Fgf20 is necessary for formation of the lateral compartment of the organ of Corti.

Phalloidin staining of P0 and P7 cochleae showing hair cells. In Fgf20^βGal/+ cochlea, (A) there is one row of inner hair cells (IHC) and three rows of outer hair cells (OHC) throughout the cochlea. At P0, apical hair cells are not fully differentiated; by P7, apical differentiation is complete. In Fgf20^βGal/βGal cochlea (B), the cochlear base (base) has one row of inner hair cells and two rows of outer hair cells. The mid-cochlea (middle) forms patches of hair cells with two rows of IHCs and three rows of OHCs separated by gaps that lack hair cells. The apex (apex) contains hair cells that are much less differentiated and the distal apex (end of apex) contains no identifiable hair cells. At P7, the hair cells in the distal apex of Fgf20^βGal/βGal and Fgf20^βGal/+ cochlea are comparable. (C) Diagram showing the cochlear regions examined in (A, B); b, base; m, middle; a, apex, a′, end of apex. (D) Quantification of hair cell numbers at P4. Total numbers of IHCs and OHCs were counted from Fgf20^βGal/+ (n = 3) and Fgf20^βGal/βGal (n = 3) cochlea. The numbers of IHCs were comparable, while the numbers of OHCs were decreased by 70% in Fgf20^βGal/βGal compared to Fgf20^βGal/+ cochlea. Total numbers of hair cells were decreased by 50% in Fgf20^βGal/βGal cochlea. (E) Length of cochlea at P4. Fgf20^βGal/βGal cochlea length (n = 3) was decreased by 10% compared to Fgf20^βGal/+ (n = 3). (F, G) Quantification of supporting cell numbers at P0. (F) Number of supporting cells, identified by staining for Prox1 (Figure S3G,H), and hair cells, were counted from base, middle, and apex regions of the OC and normalized to 100 µm from Fgf20^βGal/+ (n = 6) and Fgf20^βGal/βGal (n = 8) mice. IHC numbers were slightly increased while OHC numbers were decreased by 57% in Fgf20^βGal/βGal compared to Fgf20^βGal/+ cochlea. Numbers of Deiters' cells (DC) and outer pillar cells (OPC) were decreased by 52%, whereas numbers of inner pillar cells (IPC) were only slightly decreased in Fgf20^βGal/βGal compared to Fgf20^βGal/+ cochlea. (G) Ratio of hair cells and supporting cells in P0 cochlea. The ratio of outer compartment cells (OHC, OPC, DC) to inner compartment cells (IHC, IPC) was decreased in Fgf20^βGal/βGal compared to Fgf20^βGal/+ cochlea. Small changes in the ratio of outer compartment supporting cells (DC+OPC) to OHCs and inner compartment supporting cells (IPC) to IHCs were observed. * p<0.01.