New images reveal how the ear's sensory hairs take shape

Scientists already knew that a molecular blueprint guides the formation of upside-down V-shaped bundles on the surface of inner ear cells that detect sound, motion, and spatial orientation.

While investigating how cells draw up these blueprints, Kimberly Siletti, a graduate student in the lab, found evidence implicating a protein called Daple. It was already known to interact with a so-called compass structure, ensuring that the V-shape bundles are aligned properly to catch sound propagating through the cochlea.

Understanding these molecular orientation systems is critical because if disrupted the bundles grow facing the wrong direction, sometimes even backward. For the bundle to develop properly, the blueprint and the compass must work together.

“These two systems were discovered independently, and it isn't clear how they are coordinated,” Siletti says. “Our experiments suggest that Daple is part of the molecular machinery that links them.”

To test this hypothesis, the researchers switched off the protein in mice. The effect of this manipulation, captured in high-resolution, was conspicuous: the hair cells of animals that lacked the protein developed scrambled bundles without the distinctive V-shape (bottom image).

The scientists think Daple influences the shape of the hair bundles indirectly, by determining the position of the first filament to emerge at what becomes the apex of each bundle. If that filament is positioned improperly, the blueprint becomes warped. Their work was described in the Proceedings of the National Academy of Sciences.