Fig. 3

Hox gene expression boundaries and their relationship to the pharyngeal arches and pouches in the chick embryo. Patterns of expression of Hox genes in the developing pharynx at stage 20 (a, b, d-g, h, i, k- n) when all of the pharyngeal segments have formed. This figure additionally shows the expression of Hoxa3 at stage 17 (b, i). The expression patterns of these genes are shown in bisected embryos (a – g) and in section (h-n). The expression pattern of Hoxa2 is shown in (a, h) and this gene can be seen to be expressed in the mesenchyme of the second and more posterior arches and in the endoderm of the second pouch and posterior of that. The position of the second pouch is marked by an asterisk in (a) and an arrow in (h). The expression of Hoxa3 is shown at stage 17 (b, i) and at stage 20 (c, j). In these panels the position of the third pouch is marked by an asterisk in (b, c) and and arrow in (i, j). The expression pattern of Hoxa3 in the mesenchyme does not change between these stages and can be seen to be expressed in the third arch and more posterior. However, while Hoxa3 can be seen to be expressed in the third pharyngeal pouch at stage 17 (i), expression is lost from here at stage 20 (j). The expression of Hoxb3 is shown in (d, k) and this gene can be seen to be expressed in the mesenchyme of the fourth arch but its expression is noticeably absent from the pharyngeal endoderm (k). The position of the third pouch is marked by an asterisk in (d) and by an arrow in (k). The expression of Hoxb4 is shown in (e, l) and this gene can be seen to be expressed is not expressed in the mesenchyme of any of the arches, nor in any of the pharyngeal pouches but is expressed in the most posterior pharyngeal endoderm and the ectoderm lying posterior of the pharynx. The position of the fourth pouch is marked by an asterisk in (e) and by an arrow in (l). The expression of Hoxb5 is shown in (f, m) and it can be seen that this gene is not expressed in the pharyngeal region. Hoxb1 expression is shown in (g, n) and at stage 20 it is clear that the expression of this gene is restricted to the last formed, fourth, pharyngeal pouch, marked by an asterisk in (g) and an arrow in (n)