Fig. 2

The generation and remodelling of the pharyngeal segments. a Side view of a HH17 chick embryo, the three fully formed anterior arches can be seen while at HH21 (b) the full complement of arches have formed. PAX1 highlights the position of the intervening pouches. (c) Longitudinal section of a HH21 chick embryo, DLX2 expression shows the neural crest component of the arches. Noticeably, the caudal edge of arch 6 has no clear definition; highlighted by the black arrow while the anterior limit of this arch is delineated by the pouch of contact between the fourth pouch and the overlying ectoderm, indicated by the black arrowhead. d Longitudinal section of a HH21 embryo, TBX1 expression demonstrates the relative positions of the mesoderm components of each of the arches as well as the pharyngeal endoderm and pouches. A sizeable mesodermal population can be seen in arches 1, 2, and 3. Arch 4 by comparison only has a reduced mesoderm component; indicated by the white arrow. At HH20 (e) the second arch is beginning to expand and by HH24 (f) it has overgrown the posterior arches. The arches are numbered in all panels. Scale bar = 0.1 mm