Hoxb1 is required for the anterior-posterior identity of dB2 neurons in r4.

<p>(<b>A, B</b>) Transverse sections through r4 of E11.5 control and Hoxb1^−/− embryos immunolabeled for Phox2a and Lbx1. Phox2a expression is lost or dramatically reduced among dB2 neurons, while Lbx1 is maintained in Hoxb1^−/− embryo compared to control. (<b>C, D</b>) Transverse sections through r4 of E11.5 control and Hoxb1^−/− embryos immunolabeled for Lhx1/5 and Tlx3. dB1 Lhx1/5-positive neurons are juxtaposed to dB2-Tlx3-negative neurons in the control embryo. In the Hoxb1^−/− embryo, Tlx3 is ectopically expressed in the dB2 domain (Tlx3-positive dB2 neurons abut Lhx1/5-positive dB1 neurons). (<b>E–G</b>) Transverse sections of E11.5 embryos through r4 of control and Hoxb1^−/−, and r2/r3 of control immunolabeled for Phox2b and Lmx1b. Phox2b and Lmx1b are not co-expressed among control dB2 neurons, whereas co-expression is observed in r4 of Hoxb1^−/− and r2/r3 of control embryos. The migration of dB2 neurons in control appears to migrate in the radial direction. In r4 of Hoxb1^−/− and r2/r3 of control embryos, dB2 neurons appear to migrate in the ventromedial direction. (<b>H</b>) Quantitative analysis of the number of Phox2b/Lmx1b-double positive dB2 neurons in r4 of E11.5 control (green bar, n = 3) and Hoxb1 mutant (B1^−/−, red bar, n = 3) embryos and r2/r3 of control (r2/r3, blue bar, n = 3) embryos. The number of double-positive dB2 neurons was significantly more in r4 of Hoxb1^−/− and r2/r3 of control embryos compared to r4 of control embryos (<i>P</i><0.005). Data given as mean (cells/section) ± standard error mean. Scale bars (A–G = 50 µm).