Fig. 1. Pdgfra^-/- cells contribute minimally to limb bud and cranial ectomesenchyme. (A-G,I) Pdgfra^-/- ES cells injected into ROSA26^+/- blastocysts; chimeras after staining for ß-galactosidase activity. (A-E) E10.5 chimeras that contain a high percentage of Pdgfra^-/- cells phenocopy Pdgfra^-/- embryos (A, CH1; B,C, CH2; D,E, CH3; see Table 1). (C,E) Transverse sections through chimeras shown in B and D at the cranial and cardiac crest level, respectively. (F) Transverse section through hindlimb bud of 50% mutant cell derived chimera (CH8). Inset shows a section through forelimb bud with predominantly Pdgfra^-/- contribution to surface ectoderm (arrowhead) and wild-type contribution to the mesenchyme. (G,I) Transverse sections of branchial arches and head mesenchyme from chimeras generated by injection of Pdgfra^-/- ES cells into ROSA26^+/- blastocysts (CH11). (H,J) Transverse sections through similar regions to those in G and I for chimeras made by injection of Pdgfra^-/-; ROSA26^+/- ES cells into wild-type blastocysts (CH10). nt, neural tube; se, surface ectoderm; lb, limb bud; ov, otic vesicle; ba1, branchial arch 1; ba2, branchial arch 2; fb, forebrain. Arrows in G-J indicate regions of mesoderm-derived muscle progenitors. Arrowheads in C indicate location of occasional wild-type cells.