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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.
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