Page 727 - Atlas of Histology with Functional Correlations
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FUNCTIONAL CORRELATIONS 18.4 ■ Urinary
Bladder
The urinary bladder is a hollow organ with a thick muscular wall whose
main function is to store urine. Because the lumen of the bladder is lined with
a transitional epithelium, the wall of the organ can stretch or enlarge (change
shape) as the bladder fills with urine. When the bladder is empty, the thick
transitional epithelium may exhibit five or six layers of cells. The
superficial cells in the epithelium are cuboidal, large, dome shaped, and
bulge into the lumen. When the bladder fills with urine, however, the
transitional epithelium is stretched, and the cells in the epithelium appear
thinner and squamous to accommodate the increased volume of urine.
The changes in the appearance and cell shapes in the transitional
epithelium are due to the thickened rigid regions in the integral plasma
membrane of superficial cells called plaques that are connected to thinner,
shorter, and more flexible interplaque regions. These structures act like
“hinges,” and, in an empty bladder, the interplaque regions allow the cell
membrane to fold at the hinge regions by confining or compartmentalizing
the folded plaques in fusiform-shaped vesicles. When the bladder is filled
with urine, the apical membrane expands as the contents of the vesicles
become part of the apical membrane. The interplaque regions allow the
epithelium to expand during full stretch, changing the cell’s shape from
cuboidal to squamous shape.
The exposed or apical cell membrane of superficial cells in the
transitional epithelium is also thicker. In addition, desmosomes and
occluding junctions attach the lateral borders of the cells to each other. The
plaques are impermeable to water, salts, and urine even when the epithelium
is fully stretched. These unique properties of transitional epithelium in the
urinary passages provide an effective osmotic barrier between concentrated
urine and the underlying connective tissue.
Summary
Urinary System
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