Dll1 whole-mount RNA in situ hybridisations and somite formation. (A) Dll1 whole mount in situ hybridisations of E7.5 embryos from crosses of transgenic and wild-type mice. 24 embryos were analysed for Dll1 expression patterns. We did not observe differences in Dll1 expression patterns in these embryos. From left to right: view from the cephalic side, lateral view with the cephalic region to the left and the primitive streak side to the right, view from the side of the primitive streak, view onto the node with the primitive streak on top of the node. Each picture was taken from a different embryo. Ectopic Dll1 expression is detected at (B) E9.5 and (C) E10.5 throughout somites in transgenic embryos. Wild-type expression is restricted to caudal somite halves (asterisks). (D, E) Cryo-sections of in situ hybridised E10.5 wild-type embryos reveal expression of Dll1 in the caudal, inner epithelium. A weak staining may be present in cells of the caudal somitocoele. The red broken line in the left panel of (E) indicates the boundary between the somitic epithelium and the inner somitocoele. Transgenic embryos express Dll1 in epithelial cells without caudal restriction. Again, weaker staining is detected in the somitocoele. (F) H/E stained sections of E10.5 embryos reveal normal epithelial somites in wild-type and transgenic embryos. ac, anterior compartment of somite; pc, posterior compartment of somite; sc, somitocoele; se, somitic epithelium; TG, transgenic; WT, wild-type.