Digestion Tract: GIT

Small Intestine is the longest component (6 meters). It is the principal site of digestion & absorption. There are three segments:

• Duodenum (25 cm long)
• Jejenum (2.5 m long)
• Ileum (3.5 m long)

The small intestine has finger-like projections called villi, which completely cover it and create a velvety appearance. Core of a villus contains lamina propria, fenestrated capillaries, lymphatic capillaries (lacteal), smooth muscles (associated with lacteal), and myofibroblasts (which bridge the diameter of the villus and are involved in villus contraction).

• Villi are finger-like and leaf-life projections of the mucosa into lumen
• Microvilli of enterocytes are major amplifications of the luminal surface. Each cell has several thousand. In LM, they give the apical region of a cell a striated appearance (or BRUSH).

The intestinal glands (aka crypts of lieberkuhn) are simple tubular in shape and extend from the muscularis mucosae. They are open to the surface at the base of the villi. The glands are composed of simple columnar epithelium continuous with surface cells. The lamina propria surrounds the glands containing cells of the immune system (GALT), including plasma cells, lymphocytes, macrophages, and eosinophils. The lamina propria also contains lymphatic nodules, which are an important part of GALT. The nodules are especially large in the ileum (called Peyer’s patches). The muscularis mucosae has two thin layers of smooth muscle, inner circular and outer longitudinal.

There are five types of cells in the small intestine:

• Enterocytes, which are tall columnar cells with a basal nucleus. They have striated borders on the luminal surface due to the microvilli and increase the surface area 600X.
• Goblet cells, which increases in number from the proximal to distal small intestine. They are numerous, especially in the terminal ileum. The mucus produced is water-soluble. They have an accumulation of mucin granules, which distorts the apex of the cells and surrounding cells.
• Paneth cells, which are found in the base of the intestinal glands, and may be seen in the colon as well. They have basophilic basal cytoplasm and supranuclear Golgi. They have large apical secretory granules, which are very eosinophilic, refractile, and the granules permit their identification. The granules contain lysozyme (digests cell walls of certain bacteria) and α-defensins. Paneth cells also probably regulate normal bacterial flora of the small intestine.
• Enteroendocrine cells are similar to the ones in the stomach, and secrete into the lamina propria. They can be open or closed.
• M cells (microfold cells) are modified enterocytes, which cover enlarged lymphatic nodules in the lamina propria of the ileum. They are nearly squamous and have surface microfolds instead of microvilli. They take up macromolecules from the lumen by endocytosis, where the vesicles are transported basally for exocytosis near lymphocytes. This allows for delivery of antigens to the lymphocytes.

Submucosa consists of dense connective tissue and aggregates of adipose cells. Its conspicuous feature of the duodenum is the submucosal glands (Brunner’s glands) and branched tubulo-alveolar.