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Fig. 4. Dorsoventral specification is altered in the double-null limb buds. (A-D)
En1 expression in control (A,C) and double-null mutants (B,D): limb
buds are viewed from their distal tip with their ventral side facing upwards.
Anterior is towards the right. En1 expression level is reduced in the
limb field at 9.5 dpc (24 somites) (A,B) and is undetectable anteriorly at
11.5 dpc (C,D) in the double mutant (D, arrow). (E-I) Lmx1b
expression in control (E,G) and double mutant (F,H,I) embryos. (E,F) Limb buds
are viewed from their distal tip with the ventral side facing towards the
upper leftward side. Anterior is uppermost. The broken line indicates the
normal dorsoventral boundary of the limb bud. At 10.5 dpc, the Lmx1b
expression domain extends ventrally at the anterior aspect of the limb bud in
the double mutant (arrow). (G,H) Views of the anterior border of limb buds at
11.5 dpc showing the sharp expression limit of Lmx1b at the level of
the AER in the control (G), whereas the Lmx1b expression domain
extends ventrally in the double mutant (arrowhead) (H). The black arrow
indicates the anterior limit of the AER. (I) A section of the limb bud shown
in H, according to the plane indicated by the white arrows, that confirms the
presence of an ectopic Lmx1b expression domain in the ventral
mesenchyme (arrowhead). The broken line indicates the dorsoventral boundary.
Dorsoventral orientation is indicated by D and V.
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