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

The origin and translocation of primordial germ cells (PGCs) in goldfish Carassius auratus were studied, using both histological observations during embryonic development and by following the effects of partial elimination and duplication of blastoderm on the numbers of PGCs at the mid-blastula stage. Goldfish PGCs are distinguished from somatic cells by their large size, distinct outline and large nucleus, because they are morphologically identical with those of the other fish species. They were located in the gonadal anlage at ten days post-fertilization, and were traced back to 30-hour post-fertilization (hpf) embryos with seven to nine somites. They were distributed widely in an embryo at 30 hpf, except for brain, spinal cord, notochord and 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 eliminated at the mid-blastula stage, the number of PGCs decreased at the gonadal anlage. But, elimination of the upper part at same stage did not effect the number of PGCs. Moreover, the number of PGCs increased in the duplicated embryos, in which a normal blastoderm was transplanted onto a host embryo from which the upper part of the blastoderm had been removed. These results suggest that the PGCs-bearing-cells could be predetermined by cytoplasmic factor and were already distributed in the lower part of blastoderm at the mid-blastula stage, and that they can not regenerate after that.