Fig. 3. Tailbud-derived mesenchyme associates with the developing conduit system. (A) Whole-mount image of HH st14 embryo immediately after DiI injection into the tailbud (arrowhead). (B) The same embryo after 48 hours of incubation, including a schematic of the urinary tract. Cells deriving from the tailbud (t) are localized to the sacral aspect of the embryo and the tail. Few, if any, lineage-tagged cells were observed rostral to the hindlimb (hl). (C) Details of the schematic. At this stage, the mesonephros (ms) has formed and the nephric duct (nd) is integrated into the cloaca. The UB has not yet branched and its proximal tip is surrounded by nephron progenitors (np). Locations of transverse sections in D-F are noted. (D,E) Tailbud-derived mesenchyme associates with the distal UB as revealed by staining for Pax2 (green). The proximal UB tip (pub in D) can be identified by Pax2 expression (green), its epithelial structure and its association with Pax2-expressing nephron progenitors. Few DiItagged cells are present around the proximal UB tip. The nephric duct, which also expresses Pax2, is also visible (nd). The distal, tubular portion of the UB (dub in E) is associated with large populations of DiI-tagged cells (red). Few tagged cells deriving from the tailbud are observed near the proximal-most domain of the UB (pub). (F) The cloaca is also surrounded by large populations of DiI-tagged cells deriving from the tailbud. (G-I) The heritable lineage marker, lacZ, was transferred into the tailbud at HH st14-15 and embryos were incubated for an additional 10 days. In whole-mount preparations (G), ß-gal-expressing cells (blue in G) derived from the tailbud are present in sacral tissues, which include the cloaca (c) and the distal UB (dUB). (H) High-power examination of the cloaca indicates that lineage-tagged cells are spindle shaped and dispersed, characteristic of connective tissue. (I) Paraffin section of the ureter shows ß-gal-tagged cells derived from the tailbud in the ureteral connective tissue coat (cn). Cells deriving from the tailbud are not detected within the ureter epithelium (ep, I) or metanephros (data not shown), which are derived from rostral tissues.