Practice question

HesC is hairy and Enhancer-of-split family basic helix-loop-helix transcriptional repressor expressed broadly throughout early sea urchin embryo except in large micromeres where Pmar1 actively suppresses it. Its primary developmental role is keeping skeletogenic and signaling genes silent outside micromere lineage. Chromatin immunoprecipitation studies show HesC binds directly to cis-regulatory elements of Alx1, Tbr, Ets1 activators and Delta ligand gene, preventing their transcription in non-micromere cells. Removal of HesC by morpholino causes ectopic spiculogenesis throughout embryo. Thus direct molecular targets repressed by HesC include core skeletogenic transcription factors and Delta, restricting primary mesenchyme specification exclusively to vegetal pole.