Figure 2. Taf4 is required for correct positioning of head and heart.

(a,b). Schematic representations illustrate normal development at E8.5 and E9.5 highlighting positions of head and heart during embryo turning. (c–f) WT and mutant embryos at E9.5 before and after dissection from the yolk sac highlighting the blood-filled yolk sac vascular plexus in control embryos (red arrow in c) and the embryo protruding partially out of the yolk sac (green arrow in e). WT embryos show a strong accumulation of blood in the atrium of the heart (red arrow in d), which was absent in Taf4a^−/− embryos (f). (g–i) Schematic illustration of observed mutant phenotypes at E9.5. All mutant embryos fail to undergo turning at E9.5 and fusion of the allantois to the chorion. In addition to this, mutant embryos develop heart structures either in the exocoelomic cavity (h–i) or outside of the visceral yolk sac (k–l). (m–w) In situ hybridization of wild-type and Taf4a^−/− embryos for the cardiac transcription factors Nkx2-5 at E8.5 (m–p) or Tbx5 at E8.5 (q–t) and E9.5 (u–w). ep, ectoplacental cone; all, allantois; ht, heart; flp, forelimb precursors; ift, inflow tract. Scale bar, 500 μm (c–f), 300 μm (h-i), 200 μm (m–w).