Fig. 6. Ectopic Mesp2 expression in somites leads to the activation of Epha4. Expression of Mesp2 in 10.5 dpc wild-type (A) and CAG-CAT-Mesp2; Mox1-Cre double heterozygous embryos (B-D). In addition to the normal Mesp2 expression in the anterior PSM, ectopic expression of Mesp2 was observed throughout the entire somitic region (B-D). The expression pattern of Epha4 at 11.5 dpc in wild-type (E,G) and double heterozygous embryos (F,H) is also shown. An expression pattern for Epha4 that was similar to Mesp2 was observed in the double heterozygote (F,H). Histological analyses of 10.5 dpc wild-type (M) and double heterozygous (I-L,N) embryos. Ectopic Mesp2 protein (I,K) and Epha4 expression (J,L) were evident in serial sections of double heterozygotes. Magnified images of square parts of I and J are shown in K and L, respectively. Another consecutive section was stained with phalloidin (N) and a similar region of the wild-type embryo is shown in M. In double heterozygotes, abnormal epithelial cells were observed within the somite (N). A paraffin section stained with nuclear Fast Red revealed gaps in epithelialized somites in the double heterozygote at 10.5 dpc (O). Black arrows in I and J indicate the endogenous expression of Mesp2 (I) or Epha4 (J). Red arrows in K,L,N indicate separated cell clusters. White arrow in N indicates an abnormal epithelialized feature. Black arrows in O indicate local gaps. Scale bar: 100 µm.