Plicae circulares

The plicae circulares, or circular folds, form internal rings around the circumference of the small intestine that are found along the length of the small intestine. They are formed from inward foldings of the mucosal and submucosal layers of the intestinal wall. The plicae circulares are particularly well developed in the duodenum and jejunum and increase the absorptive surface area of the mucosa about threefold. Each plica is covered with millions of smaller projections of mucosa referred to as villi. Two types of epithelial cells cover the villi ... 

Microvilli are microscopic projections found on the luminal surface of the absorptive cells. Each absorptive cell may have literally thousands of microvilli forming the brush border. These structures increase the surface area for absorption another 20-fold. Together, these three anatomical adaptations of the intestinal mucosa — plicae circulares, villi, and microvilli — increase the surface area as much as 600-fold, which has a profound positive effect on the absorptive process. 

Figure 6.2 Section of the small intestine. (A) Section through the small intestine showing plica circulares. (B) Magnified view of a section of the intestinal wall showing the villi and the four layers. (C) Microscopic view of three villi showing the internal structure. From V.C.Scanlon and T.Sanders (1995) The digestive system. Essentials of Anatomy and Physiology, F.A. Davis Company, Philadelphia, pp. 358-385...

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