Fig. 3. Loss of Blimp1 expression causes branchial arch patterning defects. Whole-mount in situ hybridization analysis of Blimp1 expression in wild-type embryos at (A) 7.75, (B) 8.5, (C) 9.5, (F) 10.5 and (G) 11.5 dpc. (A) Blimp1 transcripts are expressed in the anterolateral definitive endoderm, most abundantly in the tissue from which the first branchial arch (ba) will emerge (arrowhead). (B) This expression domain is maintained one day later (arrowhead), when Blimp1 transcripts also appear in the ventral anterior neurectoderm. (C) At 9.5 dpc, Blimp1 is expressed in the ventral forebrain, and in the regions of the first, second and third branchial arches. (D,E) Transverse sections of embryo shown in C showing expression confined to the pharyngeal endoderm of the first branchial arch (arrow in D), while Blimp1 is broadly expressed in the ectoderm, mesoderm and endoderm, more caudally in the region from which the second and third arches arise. From this stage onwards, Blimp1 transcripts are also detected in the foregut (fg) and otic vesicles (ot). (F) At 10.5 dpc, expression is downregulated within the branchial arches, and by 11.5 dpc (G), resolves to a subdomain within the first branchial arch. (H,K) Histological analysis of 9.5 dpc wild-type (+/+; H) and mutant (-/-; K) embryos. (H) Whole-mount view of a 9.5 dpc wild-type embryo and corresponding Hematoxylin-Eosin-stained transverse sections 1 and 2 (levels indicated by dashed lines). Three branchial arches are visible (outlined). (K) In a similarly staged mutant embryo, only the first branchial arch is present (outlined). Defects are restricted to the levels of the second branchial arch (K part 2, compare with H part 2). At this site (see high magnification image in K2'), apoptotic cells with pyknotic nuclei (arrow) accumulate within the mesenchyme (compare with H2'). (I,J,L,M) Fgf8 expression domains at 9.5 and 10.5 dpc. In 9.5-10.5 dpc embryos, Fgf8 is expressed at multiple tissue sites, including the ectoderm and endoderm of the branchial arches, the midbrain-hindbrain junction, the somites, the developing limb buds (arrows) and the tail bud. The branchial arch domain in the region of the second and third arches is largely missing in mutant embryos (compare brackets in I and L). (J,M) At 10.5 dpc, Fgf8 expression is more broadly expressed in the mutant than in the wild type, and extends into the epaxial lip of the rostral dermamyotome (short arrow). Fgf8 expression at all other sites, including the AER of the limb buds (arrow), is unperturbed. ba, branchial arch; ot, otic vesicle; pc, pharyngeal cleft; fg, foregut; tgg, trigeminal ganglia; op, optic vesicle.