SPECIFICATION OF ENDODERM IN THE SEA-URCHIN EMBRYO

Expression of the endoderm specific gene Endo16, was used to monitor e ndoderm specification in developmentally arrested and in dissociated e mbryos. beta-APN treatment halts gastrulation, however, in two species of sea urchins Endo16 mRNA is still expressed, suggesting that specif ication of the endodermal lineage has taken place in these development ally arrested embryos. Endo16 mRNA is not expressed in embryos dissoci ated at the 4-8-cell stage unless they are reassociated shortly after the 16-cell stage. Interestingly, dissociation after the 16-cell stage also results in a lack of Endo16 expression. Lithium is unable to res cue Endo16 expression in these dissociated embryos. These results indi cate that early signaling events mediated by cell-cell contact are req uired for the initial specification and maintenance of the endoderm in the developing embryo. (C) 1997 Elsevier Science Ireland Ltd.