Fig. 7.

Model for PGC molecular sequestration. During the early cleavage stages, germ plasm (green) segregates asymmetrically into a few vegetal pole cells, the future PGCs. Daughter cells lacking germ plasm (yellow) are fated to become endoderm. Both PGCs and endoderm cells contain the maternal endoderm determinant VegT. At MBT, somatic cells become transcriptionally active (red nuclei) and transcribe the zygotic downstream targets of VegT, Bix4 and Xnr1, thus setting the endoderm fate. PGCs are not transcriptionally active (white nucleus) and cannot respond to VegT. Maternal transcripts, including VegT, decline dramatically by neurula, at which time PGCs initiate their own program of transcription that includes Oct-91 and Rack1. We propose that transcription is delayed in PGCs to prevent them from entering an endodermal fate.