The stomach, like all other parts of the gastrointestinal tract, is made up of four different kinds of tissues, each of which has a variety of functions associated with digestion. These parts are the outermost layer facing the stomach cavity, called the mucosa, the layer under that called the submucosa, the next layer called the muscularis externa, and the outermost-layer, called the serosa.
The mucosa of the stomach comprises a large number of gastric glands, which contain a variety of digestive juices. Specialized cells called oxyntic cells produce hydrochloric acid and a substance called gastric intrinsic factor, which is a substance that assists the ileum in absorbing vitamin B12. HCl and GIF are brought to the stomach cavity via gastric pits, connected to the gastric glands, and lined with specialized mucosal cells to prevent acidic degradation.
The underlying submucosa is lined with blood vessels that bring nutrients to the mucosa, supporting the generation of gastric juices and maintaining cell integrity. The submucosa also contains the Meissner’s Plexus, a network of interconnected, parasympathetic nerve fibers that facilitate digestion, smooth-muscle contraction, and contraction of the stomach wall.
The muscularis externa is located close to the submucosa, and it is the actual tissue to receive messages from the nerve plexus. It consists of two sets of smooth muscles—an inner circular and an outer longitudinal—which wrap around the stomach wall and facilitate the churning of the stomach contents.
The serosa contains connective tissue covered by a mesothelium (the simple squamous epithelium), which reduces friction between the walls of the stomach and the peritoneum.