Intestinal Lining of the Small Intestine

Structure and Composition

The innermost layer of the small intestinal wall, playing a crucial role in nutrient absorption and protection. It is composed of three primary layers:

• Epithelium: The single-layered columnar epithelium specialized for absorption. It contains various cell types including enterocytes (absorptive cells), goblet cells (secreting mucus), enteroendocrine cells (secreting hormones), and Paneth cells (secreting antimicrobial peptides).
• Lamina Propria: A layer of connective tissue containing capillaries, lymphatic vessels, nerve fibers, and immune cells (e.g., lymphocytes, plasma cells, macrophages). It supports the epithelium and facilitates nutrient transport.
• Muscularis Mucosae: A thin layer of smooth muscle that creates local movements, enhancing contact between the lining and the intestinal contents.

Surface Area Amplification

The surface area is significantly increased by three key structural features:

• Plicae Circulares (Valves of Kerckring): Large folds of the submucosa and lining, visible to the naked eye, that project into the lumen.
• Villi: Finger-like or leaf-like projections of the lining, extending into the lumen. Each villus contains a central lacteal (lymphatic vessel) and a capillary network.
• Microvilli: Microscopic, hair-like projections on the apical (luminal) surface of the enterocytes, forming the "brush border." The brush border contains enzymes that aid in digestion.

Cell Types and Functions

Epithelial Cells

• Enterocytes: Responsible for absorption of nutrients, ions, and water.
• Goblet Cells: Secrete mucus, which protects the lining from damage and facilitates the movement of chyme.
• Enteroendocrine Cells: Produce various hormones (e.g., secretin, cholecystokinin) that regulate digestive processes. Different types of enteroendocrine cells are located throughout the lining.
• Paneth Cells: Located at the base of the crypts of Lieberkühn and secrete antimicrobial substances, such as lysozyme and defensins, which help regulate the intestinal microbiota.
• M Cells: Specialized epithelial cells overlying Peyer's patches (lymphoid follicles) that sample antigens from the lumen and present them to immune cells.

Crypts of Lieberkühn

Invaginations of the epithelium into the lamina propria, containing stem cells and Paneth cells. Stem cells continuously divide and differentiate into the various epithelial cell types, replenishing the lining. This continuous renewal process is essential for maintaining the integrity of the lining.

Blood Supply and Lymphatic Drainage

The lining is richly supplied with blood vessels that transport absorbed nutrients to the liver via the portal vein. Lacteals (lymphatic vessels) within the villi absorb fats and other lipids, transporting them to the lymphatic system. Lymphoid follicles (Peyer's patches) in the lamina propria contribute to immune surveillance and defense against pathogens.