Fig. 4. Development of the cochlear duct and organ of Corti. Each drawing illustrates a cross-section through the developing mammalian cochlear duct. (A) At E12.5, the duct consists of undifferentiated epithelial cells. A subset of cells located within the duct expresses Fgfr1 (pink). Expression of Fgfr1 appears to correlate with the region of the duct will develop into the greater epithelial ridge (GER, green outline) and organ of Corti (OC, red outline). (B) By E16.5, individual hair cells (yellow) begin to differentiate within the OC. Fgfr1 expression continues in the GER and OC (red outline). In addition, a subset of cells within the OC begins to express Fgfr3 (blue) and Fgfr1 (overlap of Fgfr1 and Fgfr3 expression appears as purple). (C) By E18.5, the basic cellular pattern of the OC is complete. A single inner hair cell (IC, yellow) and three outer hair cells (1-3, yellow) are present at this stage. In addition, inner and outer hair cells are separated by the developing pillar cells (labeled as OP and IP) that will give rise to the tunnel of Corti. By this time, expression of Fgfr1 (pink) has become restricted to the outer hair cells and Deiter's cells (another type of non-sensory supporting hair cell), while Fgfr3 expression (blue) is restricted to the pillar cells.