HCB Objectives 19
1.
Four layers of the digestive tube:
a.
Esophagus: protection from pathogens
a. Outer layer: adventitia (loose CT) until abdomen where it turns into a
serosa (simple squamous epithelium)
b. Muscularis propria: skeletal muscle until abdomen where it is smooth
muscle
c. Submucosa: esophageal mucus glands
d. Mucosa: non-keratinized stratified squamous epithelium
b.
Stomach: mechanical and chemical breakdown
a. Outer layer: serosa
b. Muscularis propria: outer longitudinal, middle circular, and inner
oblique smooth muscle layers; circular layer enlarges at pyloric
sphincter
c. Submucosa: dense CT without glands
d. Mucosa: simple columnar epithelium invaginated to form stomach
glands
c.
Small intestine: enzymatic breakdown and absorption
a. Outer layer: serosa (except retroperiotneal regions of duodenum and
ileocecal junction where it is an adventitia)
b. Muscularis propria: outer longitudinal, inner circular; circular layer
enlarges at ileocecal valve
c. Submucosa: Brunner’s glands in duodenum
d. Mucosa: simple columnar epithelium with villi, crypts, and lymphoid
nodules (Peyer’s patches in the ileum)
d.
Large intestine: completion of absorption and protection from feces
a. Outer layer: serosa in the colon and adventitia in the rectum
b. Muscularis propria: outer longitudinal, inner circular; outer
longitudinal breaks into taeniae coli (3 bands) and inner circular breaks
into internal and external anal sphincters at the rectoanal junction
c. Submucosa: hemorrhoidal vein plexus
d. Mucosa: simple columnar epithelium with crypts and lymphoid
nodules; at rectoanal junction the epithelium changes to stratified
squamous
2.
Epithelial types in digestive tube:
a.
Esophagus: non-keratinized stratified squamous epithelium
b.
Stomach:
a. Mucous cells: light, foamy cytoplasm; secrets mucus to lubricate and
protect stomach lining
b. Parietal cells: acidophilic cytoplasm with apical invaginations;
secretes HCl and intrinsic factor (Vitamin B12 absorption)
c. Chief cells: basophilic basal cytoplasm with apical secretory granules;
produces pepsinogen
d. Enteroendocrine cells: neutrophilic cytoplasm with basal secretory
granules; releases hormones including gastrin (inducing parietal HCl
production)
e. Stem cells: undifferentiated columnar cells that will repopulate
mucous cells quickly and parietal/chief cells more slowly
 Note: As food passes through the stomach it goes through
cardiac glands (coiled glands with short pits; mucous and
enteroendocrine cells), gastric glands (uncoiled glands with
long pits; parietal to chief cells with mucous and
enteroendocrine cells), and pyloric glands (coiled glands with
long pits; mucous and enteroendocrine cells)
c.
Small intestine:
a. Absorptive cells: tall columnar with dense microvilli and small
vesicles in the apical cytoplasm, numerous mitochondria, and an
extensively folded basal membrane; involved in absorption/digestion
of carbohydrates, proteins, lipids, electrolytes, water, and
transepithelial transport of polymeric antibodies
b. Goblet cells: goblet shaped with abundant mucous granules; secrete
mucus
c. Paneth cells: tall columnar with apical, acidophilic secretory granules
located at the base of crypts; secretes lysozyme (destruction of
bacterial cell walls)
d. Enteroendocrine cells: neutrophilic cytoplasm with basal secretory
granules; releases hormones including CCK (stimulates digestive
enzyme release from pancreas) and secretin (stimulates bicarbonate
production from pancreas)
e. Intraepithelial lymphocytes: above basal lamina
f. M cells: transport antigens into lymphoid nodules
g. Stem cells: repopulate cells sloughed off in GI epithelium
d.
Large intestine:
a. Absorptive cells: fewer as large intestine moves distally
b. Goblet cells: same as small intestine
c. Paneth cells: present only in the proximal large intestine
d. Enteroendocrine cells: same as small intestine
e. Intraepithelial lymphocytes: same as small intestine
f. M cells: same as small intestine
g. Stem cells: same as small intestine
3.
Pathway of digestion/absorption of:
a.
Electrolytes: facilitated diffusion through apical membranes; Na+/K+
ATPase pumps Na+ out of basolateral membranes
b.
Fluid: High salt concentration near basolateral membranes brings in water
via an osmotic gradient
c.
Nutrients:
a. Carbohydrates: broken down into oligosaccharides and then
monosaccharides; enter apical surface via facilitated diffusion and
leave basolateral membrane via a concentration gradient
b. Proteins: broken down into oligopeptides and then amino acids; enter
apical surface via facilitated diffusion and leave basolateral membrane
via a concentration gradient
c. Lipids: monoglycerides and fatty acids diffuse across apical
membranes and are resynthesized into triglycerides in the smooth ER.
They are then packaged into chylomicrons and sent through the
basolateral membrane
4.
Pancreatic enzyme function: breakdown of proteins to oligopeptides and
carbohydrates to oligosaccharides
Brush border enzyme function: breakdown of oligopeptides to amino acids and
oligosaccharides to monosaccharides