Fig. 3. LacZ expression in the head mesenchyme and neural-crest derivatives in Ednra ^lacZ2FRT/+ mouse embryos. (A-C) Transverse sections at E10.0. LacZ expression is observed throughout the head mesenchyme adjacent to the neural tube and in the peripheral pharyngeal arch mesenchyme (A). Trigeminal ganglia are highly populated with lacZ-positive cells (B). LacZ expression is detectable within the lateral wall of the aortic sac and pharyngeal arch arteries (C). (D-H) Transverse sections at the levels of the central (D-F) and distal (G,H) regions of the lower jaw at E12.5. E,F,H are high magnification images of the boxed areas in D and G. LacZ expression is observed in mesenchyme underlying the oral epithelium in the lower and upper jaws (D,G). In the molar (E) and incisor (H) buds, lacZ expression is present in mesenchyme surrounding the dental lamina. LacZ expression is undetectable in the precartilage primordium contributing to the rod portion of Meckel's cartilage, whereas surrounding mesenchymal cells express lacZ (F). In the primordium of the rostral process of Meckel's cartilage, lacZ is highly expressed (H). Scale bars: 100 µm in A-C; 500 µm in D,G; 50 µm in E,F,H. aa2, second arch artery; as, aortic sac; da, dorsal aorta; dl, dental lamina; dm, dental mesenchyme; fg, foregut; gV, trigeminal ganglion; lj, lower jaw; nt, neural tube; pa1/2, first and second pharyngeal arches; pc, precartilage primordium of Meckel's cartilage; to, tongue; uj, upper jaw.