Fig. 7. Fgf17 treatment promotes PC differentiation and induces ectopic PCs. Cross-sections were generated using confocal reconstruction. (A) Cross-section through a control explant showing cell boundaries labeled with phalloidin (green), developing PCs (red) and pillar head (asterisk); OHCs are numbered. HeCs located adjacent to OHC4 are short and have weak p75^ntr expression. (B) An Fgf17-treated explant has a broader pillar head (asterisk), reduced OHC numbers, and three rows of PC-like cells in the HeC position (red asterisks). (C,C') High-magnification cross-section of an inner PC (red in C, and shown alone in C') from a control explant. The pillar head narrows as it approaches the lumenal surface. (D,D') Cross-section along the mediolateral plane illustrating the row of inner PCs in a control explant. (E,E') Cross-section along the mediolateral plane illustrating the row of HeCs in the same control explant. Note the shortness of the HeCs and lack of p75^ntr expression. (F) Cross-section through an inner PC from an Fgf17-treated explant. Note the increased width of the pillar head. (G,G') Cross-section of PCs from an Fgf17-treated explant, same view as in D. (H,H') Cross section of the HeC region from an Fgf17-treated explant. HeCs are taller, are strongly positive for p75^ntr and are morphologically similar to PCs (compare with D and G). (I-L) Examples of PC (I,J) and HeC (K,L) morphology in control (I) and Fgf17-treated (J-L) explants. All illustrate expression of p75^ntr. Note the increased thickness of the pillar head in J versus I, and the similarity between endogenous PCs (I,J) and ectopic PCs located in the HeC region (K,L). Asterisks indicate pillar head-like structures. Scale bars: 10 µm.