PROGRESSIVE DETERMINATION OF CELL FATES ALONG THE DORSOVENTRAL AXIS IN THE SEA-URCHIN HELIOCIDARIS-ERYTHROGRAMMA

In the direct-developing sea urchin Heliocidaris erythrogramma the fir st cleavage division bisects the dorsoventral axis of the developing e mbryo along a frontal plane, In the two-celled embryo one of the blast omeres, the ventral cell (V), gives rise to all pigmented mesenchyme, as well as to the vestibule of the echinus rudiment. Upon isolation, h owever, the dorsal blastomere (D) displays some regulation, and is abl e to form a small number of pigmented mesenchyme cells and even a vest ibule. We have examined the spatial and temporal determination of cell fates along the dorsoventral axis during subsequent development. We d emonstrate that the dorsoventral axis is resident within both cells of the two-celled embryo, but only the ventral pole of this axis has a r igidly fixed identity this early in development. The polarity of this axis remains the same in half-embryos developing from isolated ventral (V) blastomeres, but it can flip 180 degrees in half-embryos developi ng from isolated dorsal (D) blastomeres. We find that cell fates are p rogressively determined along the dorsoventral axis up to the time of gastrulation. The ability of dorsal half-embryos to differentiate vent ral cell fates diminishes as they are isolated at progressively later stages of development. These results suggest that the determination of cell fates along the dorsoventral axis in H. erythrogramma is regulat ed via inductive interactions organized by cells within the ventral ha lf of the embryo.