Practice question

Classical isolation experiments by Sven Hörstadius cultured separated micromeres from 16-cell sea urchin embryos in isolation. Even without ectodermal cues, large micromeres undergo epithelial-mesenchymal transition, migrate within culture droplet and secrete calcium carbonate forming characteristic triradiate spicules typical of larval skeleton. They autonomously express Alx1, Sm50, Msp130 and other biomineralization genes under Pmar1-HesC circuit control. This proves skeletogenic program intrinsic to micromeres, not requiring inductive interactions. Other lineages lose ability to form gut or ectoderm alone. Skeletal spicule formation thus serves definitive functional assay for micromere autonomy and specification.