Fig. 5. Novel RA-generating activity in neural tissues. Shown are vibratome sections of E10.5 Raldh2^–/– and wild-type embryos treated with RA to E8.25 showing localization of RARE-lacZ expression. (A,B) Transverse sections of the posterior spinal cord; the lower level of ß-galactosidase activity observed in the intermediate region (vi) of the wild-type relative to the corresponding region of the mutant may be due to sequestration of the substrate (X-gal) by the high level of ß-galactosidase present dorsally and in the somites, thus reducing the amount of substrate able to diffuse into the intermediate region during staining of whole-mount embryos; alternatively, the mutant may have higher expression of the novel enzyme. (C-H) Frontal sections of forebrain (C,D); frontal sections of midbrain (E,F); and oblique transverse sections through the hindbrain just rostral to the occipital somites (G,H). drg, dorsal root ganglia; f, forebrain; f/m, forebrain/midbrain junction; fp, floorplate; m, midbrain; r, rhombomere; s, somite; vi, ventral interneurons.