Fig. 1. Fgfr3 expression in transverse sections of embryonic chick and mouse cervical spinal cords. (A) Chick stage 22-24 (E3.5-4); (B) chick stage 34 (E8); (C) chick stage 35 (E9); (D) chick stage 37 (E11); (E) chick stage 35 (E9); (F) mouse E13.5; and (G) mouse E14.5. Initially, Fgfr3 is expressed in the floor plate and the ventral two-thirds of the VZ (A) and is later downregulated in part of the ventral VZ (B). Starting around stage 34 (E8) Fgfr3⁺ cells are visible in the parenchyma of the cord. By stage 37 (E11) the floor plate and VZ no longer express Fgfr3 but scattered Fgfr3⁺ cells are present throughout most of the cross-section of the cord, including both grey and white matter (D). (E) A magnified image of the ventral VZ from a stage 35 (E9) cord, showing the two spatially separated domains of Fgfr3 expression. A similar progression occurs in mouse (F,G). However, the ventral `gap' is not so pronounced in mouse (arrow in G). Scale bars: 200 µm (A-D), 100 µm (F,G), 50 µm (E).

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