Anatomy and Histology of the GIT hollow organs

Anatomy and Histology of the GIT hollow organs II Dr. Nabil Khouri. MD. Ph. D

The Small Intestine – About 18 feet long – 3 regions

Mesenteries of Digestive Organs

Mesenteries of the small intestine • The mesentery is a double fold of peritoneum attached to the posterior abdominal wall. It is fan-shaped with a root of about 15 cm extending obliquely from the left L 2 transverse process level to the right sacroiliac joint and crossing a third part of the duodenum, aorta and inferior vena cava (IVC) right ureter, and a 4 - to 6 -m periphery, which covers the entire length of the jejunum and ileum. Between the 2 leaves of the mesentery are the mesenteric vessels and lymph nodes.

Small Intestine: Gross Anatomy • Runs from pyloric sphincter to the ileocecal valve • Has three subdivisions: duodenum, jejunum, and ileum • The bile duct and main pancreatic duct: – Join the duodenum at the hepatopancreatic ampulla – Are controlled by the sphincter of Oddi • The jejunum extends from the duodenum to the ileum • The ileum joins the large intestine at the ileocecal valve

• The jejunum and ileum form the second and third part of the small intestine respectively. They collectively measure about 20 feet long • Each of jejunum and ileum has distinctive features, but there is a gradual change from one to the other and that is why it is better to describe both simultaneously. • Jejunum • The jejunum is smaller of the two and measures about 8 feet, while the ileum is about 12 feet long. • The jejunum constitutes about two fifths of the small intestine and the ileum about three fifths. The jejunum has a thicker wall and a wider lumen than the ileum and mainly occupies the left upper and central abdomen. • The jejunum begins at the duodenojejunal flexure. After its origin, it gets coiled and gradually changes its features to become ileum, which ends at the ileocecal junction. • Ileum • The ileum constitutes about three fifths of the small intestine and the jejunum about two fifths. The ileum has a thinner wall and a smaller lumen than the jejunum and mainly occupies the central and right lower abdomen and pelvis. Mesenteric fat is abundant in the mesentery of the ileum, and vessels in the mesentery are, therefore, not well seen.

Functions of the Small Intestine – Chyme is further broken down • Proteins are further broken down • Carbohydrates are broken down to simple sugars • Fats are broken down to glycerol and fatty acids – Most absorption is in the small intestine • Amino acids & simple sugars • Fats

Differences between jejunum and ileum: • Location: • The jejunum lies coiled in the • the ileum is found in the lower part of the peritoneal cavity and in upper part of peritoneal cavity below the left of the transverse the pelvis. mesocolon, while • Structural features: • The jejunum has wider bore, • as compared to ileum, where they are smaller and widely spread in thicker walls, and is redder in the upper part and totally absent color. The infoldings of the in the lower part. mucous membrane (plicae circulars) are larger, more numerous and more closely set in the jejunum • Attachment of mesentery: • the mesentery of ileum is • The mesentery of jejunum is attached to the right. attached to the left of aorta while • the root to the intestinal wall.

Differences between jejunum and ileum: • Vascular arcades: • The mesenteric vessels of jejunum form only one or two arcades, which supply the jejunal wall through long and infrequent branches. • the ileum receives numerous short terminal vessels that arise from a series of three, four or even more arcades.

Blood supply of jejunum and ileum: • Arteries: • Both jejunum and ileum are supplied by the branches of superior mesenteric artery. These branches arise from the left side of the artery (from where no other branches arise) and run through the mesentery to reach the wall of intestine. The branches anastomose with one another to form a series of arcades. • The lowest part of ileum, near the ileocecal junction, is supplied by the ileocolic artery in addition to the usual blood supply. • Veins: • The veins correspond to the arteries and eventually drain into the superior mesenteric vein.

Histology of the Small Intestine

Histology of the Small Intestine – The mucosal surface is folded into villi • Increases surface area for absorption • Interior contains blood vessels and a lacteal – Intestinal glands • Secrete intestinal juice • Contains enzymes that break down proteins, carbohydrates, fats

Microscopic Anatomy of the Small Intestine • Structural modifications of the small intestine wall increase surface area – Plicae circulares: deep circular folds of the mucosa and submucosa – Villi: fingerlike extensions of the mucosa – Microvilli: tiny projections of absorptive mucosal cells’ plasma membranes

Mucosa of the Small Intestine • The columnar epithelium of the mucosa is made up of: • Absorptive cells and goblet cells • Interspersed T cells (intraepithelial lymphocytes), and • Entero-endocrine cells – The mucosal surface is folded into villi • Increases surface area for absorption • Interior contains blood vessels and a lacteal – The mucosa and submucosa are thrown into circular folds Plicae Circulares (PC) – Microvilli are present on the luminal surface of the enterocytes. – Prominent future Peyer’s Patches – lymphoid tissue aggregates – Lamina propria – Intestinal Glands • Secrete intestinal juice – Contains enzymes that break down proteins, carbohydrates, fats

Small Intestine: Histology of the Wall • The epithelium of the mucosa is made up of: – Absorptive cells and goblet cells – Interspersed T cells (intraepithelial lymphocytes), and – Enteroendocrine cells • Intestinal crypts cells secrete intestinal juice • Peyer’s patches are found in the ubmucosa

Small intestine

• The small intestine • The mucosa includes a columnar epithelium with glands called crypts of Lieberkuhn; mucus-secreting goblet cells; Paneth cells, which secrete lysozymes; enteroendocrine cells, which secrete hormones; fingerlike (leaflike) projections called villi, which increase its absorptive surface area several times; lamina propria (connective tissue); and muscularis mucosa. • The ileum has subepithelial aggregates of lymphoid tissue along the antimesenteric border; these are called Peyer patches. Mucosa is much thicker in the jejunum than in the ileum and is arranged in spiral folds called plicae circulares, which appear as valvulae conniventes on plain abdominal radiographs. • The submucosa contains the blood vessels and the Meissner nerve plexus; the muscularis propria contains inner circular and outer longitudinal muscles and myenteric (Auerbach) nerve plexus; and the serosa covering the organs of the peritoneal cavity is called the visceral peritoneum.