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. Author manuscript; available in PMC: 2015 Sep 1.
Published in final edited form as: Wiley Interdiscip Rev Dev Biol. 2014 Jun 24;3(5):331–347. doi: 10.1002/wdev.141

Figure 6.

Figure 6

Branching versus alveolar differentiation in evolution.

(A) Top: OPT images of E11.5 control and epithelial Sox9 mutant mouse lungs and a stage 42 Xenopus embryo immunostained for SOX9. Bottom: Surfactant protein B (Sftpb) whole mount in situ hybridization of E10.5 control and epithelial Sox9 mutant mouse lungs and a stage 42 Xenopus embryo. When Sox9 is absence in the lung buds (dashed line) in the mouse mutant and Xenopus, Sftpb is expressed. Modified with permission from45. (B) The branching program may be an evolutionary addition that temporarily delays alveolar differentiation to allow an increase in lung complexity by expanding the progenitors and building the conducting airways. Genes promoting branching (for example Sox9) have a second function to suppress (blunt arrow) the ancestral program that starts alveolar differentiation immediately after lung specification (gray dashed line in Xenopus).