Primordial germ cells: the blueprint for a piscine life

Germ cells prior to sex differentiation are called primordial germ cells (PGCs). They are the progenitor cells of the germ cell lineage and possess the ability to differentiate into either spermatogonia or oogonia. In all animals examined to date, PGCs are set aside from somatic cell lineages during early embryonic development. Recent studies of fish germ cells have revealed that (1) germ plasm-like structures are found in fertilized eggs of zebrafish, and structurally and morphologically similar structures called `nuage' are found in the gonadal PGCs of various fish species; (2) RNA transcripts of the vasa and nanos genes, which are known to be components of the germ plasm of Xenopus laevis, are also present as maternally inherited molecules in fishes; and (3) PGCs emerge only from the lower part of the blastoderm and this specification occurs prior to zygotic gene expression. Based on these findings, asymmetrical distribution of maternally inherited germ plasm appears to be a major system for germ cell specification in fishes. This mechanism is different from that in mammals, in which PGCs are specified epigenetically. PGCs separate from somatic cell lineages in extragonadal areas and then migrate to coalesce with somatic gonadal support cells. These migrations are performed at least partly by active movement of PGCs themselves in zebrafish and rainbow trout. On the other hand, in medaka, the PGC migration may be the result of passive movement caused by surrounding somatic cell migration. During these migration periods, fish PGCs are mitotically silent and resume proliferation after they reach the gonadal anlagen. PGCs, in addition to being biologically important as founders of the germ cell lineage, may have many applications for bioengineering of fish because of their ability to be converted into individual fishes. Some of these potential applications are discussed.