Figure 1. Lineage labelling of the intermediate mesoderm.

(A) Panels show dorsal views of flat mounted embryos double stained for pax2a, pax8 and hnf1ba (purple) and myod1 (red) transcripts at the 0-2-somite and 5-6-somite stages. The caudalmost limit of the hnf1ba expression domain in the intermediate mesoderm is indicated with a black arrow. (B) Panels show the results of uncaging a lineage tracer in cells posterior to the end of the notochord (first panel) and the descendants of the labelled cells (purple) compared to the unlabelled cdh17-expressing kidney tubules (red; second and third panels, arrow in third panel labels cloaca cells). The fourth panel shows a cross-section, as indicated by the grey dotted line, of the embryo in panel two (red staining has been lost, but red dotted lines highlight the pronephric tubules). (C) Left panel shows lineage labelling of the ‘horseshoe’ domain at the 5-somite stage (fluorescence observed in the more rostral midline is background). Middle panel shows their contribution to non-renal cell types at 24 hpf by whole mount in situ hybridization. Labelled ventral fin cells are highlighted with black arrows and a population of mesenchyme lateral to the posterior pronephros is indicated with a white arrow, which is also highlighted in the cross-section view in the right panel. (D) Panels show lineage labelling of cloacal precursors (white arrow) located in a small cluster of cells near the end of the intermediate mesoderm. Images in (B,C,D) are shown in lateral view.