Lower GI: Stomach to Anus

Question 1

Answer 1

Stomach

• mucosa (epithelium, lamina propria and muscularis mucosae)
  
  • large folds → rugae
    
    • flatten out as the stomach fills/stretches with foodstuffs
    • adult stomach expand from ~1.5L of space in physiologically empty state to ~3L in full (distended) state

Question 2

Answer 2

Stomach

Question 3

Answer 3

Stomach Mucosa

• Mucous cells line the lumen (surface) of the stomach
  
  • simple columnar cells
  • filled with mucin granules → pale, washed-out
  • dips down at regular intervals and these in-foldings form gastric pits
    
    • lined by tall simple columnar cells
    • dip further down into gastric glands → secret mucus, acid and digestive enzymes

Question 4

Answer 4

Gastric Gland Schematic

• parietal cells
• chief cells

3 Parts of Gastric Glands

• upper part (isthmus)
  
  • opens into gastric pit
• midregion (neck)
  
  • contains mixture of mucous neck cells and parietal cells
• bottom (fundus or body/base)
  
  • mixture of parietal cells and chief cells
  • lower base mostly chief cells

Question 5

Answer 5

Stomach Mucosa - Parietal Cells

• aka oxyntic cells
• most numerous in body of a gastric gland but may also be found in neck and base
• large, round, pale-staining, eosinophilic cells
• secrete high concentrations of HCl into lumen of gastric glands
• also secrete intrinsic factor → facilitates vitamin B12 absorption in proximal small intestines

Question 6

Answer 6

Stomach Mucosa - Chief Cells

• mostly found at base of gastric glands
• round, basally located nucleus, and apical part of cells appears granular due to presence of secretory (zymogenic) granules
• Compared to parietal cells, chief cells much more basophilic
• produce pepsinogen I and II (inactive proenzymes)
• work with parietal cells
  
  • HCl secreted by parietal cells → intraluminal pH of gastric glands low → converts released pepsinogen I and II to pepsin (protein breakdown)
• produce lipase

Try to locate muscularis mucosae

• underneath gastric glands
• band of smooth muscle

Question 7

Answer 7

Stomach Submucosa

• underlying muscularis mucosae
• highly vascularized and innervated CT layer
• lymphatic vessels present

Question 8

Answer 8

Stomach Muscularis Externa

• 3 layers of smooth muscle
  
  • inner oblique
  • middle circular
  • outer longitudinal
• randomly oriented layers depending on what part of stomach → random arrangement is typical for organs that expel their contents
• muscle layers work to mix chyme and expel into small intestine

Question 9

Answer 9

Stomach Serosa

• moist, slippery surface
• mesothelial cells
• produce a thin film of serous fluid
  
  • readily identified if using electron microscopy.

Question 10

Answer 10

Duodenum

Question 11

Answer 11

Duodenum Mucosa

• epithelium, lamina propria and muscularis mucosae
• duodenal villi
  
  • projections of epithelium and lamina propria
  • broad and leaf-shaped
  • huge surface area
  • simple columnar epithelial cells (enterocytes) and goblet cells
• epithelium dips down → intestinal glands or crypts of Lieberkühn
  
  • extend to muscularis mucosae

Question 12

Answer 12

Duodenum Submucosa

• protective mechanism against HCl from stomach
  
  • glands of Brunner
    
    • mucus-secreting
    • unique to submucosa of duodenum → key identification feature
    • can clearly see underlying muscularis mucosae of duodenum
    • secrete alkaline glycoprotein → buffers HCl

Question 13

Answer 13

Duodenum Muscularis Mucosae

• 2 muscle layers

Duodenum Adventitia/Serosa

• depending on location, either serosa or adventitia
• some aspects of duodenum are lined by a serosa while others are lined by an adventitia

Question 14

Answer 14

Jejunum Mucosa

• epithelium, lamina propria and muscularis mucosae
• villi
  
  • projections of epithelium and lamina propria
  • quite tall and finger-shaped
  • simple columnar epithelial cells (enterocytes) with goblet cells
    
    • number of goblet cells increases as move through small intestine

Question 15

Answer 15

Jejunum Mucosa - Lacteal

• core of each villus contains a lamina propria that is cellular and vascularized
• each villus contains one large lymphatic lacteal
• lacteal
  
  • large lymphatic capillary that absorbs dietary fat from small intestine
  • fat is combined with lymph in lacteals → chyle
  • individual lacteals merge → larger lymphatic vessels → transport fats to thoracic duct → empties into left subclavian vein
  • capillaries and smooth muscle cells surround lacteals
• muscularis mucosae of mucosa well defined

Question 16

Jejunum

Answer 16

Mucosa

Submucosa

• typical supportive CT layer underlying muscularis mucosae
• blood vessels, lymphatics and nerves

