Transitional epithelial cells line the lumen of the ureter, according to Victoria College. These elongated cells face in one direction when the bladder is empty and turn 90 degrees as the bladder stretches when it fills. Transitional epithelium are only found in the bladder, according to the University of Michigan.
A transitional epithelial cell is specifically designed to be impermeable to water. When the bladder is empty, these specialized cells appear dome-shaped, and they bulge out into the lumen. As the bladder fills and becomes distended, the surface cells stretch and become thinner, notes the University of Michigan. Fewer layers of transitional epithelial cells appear to line the bladder when it is full. These cells form the innermost of three layers of smooth muscle that make up this lining of the bladder.
Transitional epithelial cells become flat to prevent damage to the lining of the bladder as the organ expands, according to Austin Community College. In a relaxed state, transitional epithelial cells are approximately 10 cells thick adjacent to the lumen.
The lumen of the ureter is the narrow opening that connects the ureter to the bladder in humans, notes Loyola University Chicago. In its relaxed state, the lumen appears as a star-shaped opening surrounded by transitional epithelial cells.