Expression analysis of Ptdsr during embryonic development. (a) Schematic representation of the construction of the Ptdsr gene-trap mouse line used for expression analysis at different embryonic stages. Gray and bright blue boxes represent regulatory elements of the gene-trap, and β-geo, the β-galactosidase/neomycin phosphotransferase fusion protein-expression cassette [48,51]. Restriction enzyme nomenclature is as in Figure 1 (b) Whole-mount β-galactosidase staining of heterozygous Ptdsr gene-trap embryos at mid-gestation. Expression of Ptdsr is highest in neural tissues and somites, in the branchial arches, the developing limbs, the heart, the primitive gut and the developing eye. (c-e) Sectioning of E12.5 β-galactosidase-stained embryos confirms expression of Ptdsr in (c) the neural tube; (inset in c) neural epithelium; (d) somites; and (e) eyes. Expression in the eye is restricted to developing neural retinal and lens cells. (f) Expression analysis of adult tissues by northern blot. Expression of Ptdsr in the muscle (asterisk) was detected only on long-term exposures of the filter (> 48 h). A β-actin hybridization was used to confirm equal loading of RNA samples. Scale bar, 100 μm.