upregulation of marker genes for hair cells and by the
appearance of cytomorphological specializations, in particular,
filamentous actin-rich protrusions that display
strong immunoreactivity for the hair-bundle marker espin
(Figure 1g–k). Cells that displayed hair-bundle-like structures
emerged almost exclusively on top of or closely
surrounded by large cells that expressed markers for
inner-ear supporting cells [3]. In differentiating cell populations
that are generated from ES-cell-derived progenitors,
hair-cell-like cells are rarely found in association with
large supporting-cell-like cells [4]. The in vitro generation
of morphologically mature hair cells might, therefore,
require intimate contact with cells that substitute for
inner-ear supporting cells, as is the case in vivo, suggesting
that the generation of functionally mature hair cells
in vitro requires the co-generation of accessory cell types.