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.

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

    • 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 the parietal cell and ECL cell to prevent histamine and 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. One canaliculi is for secretion of H ions and the other secretes Cl ions.

  • 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.

Ways by which proton pumps are transported from apical surface to the lumen(They stimulate the parietal cell, acting on the GPCR pathway):

  • Positive stimuli:

    • Acetylcholine release via the vagus nerve: Directly stimulates parietal cells, increasing HCl secretion through activation of muscarinic receptors, which will be at the base of the cell. Vagus determines this after distension of the stomach(when food has entered).

    • Histamine release via the ECL cells: The ECL is stimulated by the vagus nerve to release the granules with histamine.

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

    • Gastrin release from the G cells: G cells release gastrin into the bloodstream, which acts on the base of the parietal cell. Increased protein is sensed by the G cells, which will also stimulate the release of gastrin.

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.

  • 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.