Fig. 5. All-trans-RA, but not all-trans-retinal, phenocopies Cyp1B1 overexpression. Beads soaked in either RAL or RA were implanted into the neural tubes (bead-implantation site marked by a black or white arrow) of HH11 chick embryos and allowed to develop for a further 16-20 hours before being harvested. Beads soaked in retinoid carrier solution were used as negative controls and no changes in target gene expression were recorded (data not shown). (Aa-Ad) Lateral view of embryos. (Aa) Expression of Shh with RAL bead implanted in hindbrain showing no effect on expression levels. (Ab,Ac) RAL bead has no effect on Nkx6.1 and Isl1 expression. (Ad) Shh expression is completely lost (blue arrows) in response to a single RA-soaked bead implanted into spinal cord. These data demonstrate an antagonist role for RA but not RAL in the regulation of Shh expression. (Ba-Bd) Following electroporation, a Raldh2-containing construct downregulates Shh expression (Bc,Bd) when compared with the control electroporated embryos (Ba,Bb). Arrows in Bb mark the normal domain of Shh, and those in Bc and Bd mark a reduction in expression levels. (Ca-Cd) Transverse sections of Shh expression in the hindbrain of a normal (Ca), VAD (Cb,Cc) or Cyp1B1-electroporated (Cd) embryo. Arrows in Cb and Cc mark expansion domains and the arrows in Cd mark reduction domains.