What is the common purpose of each of the following modifications to the small intestine? Describe each.
All aid in absorption in the small intestine by increasing the surface area (overall: 400-600 fold)
Plicae circularis: permanent mucosal folds located primarily in the duodenum, jejunum, and proximal ileum. Increase surface area by 2-3 fold.
Villi: finger-like projections of epithelium. Increase surface area 10-fold
Microvilli: extensions of epithelial cell apical plasma membrane. Increase surface area 20-fold
What is a lacteal?
Central lymphatic channel in an intenstinal villus
Name the (3) major epithelial cell types found on the intestinal villi? Which is the most abundant?
Enterocytes (most abundant)
Golbet cells (secrete mucus)
DNES cells (neuroendocrine)
What is a Paneth cell? Where is it located?
What stains might be used to identify these cells?
Paneth cells secrete lysozyme and defensins - overall purpose is to help neutralize bacterial and viral infections
These are long-lived cells that remain resident in the base of the crypts (do not migrate like other cells).
H&E (eosinophilic) or Phloxine-Tartrazine (scarlet)
Each enterocyte contains approximately how many microvilli?
What is the glycocalyx coat? What is its purpose?
~3000 microvilli per enterocyte
Glycocalyx coat: protective layer covering the microvilli made of glycosylated membrane proteins. Its overall purpose is protective but it also assists in the digestion of disaccharides and dipeptides (breakdown into monomers)
What is the purpose of Goblet cells?
How do they appear on H&E stain? Why? What might be used to better visualize them?
Secrete 'copious' amounts of mucin (high MW glycoprotein)
H&E stain: appear unstained (clear) due to high carbohydrate content (does not stain). Use PAS instead (stains carbohydrate)
How might one identify stem cells in the crypts of Lieberkuhn?
Look for mitotic bodies
Describe the cell populations in the upper and basal halves of the crypts of Lieberkuhn.
Upper: surface absorptive (enterocytes) and goblet cells
Basal: few goblet cells. Most cells are regnerative (stem cells), DNES cells, and Paneth cells.
Approximately how often is the epithelium of the small intestine replaced?
Describe the core of intestinal villi
Core = lamina propria
Highly vascularized to accept absorbed nutrients
Abundance of lymphoid cells (protection from microbial invasion)
Muscularis mucosae (inner circular and outer longitudonal smooth muscle)
Some muscle fibers extend to the tip - these contract during digestion to shorten the villus (possibly to empty the lacteal)
What is GALT?
Gut-associated lymphoid tissue
Dense aggregates of lymphoid cells within the intestinal mucosa are called what?
What is it called when several of these coalesce? How many of these are found in the gut? Where are they found?
Peyer's Patches: 30-40 permanent patches found throughout the gut (principally in the ileum, some in the jejunum, few/none in the duodenum)
What is the key histological feature of the duodenum (not found anywhere else)?
What is its purpose?
Found in the submucosa - secrete alkaline mucous that helps to neutralize gastric chyme as it enters the duodenum from the stonach
Secretions in the small intestine are principally under the control/influence of what?
Approximately how much fluid is secreted by the small intestine per day?
Submucosal (Meissner's) plexus
Describe the (2) actions of the musclaris externa
Mixing: localized contractions that churn the chyme, increasing exposure of nutrients to digestive juices
Propulsive: peristaltic waves that propel the chyme along the gut tube (~2 cm/minute)
Approximately how much of each of the following is absorbed by the gut each day?
Fluid: 7 L
Sodium: 30 g
Carbohydrate: 0.5 kg
Protein: 0.5 kg
Fat: 1 kg
Describe the regional differences of the duodenum, jejunum, ileum
(hint: villi, goblet cells, specialized tissue)
- Broader, taller, more dense villi
- Brunner's glands
- Fewer goblet cells
- Narrower, shorter, sparse villi
- More goblet cells
- Sparse, short, narrow villi
- Abundant Peyer's patches in the lamina propria (villi may be absent here)
Describe the 2 key functions of the colon (large intestine)
Absorb water, compact remaining material into feces for excretion
Does the colon have plicae circularis? Villi?
No plicae circularis or villi (smooth)
What cell type is absent from the crypts of the large intestine?
What else is different about large intestinal crypts compared to those found in the small intestine?
Paneth cells are not present
All other epithelial features of the large intestine crypts are similar to those seen in the small intestine.
Large intestine crypts are longer and highly glandular (many goblet cells)
Compare the musclaris mucos of the large intestine to that of the small intestine
Compare the muscularis externa of the large intestine to that of the small intestine
Muscularis mucosa: better-developed with more defined layers of circular and longitudonal fibers
Muscularis externa: outer longitudonal layer is organized into three distinct bands (taenia coli). These bands remain partially contracted, puckering the gross structure of the colon into sacculations called haustra coli
How often is the epithelium of the large intestine replaced?
every 6-7 days
What is the purpose of the appendix?
Give one key histological feature of the appendix
Unknown - probably vestigial (though some animals use it for cellulose digestion)
Histology: abundant masses of lymphoid tissue and follicles with germinal centers
About how long is the rectum? What differences (histologically) does it have compared to the rest of the colon?
deeper crypts, more abundant goblet cells, recto-anal junction
Describe the recto-anal junction histologically
The epithelium transitions from simple columnar to stratified squamous
The crypts disappear and are replaced by large circumferential glands
What are hemorrhoids?
Who gets them?
Increase in the size of the vessels of the anal canal (internal and external hemorrhoidal plexi)
More common in older people and pregnant women
How many sphinters are present in the anal canal?
Anal sphincter (proximal) - involuntary
External anal sphincter (distal) - voluntary