Primordial germ-cell development: the zebrafish perspective

Primordial germ cells (PGCs) in zebrafish are specified by the inheritance of maternally provided cytoplasmic determinants, which are collectively known as the germ plasm.Several molecules that are important for PGC development in zebrafish also have a role in germ-cell development in other organisms, invertebrates and vertebrates.During the first day of embryonic development, PGCs migrate from the positions where they are formed towards their target — the somatic part of the gonad — where they differentiate into sperm and eggs.During their migration, PGCs acquire directional cues from somatic cells that attract them towards their intermediate and final targets.A crucial molecule that directs the migrating PGCs is the chemokine Sdf-1a, which is expressed in positions at which PGCs are found.Reducing the level of Sdf-1a or its receptor Cxcr4b results in a severe PGC migration defect; conversely, expression of Sdf-1a in ectopic locations leads to accumulation of PGCs in those sites.