What is the core of stereocilia?

• A. Microtubules
• B. Actin
• C. Intermediate filaments
• D. Keratin

Answer: (B)

The main idea is understanding what filament provides the structural core of stereocilia. Stereocilia are hair-like projections on sensory hair cells in the inner ear, and their stiffness and ability to bend without breaking come from a solid actin filament bundle running along their length. These parallel actin filaments are tightly cross-linked by actin-binding proteins, giving the core enough rigidity for repeated deflection as sound or head movement occurs, while still allowing slight bending to open mechanotransduction channels at the tips.

Microtubules would indicate a true cilium with an axoneme, which stereocilia are not. Intermediate filaments like keratin are important for overall epithelial structure but do not form the specialized actin-rich core of stereocilia.