HA14 shows an excellent example of transitional epithelium or urothelium which lines the bladder. Urothelium is unusual because its cells can change shape and slide over each other to form either a thick or thin epithelium depending on how much urine is in the bladder. The facet cells (f) can change their shape quickly because they store vesicles of membrane in their cytoplasm. These vesicles can be added and removed from the facet cell membrane as the bladder expands and contracts.