Fig. 5.

Model of ascidian heart specification. (A and B) Diagrams of 32- and 64-cell embryos seen from the vegetal side. In these diagrams, the PP is purple and the PVC is outlined in red. Gray represents the expression patterns of Ci-Sna and Cs-ZicL. Note that when expression is limited to the nuclei (as in the B7.5 cells at the 64-cell stage), this indicates recently initiated transcription (29). (C) Antisense Ci-Wnt5 (ci0100135747, clone ID no. GC12l09) staining of a 110-cell embryo, on the vegetal side. (D) A 110-cell embryo in which descendents of the posterior vegetal quadrant have been color-coded to correspond with the cell fates listed in the diagrams to the right. (E–G) Diagrammatic representations of the posterior vegetal quadrant of the Ciona embryo. (E) At the 32-cell stage, the posteriormost blastomere (B6.3) still contains PP, repressing transcription, and thereby inhibiting a response to PVC-mediated myogenesis, as well as inductive Fgf signaling from the endoderm (yellow arrows). (F) At the 64-cell stage, the newly born B7.5 cells have now emerged from the PP and are therefore newly competent to respond to inductive signals. Possible signaling, which may occur after the 64-cell stage, is indicated by arrows, including the expression of BMP by the B7.5 cells at the 110-cell stage (red arrow) (34), FGF-8 signaling from the mesenchyme (MES) (green arrow; unpublished data), Wnt-5 signaling from the germ cell lineage (purple), or signaling from the endoderm (ENDO) (yellow). (G) By the 180-cell stage, the TVC lineage has separated from the tail muscle (MUS) lineage (35). It is unclear to what extent the TVCs are prespecified to from the heart at this point; later positional signaling may play a significant role in heart lineage specification.