Gi 10: Normal Structure And Function Of Small And Large Bowels

Front 1

What are the specialized modifications that increase the surface area to aid absorption in the small intestine? What is the cumulative increase in intestinal surface area?

Back 1

- Plicae Circulares (increase by 2-3 fold)
- Villi (increase by 10 fold)
- Microvilli (increase by 20 fold)

*Total increase in intestinal surface area by 400-600 fold

Front 2

What is the term for the permanent mucosal folds that are found in the small intestine?

Back 2

Plicae Circulares

Front 3

What is the term for the finger-like projections of epithelium-covered lamina propria in the small intestine?

Back 3

Villi

Front 4

What is the term for the extensions of epithelial cell apical plasma membrane in the small intestine?

Back 4

Microvilli

Front 5

What are Plicae Circulares? Where are they found? Function?

Back 5

- Permanent mucosal folds present in the duodenum, jejunum, and proximal ileum
- Increase surface area by factor of 2-3
- Decrease velocity of movement of chyme along GI tract

Front 6

What are Villi? Where are they found? Function?

Back 6

- Finger-like projections of epithelium-covered lamina propria
- Greater concentration in duodenum (heigh 1.5 mm) than in jejunum or ileum (height 0.5 mm)
- Increase surface area by factor of 10

Front 7

What are Microvilli? Where are they found? Function?

Back 7

- Extensions of epithelial cell apical plasma membrane
- Increases surface area by factor of 20

Front 8

What are the structures found in between the bases of the villi?

Back 8

- Invaginations of epithelium into lamina propria
- Forms tubular glands called Crypts of Lieberkühn

Front 9

What are the components of the mucosa of the small intestine?

Back 9

- Simple columnar epithelium
- Lamina propria
- Muscularis mucosa

Front 10

What structures are found within the simple columnar epithelium of the small intestine?

Back 10

- Absorptive cells (enterocytes)
- Goblet cells
- DNES (enteroendocrine cells)
- Lymphocytes

Front 11

What is the structure of a villi?

Back 11

- Extensions of lamina propria
- Covered in epithelial cells
- Contain a central lymphatic channel (lacteal) and several capillaries
- Smooth muscle fibers, loose CT, and lymphoid cells can also be seen in the center

Front 12

What are the most abundant cells in the small intestine? Size?

Back 12

Absorptive cells / enterocytes - 25 µM high

Front 13

What is the function of enterocytes?

Back 13

- Terminal digestion
- Absorption of water and nutrients
- Re-esterify FAs into TGs
- Form chylomicrons
- Transport the bulk of absorbed nutrients into the lamina propria for distribution to the rest of the body

Front 14

What is the size and structure of an enterocyte?

Back 14

- Columnar epithelial cells
- ~25 µM high
- Elongated nucleus
- Luminal surface has brush border of ~3000 microvilli/cell covered in glycocalyx coate
- Cytoplasm rich in organelles, specifically endosomes, SER, RER, and Golgi
- Lateral edges form zonulae occludentes, zonulae adherentes, desmosomes and gap junctions

Front 15

What structures are found between adjacent enterocytes? Function?

Back 15

- Zonula occludens
- Zonula adherens
- Desmosomes
- Gap junctions

- Prevents the passage of material via a paracellular route to or from the lumen of the gut

Front 16

What structures are found on the luminal surface of enterocytes? Function?

Back 16

- Brush (striated) border
- Consists of ~3000 microvilli/cell
- Microvilli are covered in a glycocalyx coat made of glycosylated membrane proteins

- Glycocalyx layer has protective roles and also is integrally involved in the digestion of disaccharides and dipeptides into monomers for absorption through peptide hydrolases and disaccharidases

Front 17

How are disaccharides and dipeptides absorbed?

Back 17

Peptide hydrolases and disaccharidases that are found on the apical surface break down disaccharides and dipeptides into monomers for absorption

Front 18

What are microvilli attached to?

