The stomach has to produce a new layer of mucus every two weeks or else it will digest itself.
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yet it houses the most powerful protein digesting enzymes known. How can it manufacture and contain these enzymes without serious damage to its own tissue?
A microscopic examination of the stomach wall shows that its epithelial layer is not smooth. It is folded into thousands of tiny pits, called gastric pits, that penetrate into the surrounding muscle layers. On the surface, it looks rather like a well used pin-cushion, but the internal structure of these pits is quite complex.
Virtually all the epithelial cells facing the lumen (the inside of the stomach), and all those lining the upper parts of the pits, produce large quantities of mucus. Mucus is a mixture of water and a protein that causes it to become viscous and slippery. Any membrane that produces mucus is called a mucous membrane or a mucosa, and so the epithelium of the stomach is called the gastric mucosa. The gastric mucosa produces so much mucus that it insulates itself from the contents of the stomach.