Pronephros progenitors are delineated into a series of molecularly distinct regions during early somitogenesis that are retinoic acid (RA) dependent. A–C: Gene expression patterns in the nephron territory in wild-type embryos and lib mutants at the 8 somite stage (A) 15 somite stage (B), and schematized respectively (C,D). Embryos were flat-mounted to remove the yolk and are shown in dorsal views with anterior to the left. Whole-mount in situ hybridization was used to mark kidney gene expression (purple) and the somites with myoD (red). Black lines indicate areas of kidney gene expression and numbers correspond to the somite position. A: In 8 somite wild-types, pax2a transcripts marked all nephron progenitors while dlc and jag2 expression was restricted proximally and mecom expression was restricted distally; a short stretch of overlap between the jag2 and mecom domains was evident. lib embryos had reduced dlc and jag2 domains, and an expanded mecom domain. B: In 15 somite wild-type embryos, hnf1ba marked all nephron progenitors, and the other transcription factors were restricted to subsets of cells in the rostral (hnf4a, hnf1bb), central (irx3b), or caudal (pou3f3b, mecom, emx1, gata3) areas of the nephron territory. lib had a shortened rostral hnf4a area while the central and caudal were expanded. C,D: Schematic depictions of nephron progenitor domains: (top) labeled boxes indicate somite number, and (below) summary of rostral (purple), central (blue) and caudal (green) areas of the nephron progenitors. At 8 somites, the overlap between rostral and caudal areas is depicted as dark purple-colored subset of the rostral area. At 15 somites, subsets of the caudal area are depicted as dark green and light green; gray indicates divergence of nontubule populations (future podocytes and neck segment cells) at the most anterior regions of the renal progenitors. Asterisk marks the posterior limit of the rostral area, which shifted anteriorly in lib at the 8 and 15 somite stages.
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