Segmentation of the central nervous system in leech

Central nervous system (CNS) in leech comprises segmentally iterated progen y derived from five embryonic lineages (M, N, O, P and Q), Segmentation of the leech CNS is characterized by the formation of a series of transverse f issures that subdivide initially continuous columns of segmental founder ce lls in the N lineage into distinct ganglionic primordia. We have examined t he relationship between the N lineage cells that separate to form the fissu res and lateral ectodermal and mesodermal derivatives by differentially lab eling cells with intracellular lineage tracers and antibodies. Although sub sets of both lateral ectoderm and muscle fibers contact N lineage cells at or near the time of fissure formation, ablation experiments suggest that th ese contacts are not required for initiating fissure formation. It appears, therefore, that this aspect of segmentation occurs autonomously within the N lineage. To support this idea, we present evidence that fundamental diff erences exist between alternating ganglionic precursor cells (nf and ns pri mary blast cells) within the N lineage. Specifically, ablation of an nf pri mary blast cell sometimes resulted in the fusion of ipsilateral hemi-gangli a, while ablation of an ns primary blast cell often caused a 'slippage' of blast cells posterior to the lesion. Also, differences in cell behavior wer e observed in biochemically arrested nf and ns primary blast cells. Collect ively, these results lead to a model of segmentation in the leech CNS that is based upon differences in cell adhesion and/or cell motility between the alternating nf and ns primary blast cells. We note that the segmentation p rocesses described here occur well prior to the expression of the leech eng railed-class gene in the N lineage.