Blood flow does not extend into the undifferentiated nephron progenitors. A and B: representative images of E15.5 kidneys perfused with FITC-TL and colabeled for markers of nephron progenitors (Six2 and Pax2, red) and the vasculature (PECAM, blue). A–A″′: Six2 (red) is shown to mark nephron progenitors, which reside in the nephrogenic zone (marked by the dotted line). Perfused vessels (yellow arrowheads) stained with TL (green) and PECAM (blue) can be seen all the way to the nephrogenic zone, and unperfused vessels (white arrowheads) are then seen throughout the nephrogenic zone. B–B″′: a similar expression pattern is seen with Pax2 (red), although staining for Pax2 was also observed in the ureteric epithelium and developing glomeruli (G). Whereby perfused vessels (yellow arrowheads) marked by TL (green) abut the nephrogenic zone, however, the blood flow does not penetrate, leaving the vessels marked with PECAM (blue) within the nephrogenic zone unperfused (white arrowheads).