Fig. 6. Comparison of AP-2 expression in the head of lamprey and axolotl. (A) Dorsal anterior view of 7-day-old lamprey embryo showing AP-2 expression in neural tube, early migrating neural crest and ectoderm. AP-2-positive cells are excluded from a discrete region of the neural tube (arrow) and the otic cup. (B) Side view of an 8-day-old lamprey embryo head showing AP-2 staining in cranial neural crest cells. Putative streams of migrating cells are bordered by dotted lines. Rostrally, white dots indicate the anterior edge of the trigeminal stream, which splits into the ophthalmic (rostralmost white line), maxillary and mandibular streams (ventral white lines). Blue dots border the rostral edge of the putative hyoid crest stream, and yellow dots indicate the position of the common branchial stream. (C) Horizontal section through the pharynx of an 11 day lamprey embryo showing AP-2-positive cells in the mesenchyme medial and lateral to the arch mesoderm (arrows). For reference, colored lines outline the three germ layers in a single arch; mesoderm is red, ectoderm is blue and endoderm is yellow. (D) Dorsal anterior view of approximately stage 21 axolotl embryo showing AP-2 staining in early migrating neural crest. AP-2 expression is reduced in rhombomeres 3 and 5 (black arrows) and a region of rhombomere 1 (white arrow). Compared with the lamprey embryo in A, axolotl shows greater patterning of AP-2 expression along the anterior neural tube. (E) Side view of approximately stage 25 axolotl embryo showing AP-2 expression in cranial neural crest streams. Streams are marked with dotted lines as in B. White dots border the trigeminal stream, which splits into the ophthalmic, maxillary and mandibular streams. Blue dots border the hyoid stream, and yellow dots border the first branchial and common 2/3/4 branchial streams. Overall, the distribution of AP-2 positive cells in the axolotl head resembles that of lamprey (B), except ventrally, where the divisions between putative neural crest streams are much less defined in lamprey than in axolotl. (F) Horizontal sections through the pharynx of stage 30 axolotl embryo. Germ layers are marked as in C. AP-2-positive cells are observed in the mesenchyme surrounding the arch mesoderm (arrow). Both lamprey (C) and axolotl have AP-2-positive cells lying medial to the pharyngeal arch mesoderm. ot, otic pit/vesicle; md, mandibular neural crest stream; mx, maxillary neural crest stream; op, ophthalmic neural crest stream; br, branchial neural crest stream; hy, hyoid neural crest stream; tr, trigeminal neural crest stream; ph, pharynx; r3, rhombomere 3; r5, rhombomere 5.