Fig. 10. Apoptosis in Alx3/Alx4^lst-J double mutant embryos. In a restricted region of the frontonasal process of Alx3/Alx4^lst-J double mutant embryos, a significant number of mesenchymal cells undergo apoptosis compared with stage-matched control embryos. (A,D,F,H,K) control embryos (wild-type or Alx3^+/-); (B,E,G,I,L) embryos of double mutant embryos; (C,J,M) schemes depicting plane of sections of embryos shown to their left. TUNEL assays were carried out to detect apoptosis. Positive cells appear red in this assay, as seen in brightfield images (A,B,F,G). Because of difficulty with visualising TUNEL staining simultaneously with histology, we show also darkfield images where positive cells appear as white to orange spots. The yellow squares in D,E mark the crucial area where we found differences in apoptosis in E10.0 embryos. These areas are shown enlarged, and in brightfield, in F,G. No or very few apoptotic cells were detected in the vicinity of the nasal processes of embryos of E10.5 and E11.5. The yellow arrows in panels H,I indicate apoptopic areas in the maxillary component of the first branchial arch, confirming that the negative result in this experiment was not due to technical failure.