Microanatomy: lower GI tract

Question 1

Mechanisms used to increase surface area in lower GI tract

Answer 1

• 20 ft of small intestines
• Plicae circulares: permanent interior folds of mucosa and submucosa to increase surface area 3x
• Villi: finger like projections of epithelium and lamina propria, contain lacteals in the middle
• Microvilli projection on absorptive columnar epithelial cells to increase surface area 20x

Question 2

Intestinal villi and glands (digestive nzs on apical membranes, not secretory vesicles)

Answer 2

• Cells within the villi: absorptive, goblet, enteroendocrine (EECs) cells
• Surface (columnar) absorptive cell: there is both transepithelial absorption and vesicular transport
• Goblet cell: for lubrication and protection of mucosa
• Cells within the intestinal glands (btwn the base of adjacent villi): regenerative, goblet, EEC, paneth, M cells
• Regenerative (stem) cell: located close to the base of the crypt for replacing cells that are sloughed at tip of villus
• Paneth cell: located at the bottom of the crypt, contain zymogen granules (contain Zn), some anti-microbial role
• M cell: transport Ag (intact) from lumen to underlying APCs (dendritic cells/lymphocytes)

Question 3

Immune surveillance of GI tract

Answer 3

• Columnar absorptive cells interact w/ bacteria and release chemokines (IL8) in response
• M cells in the intestinal glands overlie peyer’s patches (GALT) and deliver intact Ags to the dendritic cells and lymphocytes in the peyer’s patches below
• IgA Abs are produced against Ags upon B cell stimulation (both from B cell and non-specific IgA bolus release from liver)

Question 4

Differences btwn LI and SI

Answer 4

• Duodenum has special layer of Brunner’s glands: thick band of glands w/in submucosa (just under muscularis mucosa)
• LI lacks villi, mucosa is thicker (more goblet cells) and crypts are deeper
• Fewer absorptive cells and no panted cells, slower cell turnover
• Less water absorption than SI, but sensitive to aldo
• Water absorption (surface epithelium > crypt cells) and salt absorption in LI critical to maintaining homeostasis

Question 5

Rectum and anal canal

Answer 5

• Rectal epithelium is nearly all goblet cells, thick mucosa and deep crypts
• Tortuous, thin walled veins in submucosa lack valves and contribute to hemmorhoids
• Recto-anal junction epithelium changes to stratified squamous and loses crypts
• Internal anal sphincter is SmM and external anal sphincter is SkM