Practice question

Large micromeres inherit concentrated maternal determinants such as nuclear β-catenin, Disheveled protein, and localized mRNAs that activate Pmar1 transcriptional repressor. Even when isolated and cultured in calcium-free seawater, large micromeres autonomously activate skeletogenic program and produce birefringent spicules without neighboring signals, demonstrating autonomous specification. Mesomeres, Veg1 and Veg2 remain conditional, requiring Wnt and Delta paracrine signals for endomesoderm induction and archenteron formation. This autonomy underlies organizer activity of micromeres, which can induce secondary axis when transplanted to animal pole. Hence large micromere lineage exemplifies inherited determinant-driven specification in otherwise regulative embryo.