Fig. 4

Hox gene expression boundaries and their relationship to the pharyngeal pouches in Scyliorhinus canicula. Expression patterns of Hoxa2, Hoxa3 and Hoxb1 in early (a-c), stage 20/21which have formed 3 to 4 pharyngeal arches, and later (d-f) embryos, stage 23/24 which have formed 5 to 6 pharyngeal arches. At both early and late stages Hoxa2 is expressed in the mesenchyme of the second and more posterior arches and in the endoderm caudal of the second pouch, marked by an arrow (a, c). Hoxa3 shows expression in the mesenchyme of the third and more posterior arches at both early and late stages (b, e) but only weak (b) to no expression in the pharyngeal endoderm. The position of the third pouch (b, e) is indicated by the arrow. Hoxb1 shows prominent expression in fourth pharyngeal pouch at early stages, marked by an arrow (c) and in the fifth pouch at later stages, marked by an arrow (f)