Isl-1 is expressed in the various functional classes of primary neurons at 24 hours postfertilization. This selective expression of a homeodomain protein in the primary neurons implies that these neurons share a common program of early development and that they have evolved and been selected for as a coordinated system. One of the functions of the primary neurons is to send long axons that pioneer the major axon tracts in the zebrafish embryo. The expression by the ventromedial cells is segmentally repeated and becomes restricted to the one or two cells slightly anterior to the segment borders. This raises a possibility that Isl-1 may be involved during determination of subtype identities of the primary motoneurons.
Zebrafish embryos have three or four identifiable primary motoneurons per hemisegment.
Double staining by in situ hybridization and immunohistochemistry strongly suggests that these cells are mainly rostral primary motoneurons. In zebrafish, individual primary motoneurons can be uniquely identified by their characteristic cell body positions and axonal projection patterns. The fate of individual primary motoneurons remains plastic until just prior to axogenesis when they become committed to particular identities. Distinct primary motoneurons express particular combinations of LIM homeobox genes. Expression precedes axogenesis as well as commitment, suggesting that LIM homeobox genes may contribute to the specification of motoneuronal fates.