Histology of GIT 3

-The duodenum is mostly retroperitoneal and divided into 4 parts – the ampulla (no circular folds), descending (papillae), horizontal (crossed by superior mesenteric artery), and ascending (duodenojejunal flexure and suspensoryligament) parts.

- The duodenum is supplied by anterior and posterior superiorpancreaticoduodenalarteries (celiac trunk) and anterior and posterior inferior pancreaticoduodenal arteries (superior mesenteric artery).

- The gastroduodenal junction (bottom left) connects the stomach (S) with the duodenum (D). The muscular pyloric sphincter and outer muscle layers are shown.

- A distinct characteristic of the duodenum, which differs from the other parts of the small intestine, are mucus-secreting Brunner’s glands (G) within thesubmucosa (just like the esophagus).
-Another important characteristic of the small intestine (in general) is the presence of numerous villi. These finger-like projections extend out from the mucosal surface into the intestinal lumen, increasing surface area for absorption. The inset indicates permanent folds in the intestinal wall known asplicae.

- The jejunum and ileumare attached to the posterior abdominal wall via mesentery. Within the mesentery are arcades and straight arteries.Jejunal arteries are shorter than ilealarteries. In addition, the jejunum mucosa has many more circular folds than the ileum, showing that the jejunum absorbs most of the nutrients.

- Histologically, the jejunum and ileum are very similar. Note once again the numerous villi. Extending into the lamina propria from the mucosa are intestinal glands, better known as intestinal crypts or crypts of Lieberkuhn.

- The ileum ends in the right lower quadrant of the abdomen and connects to thececum, which then leads into the ascending colon.

- The ileocecal region is supplied by theileocolic artery, which branches off the superior mesenteric artery. The ileocolicartery gives off a colic branch which supplies beginning of the ascending colon, and an ileal branch that supplies the end of the ileum.
-The first third of the colon is supplied by the superior mesenteric artery via the ileocolic,right colic, andmiddle colic arteries. The rest of the colon is supplied by the left colic,sigmoid, and rectal arteries all branching off the inferior mesenteric artery. Note also the marginal artery running the colonic border and the arteriaerectae.

-Generally, the appendix has the same histological appearance as the large intestine. The main difference is the appendix contains a complete outer layer of longitudinal muscle, instead of bands of teniae coli.

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- The mucosa resembles that of the colon. There is simple columnar epithelium with numerous goblet cells. The glands or crypts of Lieberkuhn are straight andunbranched, but there are no villi.

-The rectum differs from the rest of the colon in that the lower one-third has nothing to do with the peritoneum, and the upper two-thirds are considered retroperitoneal. In addition, the teniae coli expand and unite to form the longitudinal muscle layer.

- The external anal sphincter is composed of voluntary, skeletal muscle. In contrast, the internal anal sphincter is not under conscious control.

- The anal columns, between each are anal valves. They mark the pectinateline, where there is an abrupt transition from simple columnar epithelium of intestine to keratinized stratified squamous epithelium of skin.

- The pectinate line also divides arterial supply.Superior to the line is supplied by thesuperior rectal arteries and drained by superior rectal veins into the portal system back to the liver. Inferior to the line, the inferior rectal arteries supply blood and middle andinferior rectal veinsdrain into the cavalsystem to the vena cava.

 The large number of veins in this region, which may become dilated and varicose, commonly known as hemorrhoids. External hemorrhoids occur below thepectinate line and can be very painful. Internal hemorrhoids, on the other hand, are usually painless