blistering of the skin. The basal layer remains intact and attached to the
basal lamina because the hemidesmosomes do notcontain cadherins. In
contrast,bullous pemphigoidis a disease in which the antigen recognized
by the autoantibodies is the BP (bullous pemphigoid) antigeninvolved in
the linkage of the basal layer to the basal lamina. Bullous pemphigoid is
also a blistering disease, but the blistering occurs at the epidermal-dermal
junction.
GASTROINTESTINAL TRACT AND GLANDSThe epithelium of the gastrointestinal (GI) tract is simple and columnar
throughout, except for the stratified squamous epithelia in regions of max-
imal friction (esophagus and anus). The stomachis a grinding organ with
glands in the fundus and body that produce mucus (surface and neck
cells), pepsinogen (chief cells), and acid and intrinsic factor (parietal cells).
Intrinsic factor binds to vitamin B 12 and is required for uptake of that vita-
min from the intestine. The parietal cell functions in a similar fashion to the
osteoclast in using carbonic anhydrase to produce protons that are pumped
into the intracellular canaliculi,which are lined by microvilliin the
active parietal cell.In the inactive parietal cell, the proton pumps are
sequestered in tubulovesicles in the cytosol.
Thesmall intestineis an absorptive organ with folds at several levels
(plicae,villi, and microvilli) that increase surface area for more efficient
absorption. The microvilli also contain specific enzymes for the breakdown
ofsugars (disaccharidases), lipids (lipases), andpeptides (peptidases).
The major digestive processes in the small intestine occur through the
action of the pancreatic juice, which contains trypsinogen, chy-
motrypsinogen, procarboxypeptidases, amylase, lipase, and other
enzymes. Trypsinogen is activated by enterokinase found on the
microvilli. Lipids are broken down to triglyceridesin the small intestinal
lumen which are subsequently degraded to glycerol, fatty acids, and
monoglyceridesthat are transported into the enterocyte. Once in the
cytosol of the enterocyte, the SERresynthesizes the triglycerides,which
are coupled with protein to form chylomicra.The chylomicra are exocy-
tosed into the lactealsand travel to the cisterna chyli and through the tho-
racic duct to the venous system. They return to the liver through the arterial
system (hepatic artery). In the liver the lipid processing is similar, but
High-Yield Facts 29