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SUMMARY The cellular mechanisms controlling digit numbers and identities have remained elusive. Profiling of wild-type (pentadactyl) and Grem1 tetradactyl mouse limb buds identifies cellular changes affecting two limb bud mesenchymal progenitor (LMP) populations. In mutant limb buds, the anteriorly biased distribution of peripheral LMPs (pLMPs) is lost and the population expanded, while the distal-posterior LMP (dLMP) population is reduced from early stages onward. Analysis of LMP signature genes in wildtype and mutant mouse limb buds with digit loss or gain establishes that pLMPs are positively regulated by BMP signaling, while dLMPs require GREM1-mediated BMP antagonism. dLMPs encompass digit progenitors and altering their population size prefigures changes in digit numbers. The anteriorly biased pLMP distribution is linked to digit asymmetry as loss of this bias in tetradactyl mouse and pig limb buds underlies middle digit symmetry and identity loss. This study indicates that variable spatial Grem1 expression in mutant and evolutionary diversified limb buds tunes BMP activity, impacting both LMP populations in a complementary manner.
Palacio et al. (Mon,) studied this question.