Back 18

Intermediate filaments of the enterocyte terminal cytoskeletal web through bundles of actin filaments

Front 19

How can microvilli spread out to increase absorptive capacity?

Back 19

Contraction of the terminal cytoskeletal web on enterocytes

Front 20

What organelle is abundant below the enterocyte terminal cytoskeletal web? Function?

Back 20

Smooth Endoplasmic Reticulum - important for fat absorption because it contains enzymes required for TG synthesis

Front 21

What kinds of cells in the small intestine act as unicellular glands that secrete mucin?

Back 21

Goblet Cells

Front 22

What is the function of Goblet Cells?

Back 22

Unicellular glands that secrete copious amounts of mucin that consists of high MW glycoprotein macromolecules (20% protein and 80% carbohydrate)

Front 23

What are the contents of the mucin released by goblet cells? When and how is it released?

Back 23

- High MW glycoprotein macromolecules (20% protein and 80% carbohydrate)
- Released at reasonably constant levels by exocytosis
- After release, mucin granules become hydrated and expand several hundred fold in volume

Front 24

How do you identify goblet cells on intestinal sections?

Back 24

- With H&E stain (left) they remain unstained because they are full of carbohydrate
- With PAS stain (right) the carbohydrate can be identified as a bright pink circle

Front 25

Which type of cell in the small intestine produces paracrine and endocrine hormones?

Back 25

DNES (Enteroendocrine) cells

Front 26

What is the function of DNES (Enteroendocrine) cells? How common?

Back 26

- Produce paracrine and endocrine hormones
- Makes up 1% of the cells covering the villi and intervillar surface

Front 27

What structure extends from the base of the villi into the lamina propria?

Back 27

Crypts of Lieberkuhn

Front 28

What do the Crypts of Lieberkuhn contain? Where are they found?

Back 28

- Extend from the base of the villi into the lamina propria
- They contain enteroendocrine (DNES) cells, Paneth cells, and stem cells

Front 29

What is found on the upper half of the Crypt of Lieberkuhn?

Back 29

- Surface absorptive cells
- Goblet cells

Front 30

What is found on the lower half of the Crypt of Lieberkuhn?

Back 30

- No absorptive cells
- A few goblet cells
- Mostly regenerative cells (stem cells), DNES cells, and Paneth cells

Front 31

How frequently is the epithelial lining of the intestine being renewed by stem cells? Where?

Back 31

- Continuous renewal within the Crypts of Lieberkuhn
- The cells migrate up the villus and are finally exfoliated from tips of villus
- Intestinal epithelium is replaced every 3-6 days in humans

Front 32

How do you identify stem cells in the intestine?

Back 32

Mitotic figures in the Crypts of Lieberkuhn - they are the only cells that are proliferating

Front 33

What kind of intestinal cells are long-lived and remain in the base of the Crypts of Lieberkuhn

Back 33

Paneth Cells

Front 34

Where are Paneth cells? Why?

Back 34

They remain resident at the base of the Crypts of Lieberkuhn because they do not migrate upward

Front 35

What is the shape of Paneth cells? Organelles?

Back 35

- Pyramid-shaped
- Well-developed Golgi
- Lots of RER, mitochondria
- Large apical secretory granules

Front 36

What do Paneth cells contain? Function?

Back 36

- Full of secretory granules
- Continuously secrete Lysozyme
- Secrete defensins to neutralize bacterial and viral infections

Front 37

How can you identify Paneth cells?

Back 37

Stain intensely w/ eosin and also with Phloxine-Tartrazine which stains the secretory granules scarlet

Front 38

Where is the Lamina Propria? Structure?

Back 38

- Core of the villi
- Extends down to the muscularis layer

Front 39

How does the organization of the Lamina Propria help with its function?

Back 39

- Highly vascularized to accept products of absorption
- Abundance of lymphoid cells to protect intestinal lining from microbial invasion

Front 40

What is the organization of the Muscularis Mucosa?

