Fig. 6. Pre-midblastula transition (MBT) ß-catenin/Xtcf3-dependent transcription in the regulation of dorsal development. (A) In this model, ß-catenin/Xtcf-dependent transcription begins in early cleavage stages and continues throughout pre-MBT stages (blue line). Once transcription of these pre-MBT target genes reaches a critical level (horizontal dashed line), dorsal development at post-MBT stages can proceed. If ß-catenin or Xtcf function is inhibited early (inh, red arrow) and inhibition is maintained throughout pre-MBT stages, dorsal development is blocked. However, if introduction of the inhibitor is delayed (inh, green arrow), then sufficient ß-catenin/Xtcf-dependent transcription occurs to allow dorsal development. (B) Transient inhibition of transcription with actinomycin D (ActD) until the 32-cell stage does not disrupt dorsal development because the inhibitor is reversible and thus pre-MBT transcription resumes when ActD is removed. However, if ActD is followed directly by specific inhibition of ß-catenin/Xtcf function at the 32-cell stage (inh, red arrow). at 32-cell), then dorsal development is blocked. (C) If Xtcf function is restored at the 500-cell stage (TVGR at 500 cell), then ß-catenin/Xtcf-dependent transcription resumes and reaches the threshold required for dorsal development. Thus, Xtcf function at any time during pre-MBT stages appears to be sufficient for dorsal development. TVGR can also be activated post-MBT but does not rescue dorsal development under these conditions, indicating that ß-catenin/Xtcf activity is required prior to MBT for dorsal development.