Microscopically, the stomach consists of four layers, from interior to exterior, mucosa, submucosa, muscularis externa, and serosa.

When the stomach is empty or contains very small quantities of food, it is contracted and shrunken. The mucosa is wrinkled and consists of ridges called gastric folds (rugae). During distention of the stomach, its gastric folds disappear. Along the lesser curvature between the folds, the gastric canal is formed, which is a temporary, continuous furrow where saliva and fluids pass during swallowing.

The mucosa is lined by simple columnar epithelium, and it contains gastric pits and gastric glands. The main cells of the gastric glands are the mucous neck cells producing the mucous layer, parietal cells secreting the hydrochloric acid, and chief cells that release pepsinogen.

The submucosa lies beneath the mucosa and is formed by loose connective tissue containing blood vessels and nerves.

The third layer is the muscularis externa that is formed by three smooth muscle sub-layers. From internal to external, these are inner oblique, middle circular, and external longitudinal layers.

The serosa is the outer layer, which is the visceral peritoneum that covers the stomach.

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Pylorus and layers of the stomach