Normal Somitogenesis Despite the Absence of Snai1 in the Presomitic Mesoderm

Conditional loss of Snai1 throughout the posterior mesoderm, including the presomitic mesoderm, yields normal somite formation and segmentation clock activity despite complete absence of Snai1 expression.

• Why it matters: Snai1 oscillates with segmentation clock genes in the PSM, but its functional requirement has been unclear due to early lethality from global or epiblast-specific loss.
• What they did: The study mapped a conserved downstream Snai1 enhancer (SCDE) and found B1 and B3 modules partially redundant, accounting for most posterior mesoderm activity; deletion reduced PSM Snai1 expression while preserving dynamic expression and Lfng oscillations.
• The result: Embryos lacking Snai1 in the posterior mesoderm show normal segmentation, but complete loss causes a fully penetrant gain of one pre-sacral vertebra, implying Snai1 fine-tunes axial segment number rather than segmentation.