Fig. 4. AP-2 expression in lamprey. (A) Dorsal anterior view of 4-day-old neurula. AP-2 expression is observed in the non-neural ectoderm at the exclusion of the neural plate. (B) Dorsal anterior view of 6 day neurula showing expression confined to the neural rod. An AP-2-free gap disrupts the neural rod signal anteriorly. (C) Side view of embryo in B with head protruding upwards. Dorsal neural rod expression is indicated by the arrow. (D) Side view of 7-day-old embryo, anterior is towards the right. AP-2 expression is enhanced in the head epidermis and mesenchyme, but is excluded from the otic cup. (E) 7.5-day-old embryo showing AP-2 expression throughout the head. (F) Cross-section through the neurula in A at f. AP-2 transcripts are absent from the neural plate. (G) Cross-section through 6-day-old neurula in C at position g, showing AP-2 expression in the dorsal neural tube (arrow). (H) Cross-section through 7-day-old embryo in D at position h, showing AP-2 staining in dorsal neural tube, head mesenchyme and epidermis (arrows). (I) Cross-section of 7.5-day-old embryo in E through the otic vesicle at position i. AP-2 transcripts are detected in the dorsal neural tube and in mesenchyme above and below the otic vesicles (arrows). No staining is apparent medial to the vesicle where it approximates the neural tube. (J) Cross-section through 8-day-old embryo in K at the level of the first somite (j). AP-2 expression is observed in mesenchyme superficial to the ectoderm and adjacent to the gut (arrows). (K) AP-2 expression in the head of an 8-day-old embryo. Staining is most intense in the region of the forming pharyngeal arches. (L) AP-2 expression in the head at 8.5 days, anterior is towards the right. AP-2-positive cells are localized more ventrally than at 8 days. (M) AP-2 expression in the head at 9 days, showing extensive staining in the pharyngeal region. (N) Side view of 11-day-old embryo showing AP-2 expression in the pharyngeal arches and strong staining in the anterior lip. (O) Cross-section through the 11-day-old embryo in N at position o. AP-2 mRNA is detected in a cluster of neurons in the lateral neural tube and in mesenchyme surrounding the pharynx (arrows). (P) Horizontal section through a 7.5-day-old embryo at the approximate level of p in E, showing expression in the dorsal neural tube, mesenchyme and ectoderm (arrows). (Q) Horizontal section through the head of the embryo in L at position q. Anterior is towards the right. AP-2-expressing cells are seen in the mesenchyme surrounding the pharyngeal mesoderm (arrows). (R) Horizontal section through an 11-day-old embryo at about the level of r in N. Expression is observed in the mesenchyme of the pharynx (arrows). ar, archenteron; np, neural plate; ect, non-neural ectoderm; n, notochord; ot, otic pit/vesicle; nt, neural tube/rod; ph, pharynx; pa, pharyngeal arches; md, mandibular arch; mx, maxillary region (anterior lip); so, somite.

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