The duodenum is the first section of the small intestine. The internal surface of the duodenum is arranged in a series of folds known as the plicae_circulares (sing: plica). These folds originate in the duodenum and continue, but decrease in number and size, through the first section of the ileum. These folds are covered with finger-like extensions called villi (sing: villus).
The plicae plus the villi greatly increase the available surface area for the absorption of nutrients. Simple columnar epithelium covers the surface of these projections, and the villi are filled with supporting CT called lamina_propria.
The villi (and their epithelium) represent the bulk of the mucosal layer. At the bottom edge of the mucosal layer is the muscularis_mucosae. Below the muscularis mucosae is the submucosal layer, which contains duodenal (Brunners) glands. These glands produce mucus.