FIGURE 1.

Long cellular protrusions play a role in the spatiotemporal patterning of zebrafish spinal neuron differentiation. (A) Live in vivo imaging in the zebrafish embryonic spinal cord of a single neuron labeled with a membrane marker. The cell body positions to the basal surface of the neuroepithelium while maintaining an attachment to the apical surface (dashed line; 0:00). The neuron extends two long protrusions along the basal surface, one anteriorly and one posteriorly (0:00–5:00). Each basal protrusion spans several-cell diameters. Both are retracted into the cell body (6:30–8:00), along with the apical attachment (asterisk; 8:30), before the neuron extends an axon (arrow; 9:30–11:30). (B) Diagrammatic working model of transient long-distance lateral inhibition delivered via basal protrusions. t1: A differentiating neuron expresses Delta (gray cytoplasm) and begins to extend basal protrusions. Delta signaling from the basal protrusions induces Notch signaling in the neighboring neuroepithelial cells that they contact, inhibiting their neuronal differentiation (lateral inhibition delivered by basal protrusions is represented by pink signs). t2: The basal protrusions grow to span several-cell diameters and inhibit the neuronal differentiation of neuroepithelial progenitors at a distance. t3: Retraction of the basal protrusions occurs before axon initiation, releasing the neuroepithelial cells that receive least contact with basal protrusions to differentiate.