Most of the tissue in the digestive tract itself has four layers.  These layers are called the mucosa, the submucosa, muscularis externa, and the serosa. The layer in the innermost part of the digestive tract is called the mucosa.  The mucosa serves to line the cavity of the tract. The mucosa consists of three sub-layers called the epithelium, the lamina propria, and the muscularis mucosae.    In most of the digestive tract, the cells in the epithelium are shaped like columns.  However, in some places where food or feces is more likely to rub against the epithelium, the cells of the epithelium might be shaped differently. For instance, epithelium of the esophagus contains many layers of cells that are flat and look like the scales on a reptile or fish under a microscope. The epithelium of the lower anal canal contains similarly shaped cell layers. This shape helps to protect the tissues that are below the epithelium. The epithelium also contains many cells that secrete mucus. This mucus lubricates the digestive tract. The lamina propia is a sub-layer that is made of loose connective tissue. The muscularis mucosae is a sub-layer of smooth muscle.

The next layer after the mucosa is called the submucosa.  This layer is a thick layer of loose connective tissue. Although this layer does not have any sub-layers, it does have blood vessels, vessels belonging to the lymphatic system, and nerves, some of which control digestion. In some places, such as the small intestine, the submucosa also contains cells that secrete mucus onto the surface of the tract.

After the submucosa is a layer called the muscularis externa.  Except for the upper portion of the esophagus, this layer consists of smooth muscle. This layer has two sub-layers of smooth muscle called the inner circular layer and the outer longitudinal layer. The cells of the inner circular layer, which is closer to the submucosa layer, wrap around the digestive tract like a ring. The cells of the outer longitudinal layer, which is closer to the serosa, run lengthwise along the digestive tract. These muscles are involved in the peristaltic action that moves food through the digestive tract.

Even though the muscularis externa layer has 2 sub-layers of smooth muscle in most of the digestive tract, the muscularis externa layer varies in the esophagus and the stomach. Unlike the other organs in the digestive tract, the muscularis externa layer in the upper portion of the esophagus is made of striated muscle instead of smooth muscle. The muscularis externa layer in the stomach consists of three sub-layers instead of two.  These layers include an outer longitudinal sub-layer, a middle circular sub-layer, and an inner oblique sub-layer. The inner oblique sub-layer provides the stomach with added strength so that it can mix food better.

The outermost layer is the serosa.  The serosa begins with the lower part of the esophagus just before it becomes the stomach and ends at the sigmoid colon. The serosa consists of a thin layer of fiberlike tissue that is arranged like a meshwork or net. This tissue is covered by another single layer of tissue. This layer consists of flat cells that look like the scales of a reptile or fish under a microscope. This tissue forms a membrane around the organs. Unlike the other organs in the digestive system, the mouth, the pharynx, and the portion of the esophagus that is above the diaphragm are not lined with serosa. Rather, they are lined with a different layer of fibrous tissue called the adventitia.

Last Updated: Tuesday, February 7, 2012