![]() Scattered neuroendocrine cells can be observed usually near the base of the crypt and contain basally oriented eosinophilic cytoplasmic granules. Goblet cells interspersed between colorectal absorptive cells line the colonic tubules. The appearance of the colonic tubules is similar to rows of test tubes placed in a rack. The normal colorectal luminal surface of the mucosa is straight the glands are made up of tubules (crypts) that are tightly packed, parallel, nonbranching, and closely approximating the muscularis mucosae ( Fig. The large bowel mucosa is composed of a single-cell layer of colorectal epithelium covering the lumen and lining the crypts, the supporting lamina propria and a small smooth muscle band referred to as the muscularis mucosae. The inhibitory nervous activity is derived from the superior and inferior mesenteric plexuses. The remainder of the large bowel is supplied by pelvic postganglionic parasympathetic nerves. The vagus nerves supply stimulatory nervous activity to the right colon and proximal transverse colon. The lymph node drainage is divided into those lymph nodes close to the bowel wall (e.g., pericolic, perirectal) and those that follow the blood vessels (e.g., mesenteric). In portal hypertension, this area can serve as a portal-systemic shunt and can be a site of varices. The large bowel venous drainage enters the portal circulation except for the distal rectum, which drains into the systemic circulation through the middle and inferior rectal veins. Veins accompany the arteries and share their names. The inferior mesenteric artery and iliac vessels provide blood to the rectum. The remainder of the colon is supplied by the left colic and sigmoid branches of the inferior mesenteric artery. The proximal colon to the splenic flexure derives its blood supply from the superior mesenteric artery through the ileocolic, right colic, and middle colic branches. The inner surface of the large bowel is characterized by horizontally oriented folds (plicae semilunares) and fine innominate grooves of the mucosa. The submucosa contains Meissner's plexus, which is usually found closely juxtaposed to the muscularis mucosae. Appendages of subserosal fat typically hang from the large bowel to form the epiploic appendices.Įxtending luminally from the muscularis externa lie the fibroadipose tissue, blood vessels, lymphatics, and nerves of the submucosa. The inner and outer layers of muscularis externa are separated by the myenteric plexus of Auerbach. They flare at the rectum and incorporate into its external muscular layer. The taeniae unite at the base of the vermiform appendix. The muscularis externa of the large bowel is composed of an inner circular layer and an outer longitudinally running layer of smooth muscle that condenses into three longitudinally running taeniae coli, the mesocolic taenia and two antimesenteric taeniae. The upper one third of the rectum is covered by peritoneum the lower two thirds lies in the retroperitoneum surrounded by the fatty mesorectum.īeneath the mesothelium-covered serosa lies a subserosal layer of fibroadipose tissue. The rectum, measuring 10 cm to 15 cm in length, ends at the anal canal. The sigmoid colon arbitrarily becomes the rectum at approximately the level of the third sacral vertebra. At the distal portion of the descending colon, the large bowel once again acquires a mesentery to become the sigmoid colon, which measures approximately 40 cm in length. The descending colon begins at the splenic flexure, becomes retroperitoneal, and extends for 20 cm to 25 cm. The transverse colon averages 30 cm to 60 cm in length, runs from the hepatic flexure to the splenic flexure, and has a mesentery. The right colon lacks mesentery and lies mostly in the retroperitoneum except for its anterior and right lateral serosa. The right colon, 15 cm to 20 cm in length, extends to the hepatic flexure. The cecum is completely invested by peritoneum and contains the opening of the vermiform appendix. The cecum, the most proximal saccular part of the large bowel, lies inferior to a horizontal line defined by the ileocecal valve. It can be clinically useful to divide the large bowel into regions. The large bowel extends from the ileocecal valve to the anus and measures 120 cm to 150 cm in adults. Normal large bowel gross anatomy and microscopic anatomy 1., 2., 3.
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