Fig. 4. Blimp1 promotes expansion of the surface ectoderm and endoderm progenitors in the developing pharynx. (A) Blimp1-Cre faithfully recapitulates the endogenous Blimp1 expression pattern at E9.5. (B) Blimp1⁺ cells in the pharyngeal region (boxed in A) give rise to the surface epithelium of the pharyngeal arches and the anterior heart tube. (C,D) lacZ-marked Blimp1⁺ cells contribute to both the surface ectoderm and pharyngeal endoderm of the pharyngeal arches, and ventral endoderm including the thyroid primordium, but are excluded from the pharyngeal mesoderm. Blimp1 expressed in endothelial cells marks the pharyngeal arteries. (E,F) At E11.5, the Blimp1 mutant pharynx (F) shows severe hypoplasia of the pharyngeal endoderm (PE) and pharyngeal arteries and blood vessels (asterisks) are largely absent. Frontal sections of control (G) and mutant (H) embryos show loss of the thymus in the mutants. SE, surface ectoderm; M, arch mesenchyme; NE, neurectoderm; PA pharyngeal arch; PAA, pharyngeal arch artery; PE, pharyngeal endoderm; ThP, thyroid primordium; S, sternum; T, thymus; RA, right atrium; LA left atrium.