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It possesses following layers

I. Mucosa: It is comprised of surface epithelium, lamina propria (containing glands) and muscularis mucosa. It has characteristic: (a) permanent transverse or circular folds (8-10 X 3-4 mm) (valves of Klrkrlng or plicae clrcularls) and; (b) finger-like projections of mucosa (villi). The plicae circularis become maximum in terminal duodenum and proximal jejunum while disappear in terminal half of ileum.

Intestinal Villi are finger-like leaf-shaped mucosal projections (0.1-0.5 mm in length) covered by epithelium and having a central core of lamina propria, which contains a

lymph vessel or lacteal, blood vessels, nerves and extensions of muscularls mucosa. They number 10-40/mmJ of mucosa.

I. It is lined with simple columnar epithelium which is comprised of: (1) surface absorptive columnar cells; (2) goblet cells and; (3) M cells.

1. The adluminal surface of epithelial cells is folded too much to form the microvilli (3000/cell). which appear as brush border in microscopic examination.

2. A goblet cell is a modified type of columnar cell. It is a single-cell mucous- secreting gland, which is pyramidal in shape containing mucigen granules. In routine H & E staining goblet cells appear as empty cells due to dissolution of mucin during the process of staining.

3. M (mlcrofold) cells are the antigen-presenting cells and overlie the lymphatic nodules to form mucosa associated lymphatic tissue (MALT).

II. Lamina propria: It consists of loose connective tissue with infiltrated lymphocytes in the form of solitary, nodules (unlike Peyers patches of ileum); but in some places the lymphatic nodules are aggregated. It also contains intestinal glands.

Intestinal Glands (Crypts of Llberkuhn) are also present. These are simple tubular glands arranged perpendicular to the surface and occupy whole of the mucosa and open by small apertures between the villi. They are lined by: (1) Paneth cells (mucous and lysozyme secreting cells); (2) entero-endocrlne cells (secreting cholecystokinin and secretin) and; (3) stem cells.

III. Muscularls Mucosa: it consists of inner circular and outer longitudinal layers of smooth muscles.

II. Submucosa: The feature unique to duodenum is the extensive mass of coiled- branched tubular glands in submucosa (Brunner glands). These are compound tubulo- alveolar glands lined by mucous secreting columnar cells. Their ducts pierce the muscularis mucosa and lamina propria and usually open into the intestinal crypts.

III. Muscularls Externa: Beneath the mass of submucosal glands smooth muscle (muscularis externa) consists of an inner circular layer and an outer longitudinal layer as In the rest of the small intestine. The muscle layers are separated by connective tissue containing myenteric (Auerbach's) plexus.

IV. Serosa: it is the visceral layer of peritoneum, which covers the duodenum.

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