Y

Gastric gland

Chief

(peptic)

cells

Neuroendocrine cells

Figure 2 Structure of the body and fundus gastric mucosa. The secretory units of the oxyntic mucosa consist of apical pits with basal glands. The stem cells giving rise to all the cell types are located between the pit and the gland.

serotonin, enterochromaffin-like (ECL) cells secreting histamine, and D cells secreting somatostatin.

The antral glands lack parietal cells so no acid is secreted from this region. They do, however, contain peptic cells and therefore secrete pepsinogen. In addition, they contain endocrine G cells, which secrete the polypeptide hormone gastrin that together with histamine is involved in the control of acid secretion. G cells also contain intrinsic factor.

Throughout the gastric mucosa stem cells are continually dividing and migrating up the pits differentiating into mucus neck cells, which migrate out of the pits to replace shed surface epithelial cells. Consequently, the whole surface mucosa is replaced every 72-96 h. Stem cell progeny also migrate down the pits into the glands, differentiating into acid or pepsin secreting cells. The gastric glands do not turnover as rapidly as the surface mucosal cells with parietal cells having a life span of 150-200 days. The major secretion of the surface epithelial cells is mucus. Mucus forms a continuous surface layer and is the first line of defense against gastric juice; however, it does not extend up into the esophagus. The gastric mucus layer stops at the gastroesophageal junction, so the esophagus must have a different protection mechanism. Studies with human and animal gastric mucosa have demonstrated that the mucous layer in the stomach consists of two independent gel layers (Figure 3). The first is a firm adherent mucous layer (median thickness 150 mm) that protects the underlying epithelial cells against pepsin and acid; without this layer the mucosa would be destroyed. Bicarbonate secreted into this layer by the epithelial cells neutralizes the acid diffusing from the lumen in an unstirred aqueous environment. Consequently, a pH close to 7 is maintained at the epithelial cell surface. Overlying this firm gel layer is a sloppy (shear compliant) layer of variable thickness, which

Flows to assist movement of stomach contents

Flows to assist movement of stomach contents

Figure 3 The gastric mucous layer. The shear resistant layer of the mucous bilayer remains at close to 150 mm, whereas the shear-compliant mucous layer thickness is always changing, due to its constant removal by shear stress and subsequent replenishment.

although a gel will flow easily when force is applied to it. This property is essential in reducing shear forces on the mucosa. When the stomach contains food the forces generated will convert the sloppy layer into a viscous liquid, the ideal lubricant reducing the shear forces between the food and the mucosal cells. The thickness of the bilayer is maintained by secretion of new mucus, balancing that lost by pepsin digestion and mechanical shear. Along with mucus the epithelial cells also secrete trefoil peptides, which have growth factor-like activities (see below).

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