Fig. 1. Positive and negative feedback loops of Dll1 and Mesp2 are essential for stripe formation. (A-F) Dll1 induces expression of Dll1 itself. Expression of Dll1-lacZ mRNA was detected by in situ hybridization in Dll1^+/L (A-C) and Dll1^L/L (D-F) embryos at 9.5 dpc. (A,D) Lateral view, (B,E) dorsal view of the tail region. (C,F) Transverse section at the anteriormost PSM. In the Dll1^+/L embryo, lacZ expression reflects normal stripe pattern of Dll1, localized at the caudal half of somites (arrowheads in B). In the Dll1^L/L embryo, the stripe of Dll1-lacZ is lost at the putative somite region (anterior to the arrow in D). Ectopic strong staining in the ventral neural tube is evident (F). (G-J) Expression of Mesp2 is severely decreased in the Dll1-null embryo (G,H) while expression of Dll1 is strongly expanded in the Mesp2-null embryo (I,J). (K-Q) Mesp2-lacZ mRNA (with Dll1-lacZ in case of the double mutant) was detected by in situ hybridization. (K-M) Dorsal views and (N-Q) lateral views. After extended staining, Dll1-lacZ expression appears at the neural tube and the PSM, but not at the somite region (Q, arrow indicates the putative boundary between PSM and somite region). (R) Summary of reciprocal regulation of Dll1 and Mesp2. In the absence of Dll1, both Dll1 stripes and normal level of Mesp2 expression are lost. In the absence of Mesp2, both Dll1 and Mesp2-lacZ expressions are strongly expanded. The Dll1/Mesp2 double-null embryo is similar to the Dll1-null embryo in terms of reciprocal regulation.