Fig. 7. Effect of exogenous RA on the development of the reticulospinal and branchiomotor neurons. (A-C) Confocal micrographs of reticulospinal neurons in stage 26 lamprey larvae that had been treated with 0 µM (A), 0.01 µM (B), 0.1 µM (C) all-trans RA. (D-F) Positions of the Mth neuron of stage 25 embryos in relation to the expression of LjKrox20 after treatment with RA. The Mth neuron is visualized by whole-mount in situ hybridization with a neurofilament protein antisense riboprobe. Note that the Mth neuron is always located at the middle of r4. (G,H) Positions of Mth neurons in stage 25 embryos in relation to the expression of LjHox3 in the control (G) and embryos treated with 0.1 µM RA (H). LjHox3 domain is shifted anteriorly in the RA-treated larvae (compare arrows in G and H). (I,J) Expression of Lj Fgf8/17 in the control (I) and 0.1 µM RA-treated (J) stage 26 larvae. (K-N) Confocal micrographs of stage 26 larvae. (K,M) Control larvae in which trigeminal (Vm), facial (VIIm), glossopharyngeal (IXm) and vagus (Xm) neurons are clustered along the anteroposterior axis (K), and the trigeminal motor nuclei positioned posterior to the mid-hindbrain boundary (MHB: M). In 0.1 µM RA-treated larvae (L,N), boundaries between the branchiomotor nuclei (BM) have become unclear (L), and the presumptive trigeminal nucleus extends rostrally beyond the MHB (N).