Muscularis externa

• inner circular and outer longitudinal muscle layer

Serosa

• covers jejunum externally

Question 17

Answer 17

Ileum Mucosa

• epithelium, lamina propria and muscularis mucosae
• villi
  
  • stubby and club-shaped
  • simple columnar epithelial cells (enterocytes) with goblet cells
  • each villus has a single, large lacteal → absorption of dietary fats
    
    • lacteals are not evident, as they were in the jejunum
• distinct muscularis mucosae is easily identified

Question 18

Answer 18

Ileum Mucosa - Paneth cells

• base of crypts immediately adjacent to muscularis mucosae
• deeply eosinophilic cells
• found throughout small intestine → especially numerous in crypts of ileum → identify ileum
• secretory cells → antimicrobial agents (e.g. lysozyme) into the crypts

Question 19

Answer 19

Ileum Submucosa

• typical CT layer with nerves, blood vessels and lymphatic channels
• Peyer’s patches
  
  • histological landmark of the ileum
  • aggregates of lymphoid tissue that are important in immunity
  • organized and permanent structures
  • often contain germinal centres, while transient lymphocyte aggregations in other parts of the GIT will not have germinal centres

Ileum Muscularis externa

• inner circular muscle layer and outer longitudinal muscle layer

Ileum Serosa

• covers ileum externally

Question 20

Answer 20

Ileum Submucosa - Peyer’s Patches

Question 21

Answer 21

Ileum-Duodenum-Jejunum

• ileum → Peyers patches
• duodenum → glands in its sub-mucosa (the glands of Brunner)
• jejunum → neither.

Question 22

Answer 22

Colon

• includes cecum, appendix, ascending colon, transverse colon, descending colon, sigmoid colon and rectum
• function
  
  • stores intestinal contents before discharge
  • absorbs water and electrolytes
  • secretes mucus for protection and lubrication

Question 23

Answer 23

Colon Mucosa

• epithelium, lamina propria and muscularis mucosae
• lacks villi
• simple columnar epithelium
  
  • forms deep intestinal glands/crypts 2x-3x as long as crypts of small intestine
  • epithelium contains many more goblet cells than small intestine
    
    • number of goblet cells increases from cecum to rectum
• lamina propria
  
  • highly cellular and also contains GALT (gut-associated lymphatic tissue) and many defined lymphoid follicles → may or may not pierce
    underlying muscularis mucosae to penetrate the submucosa

Question 24

Answer 24

Colon

Question 25

Answer 25

Colon Submucosa

• typical but does contain more adipose tissue than small intestine submucosa

Colon Muscularis externa

• inner circular layer and an incomplete outer longitudinal layer that is not of uniform thickness
• Meissner’s and Auerbach’s plexuses
  
  • Meissner’s
    
    • inner surface of muscularis externa
  • Auerbach’s
    
    • between the 2 layers of muscularis externa

Colon Serosa and adventitia

• covered externally by a serosa, incomplete serosa or adventitia
• depends on its location in abdomen

Question 26

Answer 26

Appendix

• very much same structure as rest of colon with major difference being larger numbers of lymphoid follicles
• nodules are located in lamina propria of appendix and may protrude into lumen → stellate appearance in inflamed condition
• nodules also frequently pierce muscularis mucosae and protrude into submucosa
• abundance of lymphoid tissue (GALT) and defined lymphoid follicles → immunological function

Question 27

Answer 27

Acute Appendicitis

Question 28

Answer 28

Esophagogastric Junction

Question 29

Answer 29

Esophagogastric Junction

• junction between esophagus and stomach
• epithelium abruptly changes from nonkeratinized stratified squamous epithelium (in the esophagus) to simple columnar epithelium (in the stomach) → serrated border = Z line
  
  • easily recognized during endoscopy
    
    • esophagus → pale
    • stomach → deep red
• Clinical significance
  
  • Barrett’s esophagus
    
    • simple columnar epithelium of stomach replace stratified squamous epithelium in inferior portion of esophagus
    • epithelium adapting to acid exposure (over a long period of time) from reflux esophagitis
    • epithelial changes → strong association as premalignant condition of esophageal adenocarcinoma, particularly lethal
  • Mallory-Weiss Syndrome

Question 30

Answer 30

Gastroduodenal Junction

Question 31

Answer 31

Gastroduodenal Junction

• transition from gastric mucosa to villous epithelium of duodenum occurs much more gradually
• has portions of gastric epithelium that extend up into duodenum → there are regions of gastric epithelium in duodenum and regions of duodenal mucosa in stomach
• implemented in one of types of Crohn’s disease

Question 32

Answer 32

Anorectal Junction

Question 33

Answer 33

Anorectal Junction

• epithelium changes from simple columnar epithelium (rectum) → transitional zone of stratified columnar epithelium → nonkeratinized stratified squamous epithelium (anal canal)
• boundary between rectum and anus = pectinate line
  
  • common site of neoplastic changes