Fig. 6. Hoxa3 and Hoxb3 are necessary for the formation of the abducens nucleus and normal target innervation. (A-D) Transverse sections of the upper medulla in E18 control and Hoxa3^-/-b3^-/- double mutant embryos labeled for the neuron-specific nuclear marker, NeuN (A,B) and choline acetyltransferase (ChAT) (C,D). In the control embryo, the region of the abducens nucleus (dotted outline) can be identified by its stereotypic relationship with the axons of the genu of the facial nucleus (arrow; devoid of NeuN expression) and the expression of ChAT (A,C). In the Hoxa3^-/-b3^-/- double mutant embryo, the loss of ChAT expression in the region normally occupied by the abducens nucleus is not associated with the loss of NeuN-expressing neurons and the formation of the genu (B,D). (E-J) Transverse section through the proximal lateral rectus muscle in E18 control and Hoxa3^-/-b3^-/- double mutant embryos labeled for TuJ1 (green) and -bungarotoxin binding (BTX, red). (I,J) Merged images of E,G and F,H, respectively.