ARCHENTERON PRECURSOR CELLS CAN ORGANIZE SECONDARY AXIAL STRUCTURES IN THE SEA-URCHIN EMBRYO

Local cell-cell signals play a crucial role in establishing major tiss ue territories in early embryos. The sea urchin embryo is a useful mod el system for studying these interactions in deuterostomes. Previous s tudies showed that ectopically implanted micromeres from the 16-cell e mbryo can induce ectopic guts and additional skeletal elements in sea urchin embryos, Using a chimeric embryo approach, we show that implant ed archenteron precursors differentiate autonomously to produce a corr ectly proportioned and patterned gut, In addition, the ectopically imp lanted presumptive archenteron tissue induces ectopic skeletal pattern ing sites within the ectoderm. The ectopic skeletal elements are bilat erally symmetric, and flank the ectopic archenteron, in some cases res ulting in mirror-image, symmetric skeletal elements. Since the induced patterned ectoderm and supernumerary skeletal elements are derived fr om the host, the ectopic presumptive archenteron tissue can act to 'or ganize' ectopic axial structures in the sea urchin embryo.