part_of part of transitional epithelial cell urothelial cell urinary tract transitional epithelial cell Urothelial cells are a unique type of epithelial cell found lining the urinary tract system. They form the urothelium, a specialized, multi-layered epithelium that lines major portions of the urinary tract, including the renal pelvis, ureters, bladder, and the proximal part of the urethra. The distinct characteristic of urothelial cells is their ability to stretch and contract depending on the volume of liquid they contain - a feature that facilitates the essential role they play in maintaining the functionality and integrity of the urinary system. The primary function of urothelial cells is to provide an impermeable barrier to urine, preventing the toxic components present in the urine from seeping back into the body's bloodstream. The urothelium consists of a superficial umbrella cell layer, 1–2 layers of intermediate cells, and a basal cell layer. The umbrella layer is composed of large, mostly binucleated cells that are covered in an asymmetrical unit membrane, the uroplakin plaques. These plaques function to seal the apical membrane of the bladder from the toxic and highly variable contents of urine. The lipid bilayer structure of uroplakin plaques has an exceptionally high concentration of uroplakins that contribute to the barrier function of the urothelium. Beyond their mechanical function, urothelial cells also play a role in sensing and signaling changes in the urinary system. They express a number of sensor molecules or respond to thermal, mechanical and chemical stimuli and can release chemical mediators. This allows them to act as sensory transducers because they detect changes in the filling state of the bladder and transmit this information to the nervous system. Moreover, these versatile cells contribute to the defense mechanism against urinary tract pathogens. They respond to bacterial infections by releasing chemical messengers, cytokines and chemokines, to attract immune cells, and also can engulf pathogens via endocytosis. (This extended description was generated by ChatGPT and reviewed by the CellGuide team, who added references, and by the CL editors, who approved it for inclusion in CL. It may contain information that applies only to some subtypes and species, and so should not be considered definitional.) FMA:84127 A cell of a layer of transitional epithelium in the wall of the proximal urethra, bladder, ureter or renal pelvis, external to the lamina propria. ZFA:0009308 urothelium