FGF20 initiates lateral compartment differentiation before E14.5.

<p>(A–H) Immunostaining of Myo7a in <i>Fgf20^βGal/+</i> and <i>Fgf20^βGal/βGal</i> cochlear explants treated with or without FGF9 and cultured for 5 d (schematic). Treatment of <i>Fgf20^βGal/+</i> explants with FGF9, either at E14.5 (B) or E15.5 (F), did not have any effect on hair cell number compared to untreated explants (A,E). Treatment of <i>Fgf20^βGal/βGal</i> explants with FGF9 at E14.5 resulted in increased numbers of hair cells and decreased gaps (arrows) between hair cell clusters (D) compared to untreated explants (C). Treatment of <i>Fgf20^βGal/+</i> or <i>Fgf20^βGal/βGal</i> explants with FGF9 at E15.5 did not affect hair cell number or the formation of gaps (arrows) lacking sensory cells (G, H). (I) Quantitation of the number of hair cells. The number of OHCs and total hair cells were rescued by treatment with FGF9 at E14.5 but not at E15.5. * <i>p</i><0.05. (J–M) Immunostaining for Prox1 in <i>Fgf20^βGal/+</i> and <i>Fgf20^βGal/βGal</i> explants treated with or without FGF9 at E14.5 and cultured for 5 d (schematic). Treatment of <i>Fgf20^βGal/+</i> explants with FGF9 did not affect supporting cell number (K) compared to untreated explants (J). Treatment of <i>Fgf20^βGal/βGal</i> explants with FGF9 at E14.5 resulted in increased numbers of supporting cells and decreased gaps between sensory cell clusters (M) compared to untreated explants (L). (N) Quantitation of numbers of supporting cells in explants. The number of supporting cells was partially rescued in <i>Fgf20^βGal/βGal</i> explants by treatment with FGF9 at E14.5.