Fig. 8. Evolution of reticular and branchiomotor neurons in the hindbrain of the vertebrate embryo. The hypothetical rhombomeric organization of the lamprey brain is based on LjPax6, LjEphC, LjKrox20 and LjHox3 expression data. LjHox3 is strongly expressed up to the mid r4 level in this scheme. (Top) Hypothetical schema for the evolution of reticular neurons. Based on this architecture, lamprey I3 is positioned in r2, and I4 in r3. Mth and B neurons are localized in r4, and Mth' appears in r5. Thus, the developmental pattern of the lamprey reticular neurons is segmental. (Bottom) Branchiomotor patterning through phylogeny. Branchiomotor neurons, including trigeminal (V), facial (VII), glossopharyngeal (IX) and vagus (X) motor nuclei, are shown in different colors. The lamprey trigeminal motor nucleus extends posteriorly into r4, which is not seen in gnathostomes. Note that the boundary between the trigeminal and facial nuclei maps in the middle of r4 correspond to the strong LjHox3 expression boundary, and not to the r4/r5 boundary as in gnathostomes.