HISTOLOGY OF THE GIT (cont….)

HISTOLOGY OF THE GIT (cont….)

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GIT COMPOSITION

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The tubular part of the GASTROINTESTINAL

TRACT

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The tubular part of the digestive• The tubular part of the digestive system consists of

esophagus, stomach, small intestine, and large intestine.• The GIT has a common general plan of organization and

possesses four layers that are known as tunics. – Tunica Mucosa (Lining epithelium, lamina

propria, muscularis mucosa)– Tunica submucosa (In some part contains

submucosal glands)– Tunica Muscularis (Muscularis externa)– Tunica Adventitia/Serosa

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…tunica Mucosa• Lining epithelium

– Varies from region to region– Oesophagus & lower part of anal canal –

Stratified squamous non-keratinized– Stomach – Simple columnar secretory in

function– Small intestine – Simple columnar

absorptive, secretory, goblet cells present– Large intestine – simple columnar, absorptive,

secretory, goblet cells present in large numbers

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Oesophagus • It is a tube about 25cm long extending

from the pharynx to the stomach.

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Oesophagus • Tunica mucosa consists of the epithelium, underlying lamina

propria and muscularis mucosa. The lamina propria contains lymphatic capillaries, blood capilaries, and loose connective tissue. The muscularis mucosa is a thin, double layer of smooth muscle, more substantial in the lower part of the oesophagus

• Tunica submucosa is highly vascular, and contains loose connective tissue. It contains oesophageal glands, that secrete mucus to help ease the passage of swallowed food

• The muscularis externa layer in the top third of the oesophagus contains skeletal muscle, in the middle, it is a mixture of smooth and skeletal muscle, and in the bottom third it is entirely smooth

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The stomach

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Food starts to be digested and absorbed in the stomach, although absorption is mostly limited to water, alcohol and some drugs

The stomach…• The stomach is an expandable, muscular bag, and it

keeps swallowed food inside it by contracting the muscular pyloric sphincter. Food can stay in the stomach for 2 hours or more. Food is broken down chemically, by gastric juice, and mechanically, by contraction of the three layers of smooth muscle in the muscular externa layer. The broken up food at the end of this process is called chyme.

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• This shows an image through the wall of the body of the stomach at low power. You should be able to identify the three major layers seen here - the mucosa, submucosa and muscularis externa.

• The mucosa is full of gastric glands and pits, and there is a prominent layer of smooth muscle - the muscularis mucosa. The contraction of this muscle helps to expel the contents of the gastric glands.

• The muscularis externa layer has three layers of muscle. An innner oblique layer , a middle circular and an external longitudinal layer. The contraction of these muscle layers help to break up the food mechanically.

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Layers of the stomach

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• Gastric glandsChief (zymogenic) cells.– Predominantly

located in the body/base of the gland. Secretes pepsinogen.

• Parietal (oxyntic) cells– Larger than chief

cells. Concentrated in the central half of the gland.

– Secretes hydrochloric acid and gastric intrinsic factor

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…the stomach(histological features)

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• It measures approximately 6.25m and it consists of duodenum 0.25m, jejunum 2.4m, and ileum 3.6m

• Although these three parts have certain distinctive features, the pattern of organization is the same and consists of the same four coats as already described

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Small intestine

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Duodenum

• Between the intestinal villi there are the openings of simple tubular glands, the crypts of Lieberkühn. One function of the crypts of Lieberkühn is the secretion of "intestinal juice Mucosa: It is lined with simple columnar epithelium

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Duodenum

• The principal cell types of epithelium of the small intestine includes the Paneth cells, goblet cells, enterocytes, and enteroendocrine cells

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Duodenum

• 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.

• Muscularis Mucosa: it consists of inner circular and outer longitudinal layers of smooth muscles.

• Tunica submucosa:In the duodenum it contains branched tubular glands known as Brunner’s glands which secrete alkaline mucus that contains glycoproteins and bicarbonate that help to neutralize the acids produced by the stomach.

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…duodenum

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Brunner’s Glands

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• Mucosa: It is lined with simple columnar epithelium Unique features– This is very similar to the duodenum except Brunner’s

glands are absent. Mucosa consists of simple columnar epithelium with goblet

– Extensive villi are present as are the crypts of crypts of Lieberkuhn.

– The pilcae cicularis are permanent folds in the intestinal mucosa.

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Jejunum

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• Tunica mucosa: It is identified by the presence of the lymphoid nodule known as payer’s patches in the lamina propria of the mucosa.

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Ileum

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