Page 137 - Atlas of Histology with Functional Correlations

(Stretched)

When fluid begins to fill the bladder, the transitional epithelium (1) changes its
shape. Increased volume in the bladder appears to reduce the number of cell
layers because the surface cells (5) flatten to accommodate increasing surface

area. In the stretched condition, the transitional epithelium (1) may resemble
stratified squamous epithelium found in other regions of the body. Note also that
the folds in the bladder wall disappear, and the basement membrane (2) is
smoother. As in the empty bladder (see Fig. 4.8), the underlying connective
tissue (6) contains venules (3) and arterioles (7). Below the connective tissue

(6) are smooth muscle fibers (4, 8), sectioned in cross (4) and longitudinal (8)
planes. (Compare transitional epithelium with the stratified squamous epithelium
of the esophagus, shown in Fig. 4.10.)

FIGURE 4.9 ■ Transitional epithelium: bladder (stretched). Stain: hematoxylin
and eosin. High magnification.

FUNCTIONAL CORRELATIONS 4.6 ■

Transitional Epithelium

Transitional epithelium allows distension of the urinary organs (calyces,
pelvis, ureters, bladder) during urine accumulation and contraction of these

organs during the emptying process without breaking the cell contacts in the
epithelium. This change in cell shape is due to the unique feature of the cell
membrane in the transitional epithelium. Here are found specialized regions
called plaques that act like hinges during different bladder functions. When

the bladder is empty, the plaques are folded into irregular contours. During
bladder filling, these structures unfold allowing the cells to stretch and
flatten. The membrane plaques are impermeable to fluids, salts, and
hypertonic urine. Transitional epithelium forms a protective osmotic

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