All the essential elements of the hair cells in the inner ear assemble at a lightning-quick pace, at least by the standards of mouse developmental biology, report Gwénaëlle Géléoc and Jeffrey Holt in the 1 October Nature Neuroscience. Between embryonic days 15 and 17, hair cell morphology snaps into place and the hairs appear fully functional.

Credit: Courtesy of Géléoc and Holt

Shown is a scanning electron micrograph of a hair cell excised from a region of the inner ear. Numerous microvilli extend from the hair cell, which are linked at their tips by tiny molecular bridges, dubbed 'tip-links'. When the extensions are deflected—by movement of the head or by sound, for instance—the tip-links convey a rise in tension. The tip-links are linked to mechanosensory ion channels that increase the conductance of cations such as calcium in response to increased tension: the greater the tension, the greater the flow of ions.

Focusing on the vestibular region, which mediates balance, Géléoc and Holt examined various indicators of hair cell development, and by all criteria observed quick assembly. The molecular motor myosin Ic might provide the basis for speed, speculate the authors. Myosin lc, the only identified component of the tension-transducing apparatus, might head toward the tips of the microvilli, bringing components of the tip-link and associated molecules with it. The investigators are eager to find the molecular trigger that sets these events in motion.