Gross anatomy: The duodenum, jejunum and ileum constitute the small intestine. The duodenum has four parts - the first part or bulb, the second part or descending segment, the third or horizontal part and the fourth or ascending segment. It is arranged in the abdominal cavity as the letter C with the head of the pancreas filling in the concavity of the C. The duodenal bulb is attached to the hepatoduodenal ligament which connects it to the undersurface of the liver. The hepatoduodenal ligament contains the portal vein, hepatic artery and common bile duct. The duodenum continues as the jejunum at the duodenojejunal flexure which is to the right of the inferior mesenteric vein. The ligament of Treitz or the suspensory ligament of the duodenum, is a fibromuscular, double fold of peritoneum that suspends the duodenojejunal flexure from the retroperitoneum. It is a clinical landmark to separate upper from lower GI bleed with upper being proximal and lower being distal to the suspensory ligament. It’s position is used to determine whether malrotation of the gut is present or not, as a normal position of the ligament rules out malrotation.
The major and minor duodenal papilla are important landmarks in the second part of the duodenum. The major papilla, also known as the papilla of Vater, is the point where the dilated junction of the bile and pancreatic ducts enter the duodenum. It is surrounded by the sphincter of Oddi. The sphincter controls the flow of pancreatic juice and bile into the duodenum and also prevents reflux of duodenal contents back into the ducts. Block at the sphincter of Oddi can result in pancreatitis and cholangitis. ERCP is done via the sphincter of Oddi. The minor duodenal papilla is located proximal to the major duodenal papilla. It is an embryonic remnant,representing the opening of the accessory pancreatic duct of Santorini. Except for the first segment of the duodenum, all other segments are secondary retroperitoneal structures.
Histology: The duodenum is lined by simple columnar epithelium. It shows numerous finger-like projections of the epithelium and lamina propria called villi, openings of intestinal glands and semilunar folds of mucosa and submucosa called plicae circulares or valves of Kerckring. These structures increase the absorptive surface of the duodenum. The enterocytes are involved in the absorption of nutrients. They are joined together by tight junctions and have microscopic protrusions from the luminal surface of the cell composed of cell membrane and cytoplasm called microvilli which are coated by filamentous glycoproteins called glycocalyx. The epithelium also has goblet cells and Brunner’s glands, both secrete mucus. The Brunner’s glands are characteristic to the duodenum , in addition to mucus, they also secrete bicarbonate to neutralize gastric acid. The epithelium invaginates into the lamina propria to form the crypts of Lieberkuhn. They are composed of a variety of cells as follows:
i) Stem cells: They show high mitotic activity and replace enterocytes and goblet cells every 3-6 days.
ii) Paneth cells: They are immune cells present at the base of the crypts and secrete lysozyme, TNF alpha and defensins.
iii) Enteroendocrine cells: There are five types - I cells secrete cholecystokinin, S cells secrete secretin, K cells secrete gastric inhibitory peptide or GIP, L cells secrete glucagon like peptide 1 or GLP 1 and Mo cells secrete motilin.
Peyer’s patches are part of the gut associated lymphoid tissue or GALT and are seen in the mucosa and submucosa. They are aggregates of lymphoid follicles lined by antigen transporting M cells.
Jejunum and Ileum
The loops of jejunum and ileum occupy the center of the abdominal cavity and pelvis. Compared to the jejunum, the ileum has a thinner wall and smaller lumen. Histologically, they can be separated from the duodenum by the absence of Brunner’s glands. Peyer’s patches are more prominent in the ileum. The jejunum is devoid of Peyer’s patches and Brunner’s glands. Rest of the histological appearance of the jejunum and ileum is the same as duodenum.
Small intestine
histology
This photomicrograph depicts the cytoarchitecture exhibited by a normal section of intestinal epithelial mucosa. Note the numerous, robust mucosal projections known as villi, as well as the associated intestinal glands, or crypts of Lieberkühn, all in a healthy state. This section was obtained as an intestinal biopsy specimen.
Large intestine
The large intestine consists of the cecum and appendix, colon, rectum and anal canal. It is called so as its lumen is wider than the small intestine although the small intestine is greater in length. The appendix, transverse and sigmoid colon have a mesentery while the ascending and descending colon, rectum and anal canal are retroperitoneal. The cecum does not have its own mesocolon but it shares the ileocolic mesentery.
Cecum and appendix: The ileum ends at the cecum at the ileocecal junction which is guarded by the ileocecal valve. It occupies the right iliac fossa. It is the widest part of the large intestine and is mobile predisposing it to volvulus. It continues as the ascending colon. The appendix arises from the posteromedial part of the cecum, below the ileocecal junction. It can be located by tracing the teniae coli. It is a true diverticulum as it is composed of all layers of the colon. Although the appendix arises from a constant base, it can float freely in various positions like retrocecal, pelvic, subcecal, pre or post ileal. Spinal level T10 supplies the appendix and it’s dermatome is at the level of the umbilicus. Hence, appendiceal pain may be felt in the periumbilical area. Recent research has suggested that the appendix should no longer be considered a vestigial organ.
Colon: It starts at the cecum as the ascending colon located on the right side of the abdominal cavity, it then continues as the transverse colon which then bends under the spleen to continue as the descending colon occupying the left side of the abdominal cavity. It is followed by the sigmoid colon which is the narrowest region of the colon.
