SPECIFICATION OF CELL FATE IN THE SEA-URCHIN EMBRYO - SUMMARY AND SOME PROPOSED MECHANISMS

Am early set of blastomere specifications occurs during cleavage in th e sea urchin embryo, the result of both conditional and autonomous pro cesses, as proposed in the model for this embryo set forth in 1989. Re cant experimental results have greatly illuminated the mechanisms of s pecification in some early embryonic territories, though others remain obscure. We review the progressive process of specification within gi ven lineage elements, and with reference to the early axial organizati on of the embryo. Evidence for the conditional specification of the ve g(2) lineage subelement of the endoderm and other potential interblast omere signaling interactions in the cleavage-stage embryo are summariz ed. Definitive boundaries between mesoderm and endoderm territories of complex. the vegetal plate, and between endoderm and overlying ectode rm, are not established until later in development. These processes ha ve been clarified by numerous observations on spatial expression of va rious genes, and by elegant lineage labeling studies. The early specif ication events depend on regional mobilization of regulatory factors r esulting at once in the zygotic expression of genes encoding transcrip tion factors, as well as downstream genes encoding proteins characteri stic of the cell types that will much later arise from the progeny of the specified blastomeres. This embryo displays a maximal form of indi rect development. The gene regulatory network underlying the embryonic development reflects the relative simplicity of the completed larva a nd of the processes required for its formation. The requirements for p ostembryonic adult body plan formation in the larval rudiment include engagement of a new level of genetic regulatory apparatus, exemplified by the Hox gene complex.