A complete second gut induced by transplanted micromeres in the sea urchin embryo.

Founder cells for most early lineages of the sea urchin embryo are probably specified through inductive intercellular interactions. It is shown here that a complete respecification of cell fate occurs when 16-cell stage micromeres from the vegetal pole of a donor embryo are implanted into the animal pole of an intact recipient embryo. Animal pole cells adjacent to the transplanted micromeres are respecified from presumptive ectoderm into vegetal plate founder cells. These induced vegetal plate cells express the entire battery of genes characteristic of the endogenous vegetal plate cells. The ectopic vegetal plate invaginates during gastrulation to form a second archenteron which differentiates properly into a tripartite gut, as shown by the spatial pattern of expression of an endoderm-specific marker gene. Thus, transplanted micromeres can signal neighboring cells to induce them to change their fate.