Fig. 5. Descendants of Hesx1-expressing cells in the anterior NP colonise posterior regions of the neural tube and the first branchial arch. All embryos were X-Gal stained. (A,B) Higher numbers of lacZ-expressing cells reach more posterior regions of the NP in the Hesx1^Cre/-;R26^Cond-lacZ/+ mutant (B, arrowhead) as compared with a Hesx1^Cre/+;R26^Cond-lacZ/+ embryo (A) at the 6- to 7-somite stage. (C,D) This is accentuated in an 8- to 10-somite Hesx1^Cre/-;R26^Cond-lacZ/+ mutant (D) as compared with an Hesx1^Cre/+;R26^Cond-lacZ/+ embryo (C). Arrowhead in C indicates the boundary between anterior and posterior forebrain. (E-H) Lateral (E,F) and dorsal (G,H) views of a Hesx1^Cre/+;R26^Cond-lacZ/+ embryo (E,G) and a Hesx1^Cre/-;R26^Cond-lacZ/+ mutant (F,H). The brain was partially dissected to improve X-Gal staining. lacZ-expressing cells barely colonise the posterior forebrain of the Hesx1^Cre/+;R26^Cond-lacZ/+ embryo (arrowheads in E,G), but massively populate this region in the Hesx1^Cre/-;R26^Cond-lacZ/+ mutant (black arrowheads in F,H). Note that the Hesx1^Cre/-;R26^Cond-lacZ/+ mutant depicted in F,H shows asymmetric telencephalic development with a very small right telencephalic vesicle. This is concomitant with a more pronounced degree of colonisation of lacZ-expressing cells on the right side of the posterior forebrain (white arrowhead). (I,J) Hesx1^Cre/+;R26^Cond-lacZ/+ embryo (I) and Hesx1^Cre/-;R26^Cond-lacZ/+ mutant (J) at 9.5 dpc. Note the presence of lacZ-expressing cells within the first branchial arch in the Hesx1^Cre/-;R26^Cond-lacZ/+ mutant only (J). (K-N) Frontal sections of a Hesx1^Cre/+;R26^Cond-lacZ/+ embryo (K,M) and a Hesx1^Cre/-;R26^Cond-lacZ/+ mutant (L,N) at 10.5 dpc. Many more lacZ-expressing cells are localised in the frontonasal mass (arrowheads in L) of the Hesx1^Cre/-;R26^Cond-lacZ/+ mutant embryos as compared with the Hesx1^Cre/+;R26^Cond-lacZ/+ embryo (K). lacZ-expressing cells within the first branchial arch are only observed in the Hesx1^Cre/-;R26^Cond-lacZ/+ embryo. ba-I, first branchial arch; dd, dorsal diencephalon; dt, dorsal telencephalon; op, olfactory placode; rp, Rathke's pouch; vt, ventral telencephalon.