**1. Introduction**

The small intestine is a tubular structure that connects the stomach to the colon. The intestinal mucosa is the innermost layer that is in contact with the intestinal lumen.

It is constituted by a glandular epithelium supported on the lamina propria, below which, the muscular layer is located. The folds on its surface, called conniving valves or Kerckring folds, that are formed by mucosa and submucosa, while the villi that project on its surface, are covered only by the mucosal layer, being its height greater in the proximal sections of the duodenum and jejunum to decrease progressively toward the ileum. This design of the intestinal mucosa, forming folds alternating with villi, is aimed at obtaining the maximum possible nutrient absorption surface. At the base of the villi lie the crypts of Lieberkühn, which form glandular structures that extend to the muscularis mucosa [1].