Back 40

- Inner circular and outer longitudinal layer of smooth muscle fibers
- Some fibers enter the villus and extend to the tip

Front 41

What happens to the smooth muscle fibers during digestion? Function?

Back 41

- Smooth muscle fibers contract causing shortening of the villus
- This may be important for emptying the lacteal

Front 42

How does the lamina propria protect the intestine from microbial flora?

Back 42

Gut-Associated Lymphoid Tissue (GALT)

Front 43

What are the components of the Gut-Associated Lymphoid Tissue (GALT)?

Back 43

- Individual lymphocytes within mucosa and epithelium
- Dense aggregates called lymphoid nodules
- Lymphoid nodules coalesce to form Peyer's Patches

Front 44

What is a Peyer's Patch? How many are there in the intestine?

Back 44

Component of Gut-Associated Lymphoid Tissue (GALT)
- Aggregate of Lymphoid Nodules
- 30-40 permanent Peyer's Patches are found throughout the ileum with a few more in the jejunum

Front 45

What is a Lymphoid Nodule? Where are they found?

Back 45

Component of Gut-Associated Lymphoid Tissue (GALT)
- Dense aggregates of lymphocytes
- Found in the small intestine, but least numerous in the duodenum

Front 46

What are the types of Inflammatory Bowel Disease?

Back 46

- Crohn's Disease
- Ulcerative Colitis

Front 47

Who is affected by Inflammatory Bowel Disease?

Back 47

Mostly children and young adults

Front 48

What are the symptoms of Inflammatory Bowel Disease?

Back 48

- Abdominal pain
- Fever
- Bloody diarrhea
- Weight loss
- Chronic fatigue

Front 49

What is the cause of Inflammatory Bowel Disease?

Back 49

Defect in the immune reaction to bacteria colonizing the gut, which results in uncontrolled chronic inflammation of the gut mucosa

Front 50

How do you treat Inflammatory Bowel Disease?

Back 50

Inflammatory suppressors such as corticosteroids and surgery

Front 51

What are the contents of the small intestine submucosa?

Back 51

- Moderately dense CT
- Adipose cells

Front 52

Are all parts of the small intestine submucosa the same? If not, how do they differ?

Back 52

The duodenal submucosa differs from the rest of the GI d/t the presence of large numbers of glands called Brunner's Glands

Front 53

How do you identify the duodenum histologically?

Back 53

The presence of Brunner's glands

Front 54

What is the marker of the junction between the duodenum and the stomach?

Back 54

Pyloric sphincter and the appearance of Brunner's glands

Front 55

What is the structure of the Brunner's glands?

Back 55

Branched tubuloalveolar structures that are similar to mucous acini

Front 56

How do Brunner's glands move through the submucosa of the duodenum? Function?

Back 56

Glands ascend through the muscularis mucosa layer and enter the Crypts of Lieberkuhn to deposit their secretions into the lumen

Front 57

What are the contents of the secretions of the Brunner's glands in the duodenum? Functions? How much?

Back 57

- Alkaline mucus - neutralizes pH of gastric chyme and protects duodenum
- Epidermal growth factor - stem cell proliferation in Crypts
- 2L of fluid/day

Front 58

What controls the secretions from Brunner's glands int he duodenum?

Back 58

- Mainly under neural control by: Submucosal (Meissner's) Plexus
- CCK and Secretin also contribute

Front 59

What moves chyme through the intestine?

Back 59

Muscularis Externa - composed of inner circular smooth muscle and outer longitudinal smooth muscle

Front 60

What is found between layers of the Muscularis Externa?

Back 60

Myenteric (Auerbach's) plexus - provides motor innervation to Muscularis Externa and secretomotor innervation to th emucosa

Front 61

What are the types of movements within the small intestine? Functions?

