Practice question

Chordin gene transcription is strictly dependent on organizer induction requiring nuclear beta-catenin, Siamois/Twin and Nodal-related Smad2 signaling. Lithium chloride inhibits GSK3, stabilizing beta-catenin throughout embryo and converting entire marginal zone into organizer tissue expressing high levels of chordin, noggin and goosecoid. UV irradiation abolishes beta-catenin transport and eliminates chordin. Therefore lithium treated embryos exhibit dramatic enrichment of chordin mRNA radially, serving as molecular readout of ubiquitous dorsalization, opposite to ventralized UV embryos where chordin is absent. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.