Gangliogenesis in leech embryos: migration of neural precursor cells.

In the metameric CNS of leeches, identified neurons occupy highly stereotyped positions in each segmental ganglion. Although many of the neural precursor cells arise near their definitive positions, some arise outside the prospective domain of the segmental ganglia and thus must migrate into the CNS. Here, we report the results of an analysis of the role of cell migration in gangliogenesis in the leech Theromyzon rude. Segmental ganglia of the ventral nerve cord arise as laterally thickened sheets of tissue lying astride the ventral midline. Particular identified circular and longitudinal muscle fibers, visualized by indirect immunofluorescence using a monoclonal antibody against leech muscle, outline the presumptive ganglionic territories even before the ganglionic rudiments become morphologically distinct and serve as anatomical landmarks to which the cell movements are related. Cell lineage tracers microinjected into precursor blastomeres are used to visualize migratory cells. Small groups of neural precursor cells that arise outside the prospective ganglionic territories migrate with stereotyped timing along stereotyped pathways to reach their definitive positions, and each group of migratory cells gives rise to a stereotyped subset of the cells in a ganglion. No segmental or regional differences are observed in any aspect of cell migration studied here, supporting the view that segmental differences in the architecture of the leech CNS arise only after the initial condensation of the ganglionic rudiments.