Unlike birds and amphibians, mammals can't recover lost hearing. In people, the cells of the inner ear responsible for detecting sound and transmitting those signals to the brain form during early stages of development and can't be replaced if lost due to illness, injury or aging. Studying mice, scientists at Washington University School of Medicine? in St. Louis have identified two signaling molecules that are required for the proper development of a part of the inner ear called the cochlea. A normal mouse cochlea shows a characteristic spiral shape (above). Without both signals, the embryo does not produce enough of the cells that eventually make up the adult cochlea, resulting in a shortened cochlear duct and impaired hearing.