Fig. 5. Effects of modulating gene expression on development of the organ of Corti (OC). (A) Ectopic expression of Math1 (Atoh1) in individual cells within the greater epithelial ridge (GER) (green outline) results in the formation of hair cells (filled in green) in the GER. (B) Hypomorphic Fgfr1 produces small sensory islands that consist of inner hair cells (IC, yellow), an increased number of pillar cells (pink) and no outer hair cells. (C) Deletion of Fgfr1 leads to a nearly complete disruption in the formation of the OC (red outline) with no identifiable cell types. (D) Deletion of Fgfr3 or inhibition of FGFR3 activity results in a disruption in the differentiation of pillar cells (outlined in pink). Compare the shape of the pillar cells in this drawing with the normal shape of the pillar cells illustrated in Fig. 4. (E) Increased activation of Fgfr3 leads to an overproduction of pillar cells but no significant affects on formation of either inner hair cells (IC) or outer hair cells (1-3, yellow).