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Summary

In teleosts, primordial germ cells (PGC)s are formed by the inheritance of cytoplasmic determinants called ‘germ plasm’ during early embryonic development. Although several molecular components of the germ plasm have been identified, it is still unclear if cytoplasmic regions that show expression of germ cell specific marker genes such as vasa and nanos1 are necessary and sufficient for inducing PGCs in the embryo. In this study, the relationship between PGC formation and yolk granules was examined by reducing numbers of granules in the embryo shortly after fertilization. Ten minutes after fertilization, goldfish eggs were centrifuged at 700 g. This treatment resulted in the formation of a transparent blastodisc with apparently no yolk granules. When this segment was removed from the centrifuged egg and placed in culture, it exhibited a holoblastic mode of cleavage and developed into a spherical embryoid body. Maternal vasa mRNA was detected at the connections between blastomeres that developed from the separated fragment. Electron dense structures could be detected along the cleavage furrow in a position seemingly identical to vasa mRNA localization. When the spherical embryoid body from the centrifuged eggs was transplanted onto the blastodisc of a normal blastula stage embryo, PGCs from the donor embryo were detected around the genital ridge in the resulting chimeric larva at 10 days post-fertilization. These results suggest that cytoplasmic fragments from centrifuged eggs contain factors sufficient for PGC differentiation. However, transplantation of the putative cytoplasmic germ plasm into host blastodiscs did not result in the formation of PGCs.