Deafness is a common sensory impairment, and many different genes can be involved. Early research in deafness naturally focused upon the sensory hair cells in the inner ear. Hair cells have an array of finger-like extensions called stereocilia protruding from their apical surface. The tallest stereocilia touch an extracellular gel; in the cochlea, this gel is the tectorial membrane. When vibration enters the cochlea, the tectorial membrane and the hair cells are moved about different pivot points, causing a shearing motion that bends the stereocilia, opening the transduction channels and leading to hair-cell depolarization and synaptic activity.

The tectorial membrane has been a rather neglected partner in the process, despite the finding—nearly two years ago—that mutations in TECTA, which encodes a component of its matrix, cause deafness in humans1. Until recently, there were no histological studies of how defects in the tectorial membrane effect genetic deafness. Two studies remedy this deficit. In December's issue of Nature Genetics, Richard Smith and colleagues reported that mice with a targeted disruption of Col11a2, encoding a component of collagen, have aberrant tectorial membranes. Their constituent fibrils are disorganized and more widely-spaced2 than normal—features associated with moderate hearing impairment. COL11A2 mutations are also found in forms of syndromic and non-syndromic deafness (DFNA13). Now, on page 139, Marie-Christine Simmler and colleagues3 describe ultrastructural defects of the tectorial membrane in mice with a disrupted otogelin (Otog) gene, again associated with hearing impairment. In these mutants, the extracellular membranes of the ‘balance’ organs of the inner ear (the otolithic membranes and the cupulae) are even more severely disorganized and detach from the hair cells (see figure), leading to balance defects. The tectorial membrane is notoriously susceptible to histological artefact, making it difficult to study, but these reports, together with the earlier report of TECTA mutations in human deafness1, highlight its importance in auditory function and its potential involvement in human genetic deafness.

Lost without otogelin. The otolithic membrane (arrow) has lost contact with its sensory hair cells (arrowhead) in the inner ear of a mouse deficient of otogelin (upper panel).