Friday, 27 May 2011

Defense mechanism : Gastroduodenal mucosal defense

Despite the constant attack on the gastroduodenal mucosa by a host of noxious agents (acid, pepsin, bile acids, pancreatic enzymes, drugs, and bacteria), integrity is maintained by an intricate system that provides mucosal defense and repair.

The gastric epithelial lining consists of rugae that contain microscopic gastric pits, each branching into four or five gastric glands made up of highly specialized epithelial cells. The makeup of gastric glands varies with their anatomic location.
  • Glands within the gastric cardia comprise 5% of the gastric gland area and contain mucous and endocrine cells.
  • The 75% of gastric glands are found within the oxyntic mucosa and contain mucous neck, parietal, chief, endocrine, and enterochromaffin cells

Gastroduodenal mucosal defense

The gastric epithelium is under constant assault by a series of endogenous noxious factors. A highly intricate biologic system is in place to provide defense from mucosal injury and to repair any injurythat may occur. The mucosal defense system can be envisioned as a three-level barrier,composed of preepithelial, epithelial, and subepithelial elements.

1. The first line of defense, Pre epithelial is a mucus-bicarbonate layer, which serves as a physicochemical barrier. Mucus is secreted in a regulated fashion by gastroduodenal surface epithelial cells. It consists primarily of water (95%) and a mixture of lipids and glycoproteins (mucin) functions as a nonstirred water layer impeding diffusion of ions and molecules. Bicarbonate, secreted in a regulated manner by surface epithelial cells of the gastroduodenal mucosa into the mucous gel, forms a pH gradient ranging from 1 to 2 at the gastric luminal surface and reaching 6 to 7 along the epithelial cell surface.

2. Next defense is the Surface
Epithelial cells provide the next line of defense through several factors, including mucus production, epithelial cell ionic transporters that maintain intracellular pH and bicarbonate production, and intracellular tight junctions. If the preepithelial barrier were breached, gastric epithelial cells bordering a site of injury can migrate to restore a damaged region (restitution). This process occurs independent of cell division and requires uninterrupted blood flow and an alkaline pH in surrounding environment.

3. Finally, An elaborate microvascular system within the gastric submucosal layer is the key component of the
Subepithelial defense/repair system, providing HCO3 –, which neutralizes the acid generated by parietal cell. Moreover, this microcirculatory bed provides an adequate supply of micronutrients and oxygen while removing toxic metabolic by-products.

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