Fig. 1. Fate specification in the vegetal hemisphere of ascidian embryos. (A-C) Endoderm (En)-lineage cells are yellow. Mesenchyme (Mes)-lineage cells are shown in green and muscle (Mus)-lineage cells in red. Notochord (Not)- and nerve cord (NC)lineage cells are pink and purple, respectively. (A) Tailbud embryos. Lateral view. Anterior is towards the left. Upper and lower diagrams illustrate midsagittal and parasagittal sections, respectively. (B) 32-cell stage embryo. Vegetal view. Light-blue arrows indicate direction of induction of FGF signaling. Red hatching indicates the location of the posterior-vegetal cytoplasm (PVC). (C) 64-cell stage embryo. Blastomeres connected with blue bars are sister blastomeres. (D) A directed signaling and asymmetric division model of the tissue specification mechanism in the vegetal hemisphere of the ascidian embryo. The model is applicable to both the anterior and posterior margins of the vegetal hemisphere. Light-blue arrows indicate direction of induction of FGF signaling. (a) Schematic drawing representing embryo at the 32-cell stage. Endoderm precursors (En) emanate the inductive FGF signal (light-blue arrows) to neighboring anterior and posterior blastomeres and polarize them. The PVC (red hatching) brings about different responsiveness of posterior marginal cells. (b) Asymmetric divisions occur at the 64-cell stage in both the anterior and posterior marginal zones. For precise positions of blastomeres, see (C). (c) Without inductive signal, both daughter blastomeres in the anterior region assume the default nerve cord fate (NC), and those in the posterior region assume the default muscle fate (Mus). (d) When isolated blastomeres receive inducing FGF signal all over the surface, both daughter cells develop into notochord (Not) or mesenchyme (Mes), depending on absence or presence of PVC, respectively. (E,F) The results of PVC removal and PVC transplantation, respectively. Light-blue arrows indicate direction of induction of FGF signaling. (E) When the PVC is removed, ectopic notochord is induced in the position of presumptive mesenchyme in the posterior region. Mesenchyme formation is suppressed. (F) PVC transplantation to the anterior region of intact eggs results in ectopic mesenchyme and muscle formation in the anterior region. On the other hand, notochord formation is suppressed. NC, nerve cord; Not, notochord; En, endoderm; Mes, mesenchyme; Mus, muscle.