The Urinary Bladder

On slide B-75 (cat urinary bladder, contracted, H&E [2.5x-labeled, 10x, 20x, 40x]) identify the transitional epithelium. Notice that a transitional epithelium is usually thinner than a stratified squamous epithelium. The transitional epithelium of the urinary system is usually comprised of 6-8 cell layers in the contracted state with many of the cells on the luminal surface being dome-shaped. Identify the lamina propria and observe that like the ureter, there is very little (if any) muscularis mucosa. In the bladder, as in the lower 1/3 of the ureter, the muscularis externa consists of 3 layers of smooth muscle: an inner longitudinal layer, a middle circular layer, and an outer longitudinal layer. External to the muscularis, the tunica adventitia contains fat, blood vessels, and nerves of various sizes.

Compare the thickness of the contracted bladder with a section of distended bladder (B-76, H&E [2.5x, 10x, 20x, 40x]). The transitional epithelium appears almost squamous due to distension. The epithelium is only 3-5 cell layers thick. The muscular layer also is thin. Transitional epithelial cells must be able to endure a highly concentrated urine to survive. To protect them they have membrane plaques that are thick and rigid. When the bladder is empty they fold into the cell, and return to the cell surface when the bladder is stretched. These structures render the epithelium impermeable to urine, and prevent water from diffusing in. This adaptive feature can be seen in the electron micrographs of transitional epithelium (see R-P, Figs. 20.23, 20.24 & Pl. 75).

The Urinary Bladder Identifications

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-- AshleyLPistorio - 23 May 2007