Fig. 7. Model for Delta-Notch regulated specification of pMN precursors for motoneuron and oligodendrocyte fates. Primary motoneurons (pmn) are the first cells to differentiate and activate Notch in neighboring cells via Delta. Notch activation maintains these cells as precursors during the period of primary motoneuron specification. Downregulation of Delta expression in differentiating primary motoneurons results in loss of Notch activity in some precursors, which then adopt secondary motoneuron (smn) fate. These cells upregulate Delta expression, maintaining high Notch activity in remaining precursors, which are specified as OPCs. Maintenance of high Notch activity in OPCs inhibits their differentiation into premyelinated oligodendrocytes (pre oligo). In dla^-/-;dld^-/- embryos, which have reduced levels of Notch activity, excess pMN precursors are specified as primary motoneurons at the expense of secondary motoneurons and oligodendrocytes. This model does not account for possible lineage relationships between different cell types.

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