Which type of tissue is characterized by a round or flat appearance in its top layer of cells?

The tissue characterized by a round or flat appearance in its top layer of cells is transitional epithelium. This type of epithelium is unique because it has the ability to stretch and change shape, which is particularly important for organs that need to expand, such as the bladder. When the bladder is empty, the cells appear more rounded or dome-like, while when the bladder fills with urine, the cells become flatter and more stretched out.

The specific structure of transitional epithelium enables it to accommodate fluctuations in volume and provides a barrier to substances in the urine. This elasticity is crucial for its function in the urinary system, where it lines structures that undergo significant changes in size.

The other options represent different types of epithelial tissues with distinct characteristics. Stratified squamous epithelium consists of multiple layers with the topmost layer being flattened squamous cells, making it different in structure and appearance from transitional epithelium. Stratified cuboidal epithelium is primarily found in glandular tissues and generally has cube-shaped cells in multiple layers, while simple columnar epithelium features a single layer of column-shaped cells. Each of these tissues serves specific roles in the body but does not exhibit the round or flat