Hair cell development: commitment through differentiation.

The perceptions of sound, balance and acceleration are mediated through the vibration of stereociliary bundles located on the lumenal surfaces of mechanosensory hair cells located within the inner ear. In mammals, virtually all hair cells are generated during a relatively brief period in embryogenesis with any subsequent hair cell loss leading to a progressive and permanent loss of sensitivity. In light of the importance of these cells, considerable effort has been focused on understanding the molecular genetic pathways that regulate their development. The results of these studies have begun to elucidate the signaling molecules that regulate several key events in hair cell development. In particular, significant progress has been made in the understanding of hair cell commitment, survival and differentiation. In addition, several aspects of the development of the stereociliary bundle, including its elongation and orientation, have recently been examined. This review will summarize results from each of these developmental events and describe the molecular signaling pathways involved.