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Fig. 1. Temporal (A), spatial (B), and evolutionary (C) negative correlations between branching morphogenesis and alveolar differentiation. (A) Time-course
microarray expression profiling of FACS-purified distal lung epithelial cells. Gene expression values (log2) in E14 samples are used as a baseline for comparison.
Horizontal histograms for E15�E19 samples are the frequency distributions of average fold-changes for all genes; the units are shown as the number of genes
(# genes). Alveolar markers are up-regulated (red), and branching (green) and cell cycle (blue) related genes are down-regulated over time. Down-regulated
genes appear to have a smaller fold-change than up-regulated genes, possibly because of perdurance of transcripts. (B) Confocal images of immunostained
E18 lung sections in areas where the airway lumen can be continuously traced from the proximal conducting airways (black/white long dashed lines) to the
distal nonbranch tip (magenta) and branch tip (green) regions, as illustrated in the schematics. Branch tips are outlined with dashed lines. The boxed regions
are enlarged as insets outlined in corresponding colors, showing a spatial negative correlation between branching and alveolar differentiation. AQP1 labels
alveolar type I cells, the vasculature and the mesothelium, which are separated by dashed lines (Insets). There is a very low level of SOX9 expression in
nonbranch tip epithelium, possibly because of protein perdurance. (Scale bars, 20 μm.) (C) Whole-mount in situ hybridization and immunostaining of Xenopus
embryos of indicated stages (St) showing the absence of Sox9 expression and branching in the Xenopus lung. The lungs are indicated with dashed lines if
stained or arrowheads if unstained. Open arrowheads indicate Sox9 expression in the pharyngeal arches. The two Xenopus Sox9 homologs (Sox9a and Sox9b)
are 95% identical on the nucleotide level. OPT images of immunostained embryos (Left, Bottom) are shown as maximal intensity projection (Left) and
sectional view (Right). (Scale bar, 200 μm.) |
