HCl and intrinsic factor (look like pac man- many more foldings once activated to release intrinsic factor)
• Acid release can be sustained, intrinsic factor release cannot (only for 30 min)

Question 3

Enteroendocrine system

Answer 3

• EEC cells in the gastric glands have both paracrine and endocrine functions (>20 hormones)
• They can either be in the open (apical membrane reaches gut lumen) or closed (apical membrane does not reach lumen) states
• Open state EECs function as paracrine cells
• Closed state EECs function as endocrine cells
• EEC polarity is toward the basolateral membrane, instead of apical membrane
• Carcinoid tumors of the stomach arise from EECs
• Sensory function based on taste receptors in cytoplasm

Question 4

Exocrine pancreas

Answer 4

• Releases >15 inactivated digestive nzs (including trypsinogen) that are activated in lumen of SI by enterokinase
• Activated trypsin then activates the other digestive nzs
• Protection against auto digestion: tight junctions in the acini of the pancreas prevent nzs from entering the ISF, also secreting nzs in the inactive form
• Acinar cells release trypsin inhibitor, centroacinar and intercalated duct cells both release bicarb and trypsin inhibitor
• Trypsin inhibitor prevents premature activation of trypsin cascade and bicarb is to neutralize the low pH environment of the duodenum b/c of stomach acid

Question 5

Hormonal and neural regulation of exocrine pancreas

Answer 5

• Cholecystokinin (from EECs): stimulates release of pancreatic nzs and causes gallbladder contraction
• Secretin (from EECs): stimulate release of pancreatic fluid and bicarb from centroacinar and intercalated duct cells
• Neural control: SNS regulated blood flow to pancreas, vagus nerve has some minor secretomotor function on acinar and centroacinar cells