Gastric Gland Cells, Secretions, and Regulation 2

Overview of Gastric Gland Structure

• Gastric glands open into shallow surface invaginations called gastric pits.

• Epithelial lining is continuous, but specific cell types occupy characteristic zones:

  • Pit: surface mucous cells + mucous neck cells (produce soluble mucin).

  • Isthmus/neck: stem cells, some parietal cells.

  • Body/base: parietal cells, chief cells, entero-endocrine (EEC) cells.

Cell Types & Their Secretions

• Surface mucous cells

  • Coat stomach with thick, insoluble gel; protects epithelium from $\text{HCl}$ & pepsin.

• Mucous neck cells

  • Located mainly in the pit/neck.

  • Secrete a soluble mucin → allows diffusion of secreted enzymes & hormones through the gastric juice.

• Parietal (oxyntic) cells

  • Large, eosinophilic; abundant mitochondria.

  • Secrete:

  • Hydrochloric acid $\text{HCl}$ via $\text{H}^+ / \text{K}^+$-ATPase pump.

  • Intrinsic factor (required for $\text{B}_{12}$ absorption).

  • Ultrastructure: intracellular canaliculi (two main channels often seen in X-section) + dense tubulovesicular system.

• Chief (zymogenic) cells

  • Basophilic (rER); apical zymogen granules.

  • Produce digestive enzymes: pepsinogen, gastric lipase, etc.

• Entero-endocrine cells (EEC)

  • Scattered, often at base; small, clear cytoplasm, central nucleus.

  • Key subtypes highlighted:

  • G cells → gastrin.

  • D cells → somatostatin.

  • ECL (enterochromaffin-like) cells → histamine.

Parietal Cell Ultrastructure in Detail

• Schematic orientation: lateral wall → base → apical surface leading to gland lumen.

• Two prominent intracellular canaliculi extend from the apical surface deep into cytoplasm.

  • In EM, appear as circular/oval profiles (cross-sections).

  • Mitochondria densely packed around canaliculi → supply ATP for acid pump.

$\text{H}^+ / \text{K}^+$-ATPase (Hydrogen–Potassium Exchanger)

• Located exclusively in apical membrane of parietal cells.

• Exchanges intracellular $\text{H}^+$ for luminal $\text{K}^+$, driving acidification.

• Clinical significance:

  • Target of proton-pump inhibitors (PPIs); blockade → ↓ $\text{HCl}$ secretion.

  • If absent/inactive → achlorhydria.

Bicarbonate Handling & "Alkaline Tide"

• For every $\text{H}^+$ secreted into lumen, one $\text{HCO}_3^-$ enters bloodstream → transient post-prandial rise in systemic pH. (Detailed in goodnotes)

Regulation of Gastric Acid Secretion

1. Cephalic ("ophthalmic") phase

   • Trigger: sight, smell, thought of food.

   • Neural pathway: vagus nerve (parasympathetic).

   • Neurotransmitter: acetylcholine (ACh).

   • Positive stimuli for acid secretion.

   • Effects:

     • Directly stimulates parietal cell (ACh → M3 receptor → ↑ $\text{Ca}^{2+}$ → activate pump).

     • Stimulates ECL cells → histamine release → H2 receptors on parietal cell → ↑ cAMP → increased acid secretion

     • Stimulates G cells → gastrin release into blood → acts on parietal & ECL cells.

2. Gastric phase (not fully detailed in clip but implied)

   • Protein in chyme acts on G cells → further gastrin secretion.

   • Distention triggers vagovagal reflexes (reinforces above).

3. Modulators & inhibitors

   • Somatostatin (from D cells) → negative feedback on G, ECL, and parietal cells. Increase in HCl is sensed by the D cells and somatostatin is released, which inhibits the parietal cell, G cell and ECL.

   • See Goodnotes

   • PPIs block the acid pump; H2 blockers antagonize histamine pathway.

   • Getting a vagotomy, to get rid of vagus effect, which will then get rid of stimulation of the parietal cell, G cell & the ECL, as vagus nerve can stimulate all of them. But only considered in extreme conditions of gastritis.

Stem Cells & Epithelial Turnover

• Stem cells reside predominantly in the isthmus region, which is below the mucous neck cells.

• Cycle time: $4\text{–}7$ days → regenerate mucous neck cells & surface mucous cells.

• Parietal & chief cells also ultimately derive from these stem cells (longer maturation path).

• Enteroendocrine cells take months to years to generate

Pathology & Clinical Correlates

• Excess vagal stimulation → hyperacidity → mucosal injury.

• Brain injury can cause dramatic vagal activation → stress ulcers; lecturer referred to "Curling" ulcers (clinically, Cushing ulcers are associated with intracranial lesions).

• Recognize difference among stress ulcers:

  • "Curling’s" (severe burns) vs "Cushing’s" (CNS trauma).

Key Numerical / Formula References

• Cell turnover for surface epithelium: $\approx 4\text{–}7\ \text{days}$.

• Proton pump stoichiometry: $1\,\text{H}^+\; \text{out} \leftrightarrow 1\,\text{K}^+\; \text{in}$ per ATP hydrolyzed.

Connections & Real-World Relevance

• Importance of intrinsic factor → deficiency → pernicious anemia.

• PPIs are mainstay therapy for GERD and peptic ulcer disease because they irreversibly inhibit the $\text{H}^+ / \text{K}^+$-ATPase