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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.
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