Fig. 1. The temporal regulation of Mesp2 transcription by Notch signaling. (A-D) Representative Mesp2 transcription states revealed by high-resolution in situ hybridization with combined antisense probes corresponding to an intronic region and exons of mouse Mesp2. (A) No transcription, (B) primary transcription, (C) active transcription and cytoplasmic accumulation of transcripts, and (D) transcriptional termination. Magenta, Mesp2 transcripts; blue, DAPI staining. The subcellular localization of the Mesp2 transcripts revealed by these images is depicted schematically below each panel. (E-J) Double staining of Mesp2 transcripts (in situ hybridization) and Notch activity (anti-NICD antibody) during one cycle of somitogenesis. Mesp2 transcription was not detected in phase II (E,F, n=5), was initiated during phase III (G,H, n=6), and was further upregulated in phase I (I,J, n=7). Arrowheads in G-J indicate anterior limits of Mesp2 transcription. (K-N) Higher magnification of phase III (K,L) and phase I (M,N) images. Mesp2 transcripts were detectable in the posterior half of the active-Notch domain with a clear anterior boundary (dotted lines). The actual numbers of cells showing different subcellular localization of Mesp2 transcripts are shown on the right of the panels for phase III and I.