Stomach (Gastric Cells and Secretions)

Epithelial Cells and Mucin Secretion

• Mucous neck cells:

  • Location: Present in the gastric pits, specifically in the neck region. These are distinct from surface mucous cells, which reside more superficially.

  • Synthesis of soluble mucin: Produce soluble mucins (e.g., MUC5AC) that are less viscous than the insoluble mucins produced by surface cells.

  • Function: Secrete mucin to provide lubrication and protection, aiding in the movement of chyme and preventing damage of the stomach epithelium from stomach acid.

• Soluble mucin:

  • Transportation of secretions: Facilitates the movement of secretions from parietal and chief cells into the gastric lumen.

  • Characteristic: Less viscous, allowing for easier mixing and transport of stomach contents.

Cells in the Body of the Gastric Gland

• Parietal cells:

  • Synthesis: Produce HCl (hydrochloric acid) and intrinsic factor.

  • Regulation: HCl production is stimulated by histamine, acetylcholine, and gastrin.

  • Intrinsic factor: Essential for the absorption of vitamin B_{12} in the ileum.

• Chief cells:

  • Synthesis: Produce enzymes, primarily pepsinogen and gastric lipase.

  • Pepsinogen: An inactive zymogen that is converted to pepsin in the presence of HCl, initiating protein digestion.

  • Gastric lipase: Digests fats, especially triglycerides, in the stomach.

• Enteroendocrine cells:

  • G cells:

  • Secretion: Secrete gastrin into the bloodstream.

  • Gastrin function: Stimulates parietal cells to produce HCl and promotes gastric motility.

  • D cells:

  • Secretion: Secrete somatostatin.

  • Somatostatin function: Inhibits the release of gastrin, HCl, and other gastrointestinal hormones, thereby regulating gastric acid secretion.

Parietal Cell Structure

• Prominent nucleus and canaliculi:

  • Indicates high metabolic activity required for acid production.

• Canaliculi:

  • Extension: Extend deeply into the parietal cell, increasing the surface area for acid secretion.

  • Number: Typically two canaliculi are extensively branched, maximizing secretory capacity.

• Lateral wall, base, and apical surface:

  • Structure: Key structural components involved in maintaining cell polarity and efficient secretion.

• Small circular areas:

  • Cross-sections: Represent cross-sections of the canaliculi, highlighting their abundance within the cell.

• Abundant mitochondria:

  • Energy supply: Provide the energy (ATP) needed for the active transport of ions during HCl production.

Hydrogen Potassium Exchanger

• Significance: Essential for H^+ secretion into the gastric lumen.

• Location: Found exclusively in the stomach's parietal cells.

• Omeprazole:

  • Mechanism: Blocks the hydrogen-potassium ATPase pump, thereby inhibiting HCl secretion.

  • Clinical Use: Used to treat conditions like acid reflux and peptic ulcers by reducing gastric acid production.

Bicarbonate and Alkaline Tide

• Bicarbonate (HCO_3^-):

  • Production: Produced during HCl synthesis within parietal cells.

  • Transport: Transported back into the capillary via chloride-bicarbonate exchanger.

• Alkaline tide:

  • Definition: Increased secretion of HCl leads to elevated bicarbonate levels in the blood.

  • Effect: Temporarily increases the blood's pH, creating a postprandial alkaline tide.

Cephalic Phase

• Stimulation:

  • Triggers: Initiated by olfaction, taste, and visual stimuli related to food.

  • Signaling: Signals are transmitted via the parasympathetic nervous system (vagus nerve) to the brain.

• Vagus nerve stimulation:

  • Acetylcholine release:

  • Direct stimulation: Directly stimulates parietal cells, increasing HCl secretion through activation of muscarinic receptors.

  • ECL cells stimulation:

  • Histamine release: Stimulates enterochromaffin-like (ECL) cells to release histamine.

  • Histamine effect: Positive stimulant on parietal cells, leading to increased HCl secretion via histamine H_2 receptors.

  • G cells stimulation:

  • Gastrin release: Stimulates G cells to release gastrin into the bloodstream.

G Cells and Gastrin

• G cells:

  • Stimulation: Stimulated by the vagus nerve and the presence of peptides and amino acids in the stomach.

• Gastrin:

  • Protein digestion: Increases protein digestion by stimulating parietal cells to produce HCl, which activates pepsinogen to pepsin.

Mucus and pH

• Mucus:

  • Mixing of food: Aids in the mixing of food in the pylorus region, ensuring thorough digestion prior to entry into the duodenum.

Stem Cells and Regeneration

• Location: Isthmus of the gastric gland, situated between the base and the neck of the gland.

• Function:

  • Cell division: Divide approximately every week to regenerate mucous neck cells and surface mucous cells.

  • Differentiation: Stem cells also differentiate into parietal cells, chief cells, and enteroendocrine cells, maintaining the gland's cellular composition.

• Turnover:

  • Replenishment: Replenishment of mucous neck cells and surface cells occurs approximately every 4-7 days, ensuring the integrity of the gastric mucosa.

Curling's Ulcers

• Association: Linked to brain injury and increased vagus nerve stimulation.

• Mechanism:

  • Increased HCl secretion: Elevated vagus stimulation leads to increased HCl secretion, overwhelming the protective mechanisms of the gastric mucosa.

  • Ulcer formation: The resulting acidic environment contributes to the formation of ulcers in the stomach and duodenum.