They extend through the muscularis mucosae, a thin layer of smooth muscle separating the mucosa from the submucosa and muscularis propria. Because they extend into the bowel wall, peptic ulcers have depth when viewed through an endoscope. Most ulcers are round or oval, but some are linear, triangular, or irregular in shape. In the vast majority of cases, a single ulcer is present at any one time. When there is only focal loss of superficial epithelial cells, without extension through the muscularis mucosae, the mucosal lesion is referred to as an erosion.
Erosions appear to the endoscopist as breaks in the mucosal lining, without depth.