Figure 8. ActivinB-ALK4/5/7 Signaling Activates the Pmar1-Responsive E-EM/En-GRN during Normal Endomesoderm Specification.

(A) Embryos were treated with either (a–c) DMSO (Control) or (g–i) SB-431542 (SB; 5 μM) and analyzed by WMISH to detect (a and g) z13 at 8 h , (b and h) eve at 8 h, (c and i) foxA at 18 h. SB treatment strongly inhibits the accumulation of each of these transcripts in veg2 blastomeres at the stages assayed (g–i) compared to DMSO-treated controls (a–c). The same mRNAs were detected in embryos injected either with buffer (d–f) or ActivinB MO1 (ActBMO1; 1. 2 mM) (j–l) at 14 h (d and j), 10 h (e and k), or 20 h (f and l), respectively. z13 and foxA mRNA expression in veg2 blastomeres was strongly reduced in ActivinB MO1-injected embryos (j and l) compared to buffer-injected controls (d and f) assayed at corresponding stages. At the late cleavage stage, eve mRNA expression in the veg2 tier was unaffected by the presence of ActivinB MO1 (k vs. e).

(B) Embryos were treated with either (a–c) DMSO or (g–i) SB (5 μM) and analyzed by WMISH to detect (a and g) brachyury at 18 h, (b and h) blimp1 at 14 h, and (c and i) wnt8 at 14 h. The same mRNAs were detected in embryos injected either with buffer (d–f) or ActBMO1 (1.2 mM) (j–l) at 20 h (d and j), 14 h (e and k), and 14 h (f and l), respectively. The accumulation of each of these transcripts in veg2 blastomeres was independent of ALK4/5/7 (g–i vs. a–c) and ActivinB (j–l vs. d–f) function at the stages assayed.

Images in (A) and (B) represent the molecular phenotypes seen in over 80% of at least 50 embryos in three separate experiments.

(C) Fate map of blastula stage embryo.