Back 61

- Mixing contractions - localized and expose chyme to digestive juices
- Propulsive contractions - peristaltic waves to transport chyme through the intestine at speed of 2cm/minute

Front 62

What happens if the intestinal mucosa is assaulted w/ irritable substances?

Back 62

Enteritis - can trigger a peristaltic rush in which strong contractions move chyme into the colon within a few minutes and induce excretion as diarrhea

Front 63

How much fluid, sodium, carbohydrate / protein, and fat are absorbed by the small intestine daily?

Back 63

- Fluid: 7L
- Sodium: 30g
- Carbs and protein: 0.5kg
- Fats: 1kg

Front 64

Which components of foodstuff can you follow through the digestive tract? Why or why not?

Back 64

- Most foodstuffs that are broken down into individual molecules and are absorbed by enterocytes are difficult to trace by microscopy
- Fat is easily followed because it is not water soluble

Front 65

What is the shortest segment of the small intestine? Length?

Back 65

Duodenum - 25 cm

Front 66

What does the duodenum receive? Through what?

Back 66

Via the Duodenal papilla of Vater:
- Bile from liver via common bile duct
- Digestive juices from pancreas via pancreatic duct

Via pyloric sphincter:
- Chyme from stomach

Front 67

How does the wall of the duodenum compare to the jejunum and ileum?

Back 67

In the duodenum the villi are:
- Broader
- Taller
- More numerous per unit area

Fewer goblet cells per unit area

Brunner's glands present in submucosa

Front 68

How does the wall of the jejunum compare to the duodenum?

Back 68

Villi are:
- Narrower
- Shorter
- Sparser

More goblet cells per unit area

Front 69

How does the wall of the ileum compare to the duodenum and jejunum?

Back 69

Villi are:
- Sparsest
- Shortest
- Narrowest

Lamina propria houses Peyer's patches in the wall opposite the attachment of the mesentery
- Villi in Peyer's patches are even more reduced in height and may even be absent

Front 70

Where are the greatest number of Peyer's patches in the small intestine? Where are they located within that segment?

Back 70

Ileum:
- Wall that is opposite the attachment of the mesentery

Front 71

How long is the large intestine?

Back 71

1.5 meters

Front 72

What are the sections of the large intestine?

Back 72

- Cecum
- Appendix
- Colon: Ascending, Transverse, Descending, Sigmoid
- Rectum
- Anus

Front 73

What are the main functions of the large intestine?

Back 73

- Absorbs most of the water from chyme
- Compacts remaining material into feces for excretion

Front 74

Can you tell the difference between the colon and cecum histologically? If so, how?

Back 74

The cecum and colon are histologically indistinguishable

Front 75

Where does the ileum enter the cecal-colonic boundary? What is found at this connection?

Back 75

- Ileum enters on medial side of cecal-colonic boundary
- Closed by the ileocecal valve to prevent reflux of cecal / colonic content back into the ileum

Front 76

What is the function of the ileocecal valve?

Back 76

Prevents reflux of cecal / colonic content back into ileum

Front 77

Are there plicae circulares or villi in the colon?

Back 77

Neither is found in the colonic mucosa, which gives it a smooth appearance

Front 78

How do the cells of the colonic epithelium compare to the intestine?

Back 78

Cells in colon are similar to the small intestine
- No Paneth cells in the colonic crypts
- No villi in colon
- Colonic mucosa is full of Crypts of Lieberkuhn which are deeper
- Crypts are more glandular w/ higher abundance of goblet cells

Front 79

What is the appearance of the Crypts of Lieberkuhn in the colon?

Back 79

- Crypts are longer and more abundant with glands, especially goblet cells
- Crypts open into the lumen at the surface and appear as a regular array of holes across the mucosa

Front 80

Where do the mucosal cells of the colon originate? What happens to them?

Back 80

- Mucosal cells are born from stem cells at the base of the crypts
- Mucosal cells migrate up the walls of the crypts until they sloughed off from the apex
- Colonic mucosa is replaced every 6-7 days

Front 81

How often does the colonic mucosa replace itself?

