The elimination and duplication of lower part of blastoderm effects on thenumber of primordial germ cells in goldfish

The origin and translocation of primordial germ cells (PGCs) in goldfish Ca rassius auratus were studied, using both histological observations during e mbryonic development and by following the effects of partial elimination an d duplication of blastoderm on the numbers of PGCs at the mid-blastula stag e. Goldfish PGCs are distinguished from somatic cells by their large size, dis tinct outline and large nucleus, because they are morphologically identical with those of the other fish species. They were located in the gonadal anl age at ten days post-fertilization, and were traced back to 30-hour post-fe rtilization (hpf) embryos with seven to nine somites. They were distributed widely in an embryo at 30 hpf, except for brain, spinal cord, notochord an d the enveloping layer. The numbers of PGCs did not increase from 30 hours to ten days post-fertilization. When the lower part of the blastoderm was e liminated at the mid-blastula stage, the number of PGCs decreased at the go nadal anlage. But, elimination of the upper part at same stage did not effe ct the number of PGCs. Moreover, the number of PGCs increased in the duplic ated embryos, in which a normal blastoderm was transplanted onto a host emb ryo from which the upper part of the blastoderm had been removed. These res ults suggest that the PGCs-bearing-cells could be predetermined by cytoplas mic factor and were already distributed in the lower part of blastoderm at the mid-blastula stage, and that they can not regenerate after that.