In pigs, the entrance of the esophagus into the stomach is controlled by a sphincter. This region is called the cardia (not to be confused with the cardiac gland region farther into the stomach).
The esophageal region of the stomach (Figure 1-1) receives incoming food and is lined by stratified squamous epithelium.
An epithelium is a sheet of cells; squamous cells are flattened in shape; stratified tissues have more than one layer of cells.
In the adjacent cardiac gland region, the epithelium is supplemented by simple and by compound tubular glands which produce mucus to keep the food sliding through. Tubular glands are formed from a layer of cells rolled into a tube. Each cell secretes its products into the lumen of the tube which then opens into the stomach. Compound tubular glands are formed by branching tubes.
This shows the fundic gland region of the porcine stomach, with the lumen of the stomach towards the top of the window. Simple (unbranched) tubular glands open into pits in the stomach wall (Figure 1-2).
Cells in the neck or towards the opening of these fundic glands produce mucus.
Parietal cells in the body of the gland produce hydrochloric acid.
Other cells called chief or zymogen cells produce pepsinogen which is split by hydrochloric acid to release the digestive enzyme pepsin. The zymogen cells of milk-fed young animals produce rennin which initiates the digestion of milk.
The pyloric glands are deeper and more branched, and they produce a small amount of protease and a lot of mucus.