Immunofluorescence (A-D, G-J) and Immunohistochemistry (E, F) of transverse sections at E18.5. Cre-expressing lineages are marked by ZsGreen (ZsG; Green) and nuclei are counter-stained with Hoechst (Blue). The Tbx18:Cre allele gives rise to strong and widespread ZsG labeling in the anterior UGS mesenchyme (A) in contrast to Tbx18:BAC-iCre which labels cells only in regions dorsal to the UR (B). White arrowheads mark anterior buds (A, B). Surface urethral epithelial cells are labeled by the Tbx18:Cre (C) but not by Tbx18:BAC-iCre allele (D). Tbx18:Cre lineage labeling is intense in a ring of posteriorly located muscle cells that becomes the rhabdosphincter (E) but Tbx18:BAC-iCre does not label these cells (F), which stain positive for the Myosin heavy chain antibody (MHC; in red). The loose mesenchyme surrounding the UGS is labeled with Tbx18:Cre (G) but not Tbx18:BAC-iCre (I); see Fig. 6A, 6B. Mesothelium, marked by white arrowheads (H, J) is marked by both Cre alleles. Abbreviations DD: Ductus deferens; PG: Pelvic ganglia; SV: Seminal vesicles; UR: urethra. Scale bars are 100μm.