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Small Intestine

The small intestine is divided into

a. duodenum
b. jejunum
c. ileum.

The duodenum

1) The duodenum is the first section of the small intestine and has a thicker layer of tissue than the other areas of the small intestine.
2) It neutralizes stomach acids and breaks down carbohydrates and fats. The duodenum is about 2 feet long.

Jejunum

The jejunum is the main section of the small intestine. It covers about 15 feet and is responsible for the absorption of almost all nutrients except water.

Ileum

The ileum is the last section of the small intestine and spans about 6 feet. Its function is to absorb water and vitamins.

The Mucosa

a. Plicae circulares (valves of Kerkering) are macroscopically visible, crescent-shaped folds of the mucosa and submucosa.
b.  Plicae circulares extend around one-half to two-thirds of the circumference of the lumen of the lumen of the small intestine

Plicae circulares

a. are permanent structures, i.e. their presence does not depend on the state of distension of the small intestine.
b. are absent from the first few centimetres of the duodenum and the distal part of the ileum.
c. Are particularly well developed in the jejunum.
d. increase the surface area of the mucosa by a factor of ~ three

Intestinal villi

1) The entire intestinal mucosa forms intestinal villi (about one mm long), which increase the surface area by a factor of ~ ten. The surface of the villi is formed by a simple columnar epithelium.

2) Submucosa of each villus contains a capillary network & a “lacteal” (lymphatic capillary) for absorption of nutrients

CRYPTS OF LIEBERKÜHN

a. Between the intestinal villi we see the openings of simple tubular glands, the crypts of Lieberkühn.
b.  Intestinal glands within intestinal crypts secrete “intestinal juice” – provides watery medium to keep enzymes & digestive products in solution for help with absorption

CELLS OF CRYPTS

a. In addition to enterocytes, the epithelium is composed of mucus-secreting goblet cells and endocrine cells
b. One exception are Paneth cells which are located at the bottom of the crypts.

Intestinal Absorptive Cells

1) Absorptive cells, or enterocytes, are the predominant epithelial cell type lining the lumen of the small intestine and colon.
2) In the small intestine, brush border
3)  enzymes contribute to digestive breakdown.

Enterocyte

Each absorptive cell or enterocyte of the epithelium forms numerous microvilli   (1 µm long and about 0.1 µm wide). Microvilli increase the surface area by a factor of ~ 20.

Goblet cells

1)The apical end of each goblet cell is occupied by a large mass of mucus, which compresses adjacent cells.
2)The nucleus toward the basal end of the cell.
3)Attached by junctional complexes (evidenced in light microscopy as the “terminal bar”) to adjacent absorptive cells.

Paneth Cells

a. Paneth cells are secretory epithelial cells located at the ends of intestinal crypts.  The function for these cells is secretion of anti-bacterial proteins into the crypt lumen, thereby providing protection for the stem cells which line the crypt walls.

b. Paneth cells have typical serous-secretory appearance, with basophilic basal cytoplasm (containing protein-synthetic rough endoplasmic reticulum) and apical secretory vesicles (zymogen granules).

Lamina Propria

The lamina propria is, similar to the lamina propria of the stomach, unusually cell rich. Lymphocytes often invade the epithelium or form solitary lymphoid nodules in the lamina propria. Lymph nodules may form longitudinal aggregations of 30-50 nodules in the lamina propria of the ileum. These large aggregations are called Peyer’s patches

Muscularis mucosae

The muscularis mucosae has two layers and extends into the intestinal villi, where the smooth muscle cells form a longitudinal bundle in the centre of the villi

Submucosa

a.The submucosa contains glands only in the duodenum. Submucosal glands of the duodenum are also called Brunner’s glands.
b. the secretion of Brunner’s glands protects the duodenal mucosa – similar to the mucus which protects the gastric mucosa.

Histological slides are available below.

Small Intestine

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