Histology of the colon: The colon has four layers, from inside to outside these are mucosa, submucosa, muscularis externa and serosa. The muscularis externa has an inner circular and outer longitudinal layer which is arranged in three compact bands called teniae coli. The teniae coli converge at the base of the appendix. Contractions of the teniae coli lead to the appearance of colonic haustrations, an important radiological feature to identify the colon. The outer surface of the serosa shows outpouchings of peritoneum filled with fat called appendices epiploicae. The luminal surface of the colon shows numerous openings of colonic glands. Rugae, villi and plicae circulares are absent. The epithelium shows enterocytes and mucus producing goblet cells. GALT can be seen in the lamina propria. Enterocytes, goblet cells, Paneth cells, endocrine cells and stem cells are located in the crypts of Lieberkuhn. Stem cells regenerate the epithelium with about 10 billion cells being replaced each day in the colon alone! Gli1 positive cells, which are located adjacent to the stem cells, secrete activator proteins called Wnt proteins which are important in the pathogenesis of colon cancer and in everyday regeneration and repair of the epithelium.
Rectum: The rectum starts at the level of S3 vertebra and continues as the anal canal. Transition from rectum to anal canal occurs at the level of the levator ani muscle. Puborectalis, which is a component of the levator ani muscle, forms a sling at the anorectal junction. When the puborectalis relaxes, it straightens out the rectum facilitating defecation while when it contracts, it angulates the rectum preventing defecation. Houston’s valves of the rectum are transverse submucosal folds comprised of circular muscle, usually three or four in number. Taeniae coli are absent on the rectum. The prostate plus rectovesical pouch and the rectouterine pouch of Douglas can be seen in anterior relation to the rectum in males and females respectively. Waldeyer’s fascia is located posterior to the rectum. Histologically rectum is similar to the rest of the large intestine, lined by simple columnar epithelium and rich in goblet cells.
Anal canal: It has interesting anatomy due to the fact that it has two separate embryological origins. The anal canal is divided into upper two-thirds and lower one-third by the pectinate (dentate) line. The upper part has longitudinal mucosal columns called columns of Morgagni or anal columns which lie at the pectinate line. At the base of the columns are transverse mucosal folds forming the anal valves. The anal valves demarcate the anal sinuses which receive the openings of the anal glands. It is lined by simple columnar epithelium with plenty of mucous secreting goblet cells. About 1-2 cms above the pectinate line is the transitional zone with cuboidal epithelium, as the mucosa slowly transitions from a simple columnar to stratified squamous epithelium. The lower anal canal is lined by nonkeratinized stratified squamous epithelium. The junction of the anal canal with the perianal skin is the anal verge which, like any other area of skin, is lined with keratinized stratified squamous epithelium. Above the pectinate line, nerve supply is by the autonomic inferior hypogastric plexus, while below the pectinate line nerve supply is by the somatic inferior rectal branch of the pudendal nerve.
Blood supply of the gastrointestinal tract
The gastrointestinal tract is supplied mainly by the celiac trunk, superior (SMA) and inferior (IMA) mesenteric arteries. The celiac trunk supplies foregut structures stomach and up to the second part of the duodenum. The SMA supplies the distal part of the duodenum, the rest of the small intestine and proximal colon up to proximal two-thirds of the transverse colon while the IMA supplies distal colon starting from distal one-third of the transverse colon to the upper part of the rectum. There is extensive anastomoses between arteries supplying the gastrointestinal tract. This results in the formation of arterial arcades along the mesenteric border of the stomach, small intestine and colon. Quick note: Remember that the mesenteric border is where the mesentery attaches to the intestinal wall and the antimesenteric border is the region opposite to it. Perpendicularly placed arteries called vasa recta emerge from these arcades and penetrate the walls of the supplied hollow viscera, ultimately forming a plexus within the submucosa.
Arteries of the bowel
on CT
Anatomy. Sagittal CT MIP (maximum intensity projection) image shows three major arteries that supply the bowel, coeliac trunk (asterisk), superior mesenteric artery (white arrow) and inferior mesenteric artery (orange arrow), which are visceral branches of the abdominal aorta (+)
Celiac trunk: It is the first major branch of the abdominal aorta, arising at the level of T12 vertebra. It gives off three branches - left gastric, common hepatic and splenic arteries. Ultimately the celiac trunk and branches supply the stomach, lower esophagus, up to second part of duodenum, spleen, liver and gallbladder and pancreas.
Superior mesenteric artery: It is a branch of the abdominal aorta arising at the level of L1 vertebra. It gives rise to inferior pancreaticoduodenal, jejunal and ileal branches, ileocolic, right colic and middle colic arteries.
Inferior mesenteric artery: It arises from the abdominal aorta at the level of L3 vertebra. It gives rise to the left colic artery, sigmoid arteries and superior rectal artery.
Marginal artery of the colon or artery of Drummond: It is formed by the anastomoses of SMA and IMA arteries via all the colic and sigmoid branches. It runs in close proximity to the colon.
Arterial supply of the stomach: It is mainly by five arteries. Left gastric artery which is a branch of the celiac trunk and the right gastric, a branch of the common hepatic artery anastomose with each other near the lesser curvature of the stomach. Left gastroepiploic artery, a branch of the splenic artery and the right gastroepiploic artery, a branch of the gastroduodenal artery, anastomose near the greater curvature of the stomach. The gastroduodenal artery is a branch of the common hepatic artery. It is situated close to the posterior wall of the first part of the duodenum and can be eroded by a duodenal ulcer resulting in life threatening hemorrhage. Short gastric arteries branch out from the splenic artery and supply the fundus.
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