Dissociated Blastomeres with High Concentration of Activin Contribute Endoderm Lineage When Transplanted into Blastula in Zebrafish (Danio rerio), and Goldfish (Carassius auratus)

In teleost development, most of the blastomeres, except for the germline lineage, show pluripotency of differentiation in somatic lineages during early stages of development before the late-blastula stage. Differentiation of blastula cells into the mesendoderm is due to the induction by Activin/Nodal, belonging to the transforming growth factor ß (TGF-ß), secreted by the yolk syncytial layer. In the present paper, the differentiation potential of blastomeres treated with recombinant human Activin (rhAct) proteins was examined. When rhAct A, B and AB were injected into the blastula, mesodermal marker genes goosecoid (gsc) and no tail (ntl) were expressed throughout the blastoderm. Furthermore, blastomeres from injected blastoderm contributed to the mesendoderm lineage in the host embryos implanted at the blastula stage. In experiments where dissociated blastomeres treated with rhAct B were transplanted into host embryos, higher concentrations of rhAct B promoted differentiation into the endoderm lineage more effectively than lower concentrations. In addition, the resultant embryos with donor cells in endodermal organs had a lower incidence of malformations. When the donor cells were distributed dorsally at the gastrula stage, they differentiated into the pharynx. When they were distributed ventrally or laterally, they differentiated into the endodermal organs of the trunk. Moreover, when rhAct B-treated blastomeres were xenotransplanted between goldfish and zebrafish, the xenodonor cells were incorporated into the endoderm of the resulting normal embryos.