Fig. 5. Segregation of mutant Pax3^-/-from wild-type cells in the neural tube and dermomyotome. Whole-mount X-gal staining of several chimeric embryos of different stages and transverse sections of chimeric embryos at the limb and trunk level. The embryos shown in A-C are derived from ES^+/- cells and exhibit no segregation of wild-type and mutant cells in the dermomyotome; corresponding embryos from ES^-/- cells, however, exhibit segregation in the dermomyotome after E10 (G,H,K,L,O) and in the neural tube in J,M-O. The few white cells (+/+) in the neural tube are indicated by double white arrows on a transverse section of E10.25 embryo (O). The chimeras shown in D,E,I,J were generated according to the scheme in Fig. 2 using Sp^2H and Sp^2G mice. From this aggregation, segregation of wild-type and mutant -/- cells is shown at E12.5 (J). Black arrows in D,E,I,J indicate the DRG. In I, the DRG are affected and smaller in size. In the older embryo (J), wild-type cells rescue them, shown in transverse section of a highly chimeric embryo of E12.5 (chimerism in this embryo was determined by eye pigmentation, data not shown). DM, dermomyotome; DRG, dorsal root ganglia; +/-, heterozygous; -/-, homozygous. Scale bars: 400 µm in A-C,F-H,L,N; 200 µm in D,E,I,J; 300 µm in K; 600 µm in M; 100 µm in O.