Practice question

Dorsal ectoderm fate choice depends on BMP signaling levels. In presence of BMP2/4/7, ectoderm expresses epidermal keratin via Dlx3, AP2 and p63 activation downstream of phospho-Smad1. Organizer secreted Noggin, Chordin and Follistatin bind BMPs extracellularly preventing receptor activation, lowering Smad1 activity. This blocks epidermal differentiation program, allowing neural genes Sox2, Sox3 expression revealing default neural fate. Therefore organizer factors prevent epidermal lineage adoption, not mesodermal or endodermal. Mesodermal and endodermal fates require separate Nodal and VegT signals, not simply BMP inhibition, so epidermal prevented specifically during neural induction.