Practice question

Sea urchin embryos exhibit remarkable regulative capacity after early blastomere removal because conditional specification dominates early patterning. Remaining cells sense altered neighbor relationships and remodel signaling landscapes, particularly nuclear beta-catenin localization, Delta-Notch activation and Nodal expression, to respecify toward missing micromere-derived lineages including skeletogenic mesenchyme. Intercellular communication and community effects restore correct proportion of endomesoderm and ectoderm, yielding normal albeit smaller pluteus larvae. If development relied solely on segregated determinants, compensation would be impossible. Regulative signaling ensures robustness against cell loss and experimental perturbation.