Back 81

Every 6-7 days

Front 82

How do the lamina propria and muscularis mucosa in the colon compare to the small intestine?

Back 82

- Lamina propria is similar in the two
- Muscularis mucosa is better developed in the colon with clear circular and longitudinal fibers

Front 83

How do the submucosa and muscularis externa in the colon compare to the small intestine?

Back 83

- Submucosa is not distinctive
- Muscularis externa is unusual in that the longitudinal muscles are arranged in three distinct bands called Taenia Coli

Front 84

What is the organization and function of the Taenia Coli?

Back 84

- Three distinct bands of longitudinal fibers of the muscularis externa
- Remain partially contracted and causes the large intestine to be puckered into sacculations called haustra coli

Front 85

What are Haustra Coli?

Back 85

The sacculations of the large intestine caused by the Taenia Coli

Front 86

What nervous tissue is found in the colon? Organization

Back 86

- Both submucosal and myenteric plexuses extend the length of the GI
- Myenteric plexus is the most predominant in the colon
- Consists of ganglia containing 3-50 nerve cell bodies w/ bundles of non-myelinated axons forming a network

Front 87

What is the structure of the appendix?

Back 87

- Blind-ended tube
- Extends from cecum distal to the ileocecal junction

Front 88

What is the function of the appendix?

Back 88

- In some animals, it is involved in digestion of cellulose
- In humans, it has an unknown function

Front 89

What is a characteristic feature of the appendix?

Back 89

- Presence of masses of lymphoid tissue in the mucosa and submucosa
- Lymphoid tissue often forms follicles (F) w/ germinal centers
- Mucosal glands are more disperse

Front 90

How long is the rectum?

Back 90

Final 12 cm of GI tract

Front 91

What are the characteristics of the rectal mucosa?

Back 91

Similar to the colon, except:
- Crypts are somewhat deeper
- Goblet Cells are more abundant

Front 92

Where does the rectum join the anal canal? Characteristics of this junction?

Back 92

Recto-Anal Junction:
- Epithelium transitions from simple columnar to stratified squamous
- Crypts disappear just prior to the junction and are replaced by large circumanal glands

Front 93

What kind of epithelium is in the colon?

Back 93

Simple columnar epithelium

Front 94

What kind of epithelium is in the rectum?

Back 94

Stratified squamous epithelium

Front 95

What venous supply is in the rectum? Location?

Back 95

Two venous plexi are found in the submucosa of anal canal:
- Internal hemorrhoidal plexus
- External hemorrhoidal plexus

Front 96

What pathology can affect the internal and external hemorrhoidal plexi? Who is most likely to be affected by this?

Back 96

Hemorrhoids:
- Older people
- Pregnant women

Front 97

What happens in hemorrhoids?

Back 97

Vessels in the internal or external hemorrhoidal plexi increase in size

Front 98

What is found at the end of the anus?

Back 98

- Ring of thickened smooth muscle called the anal sphincter
- Distal to this is a circular band of striated muscle called the external anal sphincter

Front 99

What kind of muscular tissue is found in the anal sphincters?

Back 99

- Anal Sphincter: smooth muscle
- External Anal Sphincter: striated muscle

Front 100

What does this slide show? Which part of the GI tract is it characteristic of?

Back 100

Brunner's Glands - found exclusively in Duodenum

Front 101

What does this slide show? Which part of the GI tract is it characteristic of?

Back 101

Villi and Crypts
Peyer's Patches
Found in in Ileum

Front 102

What does this slide show? Which part of the GI tract is it characteristic of?

Back 102

Just Crypts, no Paneth cells
Found in colon

Front 103

What does this slide show? Which part of the GI tract is it characteristic of?

Back 103

Lymphoid Tissue, found in Appendix

Front 104

What does this slide show? Which part of the GI tract is it characteristic of?

Back 104

Squamous epithelium, glandular - found in Anus