Practice question

Conditional induction requires signaling molecules operating over short range or requiring direct membrane contact to restrict fate changes. Sea urchin micromeres emit Wnt8 as paracrine diffusible factor activating β-catenin in adjacent vegetal blastomeres and simultaneously present Delta ligand anchored on surface activating Notch receptor on Veg2 neighbors. This juxtacrine Delta-Notch interaction combined with Wnt and other secreted factors together reprogram neighboring cells toward endomesoderm fate. Pure autocrine signaling would limit effect to same lineage, while hormones and gap junction proteins alone do not carry fate-specifying information. Thus micromeres produce both juxtacrine and paracrine signals for robust induction.