In the vertebrate nervous system, specialised cell types develop at characteristic positions. To investigate how the fine patterning of motoneurone subtypes is specified, Lewis and Eisen turned to zebrafish embryos in which three subtypes of primary motoneurones (PMNs) occur in a segmentally reiterated pattern along the spinal cord (see ). Two of these PMN subtypes - middle primary (MiP) and caudal primary (CaP) - can be identified molecularly and by their axonal trajectories, i.e. by which tissue they innervate. Lewis and Eisen report that in zebrafish mutants that lack the overlying paraxial mesoderm, PMNs adopt a gene expression profile that has characteristics of both MiPs and CaPs. Furthermore, in mutants with severely disrupted paraxial mesoderm, although the PMNs have a hybrid gene expression profile, their axonal trajectories resemble those of CaP motoneurones. These results are consistent with the hypothesis that signals from the paraxial mesoderm specify the segmental pattern of PMN subtype identity in zebrafish.
