The physiological function of the urothelium--more than a simple barrier

The urothelium, the epithelium lining the surface of the urinary bladder, is a unique cell type with high plasticity and a variety of cellular functions. The urothelium represents the first line of bladder defense and an interface between pathogens and defense mechanisms. Functions of the urothelium include control of permeability, immune responses and cell-cell communication, which seems to play a pivotal role in responding to injuries and infections. The urothelium responds to stretch, during the filling phase of micturition reflex, by increasing the size of apical umbrella cells and by releasing mediators which activate the sensory fibers. For this reason the concept of 'neuron-like properties' was suggested. Finally, despite the fact that the urothelium is a frequent site of cancer formation, few experimental model systems are currently available or well characterized for studying urothelial cancer in the era of genomics and proteomics. The purpose of this review is to give emphasis to urothelial physiology and pathophysiology in different bladder disorders and to offer an up-to-date contribution to the field of urothelial research.