Commitment of cell fate in the early zebrafish embryo.

When do single cells in the early zebrafish embryo become irreversibly committed to a specific fate? Work with lineage tracing and fate mapping has shown that the marginal cells of the blastoderm give rise to hypoblast-derived fates (mesoderm and endoderm). However, experiments described here show that these marginal blastoderm cells remain pluripotent and uncommitted throughout the late blastula and early gastrula stages. Embryonic cells become committed to a hypoblast-derived fate at mid-gastrulation. Time-lapse photographic analysis reveals that committed cells, when transplanted heterotopically and heterochronically, can migrate along atypical pathways to reposition themselves within a more correct environment.