Fig. 3. Formation of the fourth pharyngeal arch arteries (PAAs) is abnormal in Fgf8^H/– mutants. (A-F) Left lateral view after intracardiac ink injection to visualize the developing PAAs at E9.5 (A,B), 10.5 (C,D) and 11.5 (E,F) in Fgf8 mutants (B,D,F) and somite/stage matched controls (A,C,E). The PAAs are numbered and the aortic sac (as) and dorsal aorta (da) labeled. Note complete absence of the fourth PAAs in the mutants at E10.5 and E11.5, although the third and sixth PAAs are present. (G-J) Formation of the third pharyngeal pouch and fourth pharyngeal arch is disrupted in Fgf8^H/– mutants. Immunohistochemistry for PECAM to stain the endothelium of the PAAs; plane of transverse sections through the arches is shown in inset of H. (G,I) control embryo, ventral (G) and dorsal (I); the ventral section is at the junction of the aortic sac with the origins of the second, third and fourth arch arteries. (H,J) Mutant embryo, ventral (H) and dorsal (J) transverse sections; the fourth pharyngeal arch tissue is hypoplastic, fused to the third arch and avascular on the left, although disorganized PECAM-positive endothelial cells are present (arrowheads); there is no third pouch on the left. The right fourth PAA is minimally endothelialized and never connects to the dorsal aorta (data not shown). The sixth arch arteries are larger than normal and have already fused with the dorsal aorta in section J. The rostral pharynx (rp) and caudal pharynx (cp) are labeled and the pharyngeal arches are numbered and labeled left (l) or right (r).

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