Light micrograph of a section through the wall of the urinary bladder in its contracted (empty) state. The numerous folds, or rugae, allow the bladder to expand as it fills. Lining the interior of the bladder is transitional epithelium (pink), which is able to expand, but also forms an impermeable barrier to prevent toxins from the urine entering the blood stream. Beneath the epithelium is connective tissue (blue) with blood vessels and smooth muscle cells (red, across bottom). Magnification: x40 when printed at 15cm wide.