What is the structure of transitional epithelium?

Transitional epithelium is made up of multiple layers of specialized cells that are capable of expanding and contracting to accommodate distention in the underlying organ. 

The cells in the transitional epithelium have a rounded pear-shaped appearance when the underlying organ is empty. In this state, it resembles a stratified cuboidal epithelium. As the organ fills with fluid, the apical or surface cells unfold and stretch, becoming more flattened. In this distended state, the transitional epithelium appears as a stratified squamous epithelium. The cells are connected to each other via gap junctions. 

The cell layers in the transitional epithelium are divided into three distinct groups – the basal layer, the intermediate layer and the apical layer. The basal or lowermost layer is attached to the basement membrane and plays a role in replenishing the detached or dead cells in the upper layers. The intermediate or middle layer is made up of cells that are capable of dividing quickly, providing urgent regeneration when the existing cells are injured or damaged. The apical or topmost layer lines the lumen and acts as a barrier, preventing leakage of toxic waste material. Some cells in the apical layer are covered with microvilli.  

The transitional epithelium does not have its own blood supply (avascular) but it has a distinct nerve supply.