APPROPRIATE PATTERN-FORMATION FOLLOWING REGULATIVE REGENERATION IN THE HINDBRAIN NEURAL-TUBE

Patterns of neuronal differentiation and axon outgrowth were examined in regenerated neural tissue of the hindbrain of the chicken embryo. S pecific regions of the hindbrain neural tube corresponding to identifi ed rhombomeres were selectively removed unilaterally by microsurgery, Of the embryos that survived, about 35% exhibited regulative regenerat ion, wherein the missing hemi-rhombomere was reconstituted to normal s ize and morphology through compensatory proliferation and migration fr om adjacent tissue. The capacity for regeneration depended on the embr yonic stage at which the ablation was performed and on whether the mid line floor plate was included in the ablation. Ablations performed pri or to the formation of morphologically overt rhombomere boundaries led to significantly lower frequencies of regulative regeneration than ab lations performed after the formation of boundaries. Ablations that in cluded the floor plate led to significantly lower frequencies of regul ative regeneration than ablations sparing the floor plate. Regulative regeneration was also observed at low frequency following excision of an entire rhombomere. Within the regenerated hemi-rhombomere, identifi ed neuron groups appeared in their normal positions and differentiated normal patterns of migration and axon outgrowth, The fidelity of this patterning, however, depended on the mediolateral position of the neu ron group, being more reliable for neuron groups in the basal plate th an in the alar plate. These results confirm the extensive regulative p otential of the neural tube and demonstrate the capacity for appropria tely patterned cellular differentiation within the regenerated tissue.