Fig. 4. Hox3 and Pax6 are necessary for the development of somatic motoneurons in r5. (A-C) Transverse sections of ventral r5 in E11.25-E11.5 control, Hoxa3^-/-b3^-/- double and Pax6^Sey/Sey mutant embryos labeled for HB9 (green) and Phox2b (red). The SMNs are missing in both Hoxa3^-/-b3^-/- double and Pax6^Sey/Sey mutant embryos. The ventrolateral expression of Phox2b in both mutant embryos is spared, suggesting that the early differentiation of visceromotoneurons in r5 is unaffected by either mutations. (D-I) Characterization of ventral progenitors and neuronal subtypes in transverse sections of ventral r5 in control E10.25 embryos prior to the migration of r4-derived BMNs. The progenitors for V3 interneurons, SMNs and V2 interneurons are characterized by the expression of Nkx2.2 (red), low and high Pax6 (green) levels, respectively (D). Olig2 (green) labels the pSMN domain (red; green fluorescent intensity of Olig2 masks the low red fluorescence of Pax6 staining), immediately ventral to the high Pax6 pV2 domain (E). pSMN domain and SMNs are labeled by HB9 (red) and Olig2 (green), respectively (F). The SMNs and the V3 interneurons can be distinguished from each other by their non-overlapping expression of HB9 (red) and Phox2b (green), respectively (G). The V2 interneurons can be distinguished from the HB9-expressing SMNs (red) by their specific expression of Chx10 (green). A CFP reporter for Hoxa3 (green) labels all cells in r5 with relatively high expression levels among postmitotic cells, which include the HB9-expressing SMNs (red, I).