Where is transitional epithelium mainly located?

Transitional epithelium is primarily found in the urinary system, particularly in structures that require the ability to stretch and contract without losing integrity. The urinary bladder is a prime example, as it needs to accommodate varying volumes of urine. Transitional epithelium has a unique ability to change shape; when the bladder is empty, the cells appear more cuboidal, but as the bladder fills, the cells flatten and can appear squamous. This adaptability is crucial for the bladder's function in storing urine and preventing leakage.

In contrast, the other options provided do not feature transitional epithelium. The upper respiratory tract typically contains ciliated pseudostratified columnar epithelium, which helps in the passage of air and trapping pathogens. The pancreatic ducts are lined with simple cuboidal epithelium, which is better suited for secretion and transport. Skin primarily consists of keratinized stratified squamous epithelium, which provides a protective barrier against environmental damage. This specificity of transitional epithelium to the urinary bladder aligns with its functional requirements in the body.