Like all portions of the gastrointestinal tract, the stomach wall consists of four distinct layers.  Beginning with the outside, is the serosa or peritoneal layer.  This is an extremely thin layer (which makes it difficult to see on the image) consisting of squamous epithelial cells on the outside and some inner connective tissue. Next is the muscularis externa which consists of several layers of smooth muscle.  The submucosa extends inward into the rugae (large folds) of the stomach and is therefore irregular in outline. The surface of the mucosa lining the stomach is comprised of mucus-secreting simple columnar epithelial cells. This epithelium extends (invaginates) down into the lamina propria to form the gastric pits which then lead into the gastric glands.