Fig. 1. X-Gal staining and Islet1 in situ hybridisation or immunofluorescence in Mlc1v-nlacZ-24 and Mlc3f-nlacZ-2E mouse embryos between the 4- and 7-somite stages. (A-F) X-Gal staining for β-gal (blue) and whole-mount in situ hybridisation for Islet1 (Isl1) transcripts (purple) on Mlc1v-nlacZ-24 (A,C,E) and Mlc3f-nlacZ-2E (B,D,F) embryos. β-gal activity in the Mlc1v-nlacZ-24 line marks the AHF and its myocardial derivatives, whereas in the Mlc3f-nlacZ-2E line it marks differentiated myocardial cells. Islet1 is transcribed in splanchnic mesoderm. Labelling by the Islet1 probe, rostral to the heart-forming region, under the head folds, marks neurectoderm of the future central nervous system. Islet1 is also expressed in endoderm. Embryos at the 4-(A,B), 6-(C,D) and 7-(E,F) somite stages. Lines in A,C,E indicate plane of sections in A' and A'', C' and C'', E'. (A'') Merge of Hoechst and Islet1 immunofluorescence. (C',C'') Merge of Hoechst, β-gal and Islet1 immunofluorescence. The arrow in C' points to a double-positive cell co-expressing Mlc1v-nlacZ-24/Fgf10 and Islet1. PC, pharyngeal cavity. The arrows in C'' indicate the Islet1-positive mesoderm near the intraembryonic coelomic cavity. The arrows in E indicate the sinus venosus (sv) and the underlying endoderm (En). The arrowhead in E' indicates the heart tube that is negative for β-gal, and the arrow points to the endoderm adjacent to the foregut, which is also positive for Islet1 transcripts. CC, cardiac crescent; HT, heart tube; NT, neural tube. Black rectangles in A,B,C,D indicate the regions used for explants. L, left side of embryo; R, right side of embryo. Beneath is shown a schematic summary of expression of Islet1 and the Mlc1v-nlacZ-24 (Fgf10) transgene in the second heart field (SHF) and anterior heart field (AHF), respectively, and of myosin in cardiomyocytes of the cardiac crescent (CC) and heart tube (HT); p, posterior.