Fig. 3. Whole-mount in situ hybridization of Tg46 rescued mutant embryos reveals abnormalities in R-C somite patterning. Embryos, derived from Tbx6^tm1Pa/+ Tg46/+ x Tbx6^tm1Pa/+ crosses, were dissected at E10.5 and hybridized with Uncx4.1 (A-D), Mesp2 (E), Cer1 (Cerr-1 in figure; F), neurofilament l (G-H), Tbx18 (I-L), Dll3 (M) and Dll1 (H-M) antisense riboprobes. The genotypes of the embryos are as indicated. The designation of `normal' is given to embryos that are wild type or Tbx6^tm1Pa/+ with or without the transgene, as no differences were observed between these genotypes when hybridized with the specified probes. (A) Uncx4.1 is normally expressed in the caudal region of each somite along the AP axis. (B-D) By comparison, Uncx4.1 transcripts are found throughout the somites of the Tbx6^tm1Pa/Tbx6^tm1Pa Tg46/+ embryos. (C,D) In some cases, the posterior-most somites express stripes of Uncx4.1 transcripts; however, these stripes (red arrows in D) are found on a background of low uniform levels of Uncx4.1 expression. (E) Mesp2 is expressed in the rostral region of the forming somite in both normal and Tbx6^tm1Pa/Tbx6^tm1Pa Tg46/+ embryonic tails (dorsal view) at comparable levels. In the tail of the top Tbx6^tm1Pa/Tbx6^tm1Pa Tg46/+ embryo, very low levels of Mesp2 transcripts are also detected in the last somite to form (red arrow). (F) Cer1 is expressed as two stripes in the anterior PSM and in the rostral region of the newly formed somite (red arrow) in normal embryos. Cer1 transcripts are downregulated in the anterior PSM and absent in the newly formed somite of Tbx6^tm1Pa/Tbx6^tm1Pa Tg46/+ embryos. (G) Neurofilament L (NF in figure) is expressed in the spinal ganglia and neural tube of normal embryos. Expression in the spinal ganglia appears as evenly distributed blocks along the axis, reflecting their migration route through the rostral halves of the somites. (H) By contrast, Neurofilament L expression in the spinal ganglia of the Tbx6^tm1Pa/Tbx6^tm1Pa Tg46/+ embryo is irregularly distributed along the axis, often appearing as fused blocks. (I-L) Lateral views of normal and Tbx6^tm1Pa/Tbx6^tm1Pa Tg46/+ embryos showing Tbx18 expression. The heads of the embryos in have been removed for genotyping. Tbx18 is normally expressed in the rostral halves of somites along the AP axis, as well as in the heart and limb buds (I,K). In the Tbx6^tm1Pa/Tbx6^tm1Pa Tg46/+ embryos, Tbx18 is either not expressed in the paraxial mesoderm (J) or is expressed in the newly formed somites, eventually being lost in more anterior regions of the embryo (L). (M) Dorsal view of Dll3 expression in the tailbuds of normal, Tbx6^tm1Pa/Tbx6^tm1Pa and Tbx6^tm1Pa/Tbx6^tm1Pa Tg46/+ embryos. (N,Q) In normal embryos Dll1 is expressed in the tailbud and PSM, and is upregulated in the next-to-form somite. Low levels of Dll1 are later detected in the caudal compartment of the formed somites (Q). (O,P,R,S) The level of Dll1 transcripts in the tails of Tbx6^tm1Pa/Tbx6^tm1Pa Tg46/+ embryos is reduced compared with Tbx6^tm1Pa/+ embryos. Weak upregulation of Dll1 expression can be seen in the anterior region of the PSM prior to somite segmentation. The expanded tail region of the Tbx6^tm1Pa/Tbx6^tm1Pa Tg46/+ embryos can be seen in R and S (broken blue outline in R), where Dll1 transcripts appear dorsally localized. Low levels of Dll1 transcripts are later localized to the caudal compartment of the formed somites similar to the Tbx6^tm1Pa/+ embryos. (P) Dll1 expression in the flank of the Tg46 rescued embryo marks ectopic neural tissue (red arrowhead).