Fig. 5. Specification of somatic motoneurons in r5 is regulated by Hoxa3 and Hoxb3 in a gene dose-dependent manner. (A-D) Transverse sections of ventral r5 in E10.25 embryos harboring various mutant alleles in Hoxa3 and Hoxb3 labeled for Olig2 (green) and HB9 (red); (E-H) TUNEL (red), activated caspase 3 (green, CASP3) and the mitotic marker, MPM2 (blue). (I-P) Transverse sections of ventral r5 in E10.25 control and Hoxa3^-/-b3^-/- double mutant embryos labeled for Pax6 (green) and Nkx2.2 (red) (I,M), Phox2b (green) and Isl1/2 (red) (J,N), Chx10 (green) and Isl1/2 (red), and Phox2b (blue) (K,O); and E11.5 control and Hoxa3^-/-b3^-/- double mutant embryos labeled for HB9 (green) and Chx10 (red) (L,P). Cell counts for Chx10-expressing V2 interneurons were determined for representative control (Hoxa3b3-double heterozygote) and mutant (Hoxa3b3-double homozygote) embryos. Values for the control, 40.0± 1.9 (mean/side± s.e.m.), versus double-mutant, 61.6± 2.3, were statistically significant (P<0.005). The arrows in panels L and P represent clusters of HB9- and Chx10-expressing SMNs and V2 interneurons, respectively.