What modification found on the apical surface of epithelial cells is primarily responsible for movement?

The modification on the apical surface of epithelial cells that is primarily responsible for movement is cilia. Cilia are hair-like structures that extend from the surface of certain epithelial cells and can beat in coordinated waves to propel substances across the epithelial layer. For example, in the respiratory tract, cilia play a crucial role in moving mucus and trapped particles out of the airways, helping maintain clear passages for air flow.

In contrast, microvilli increase the surface area of epithelial cells for absorption but do not contribute to movement in the same way as cilia. Stereocilia, while capable of movement, are primarily involved in sensory functions and absorption, particularly in the inner ear and the male reproductive system. Flagella are similar to cilia but are typically longer, primarily found in sperm cells, and are specialized for locomotion, rather than for moving substances along epithelial surfaces.