Hair cells without supporting cells: further studies in the ear of the zebrafish mind bomb mutant.

Each sensory hair cell in the ear is normally surrounded by supporting cells, which separate it from the next hair cell. In the mind bomb mutant, as a result of a failure of lateral inhibition, cells that would normally become supporting cells differentiate as hair cells instead, creating sensory patches that consist of hair cells only. This provides a unique opportunity to pinpoint the functions for which supporting cells are required in normal hair cell development. We find that hair cells in the mutant develop an essentially normal cytoskeleton, with a correctly structured hair bundle and well-defined planar polarity, and form apical junctional complexes with one another in standard epithelial fashion. They fail, however, to form a basal lamina or to adhere properly to the adjacent non-sensory epithelial cells, which overgrow them. The hair cells are eventually expelled from the ear epithelium into the underlying mesenchyme, losing their hair bundles in the process. It is not clear whether they undergo apoptosis: many cells staining strongly with the TUNEL procedure are seen but do not appear apoptotic by other criteria. Supporting cells, therefore, are needed to hold hair cells in the otic epithelium and, perhaps, to keep them alive, but are not needed for the construction of normal hair bundles or to give the hair bundles a predictable polarity. Moreover, supporting cells are not absolutely required as a source of materials for otoliths, which, though small and deformed, still develop in their absence.