Autoradiography of 3H-thymidine incorporation was combined with horseradish peroxidase (HRP) transport to distinguish the birthdates of motoneurons and internuclear neurons of the abducens nucleus, and of specific motor pools within the oculomotor nucleus. Motoneurons were identified by their retrograde transport of HRP from the extraocular muscles. In other experiments, internuclear neurons of the abducens nucleus were identified by their retrograde transport of HRP from the oculomotor nucleus. We found that motoneurons and internuclear neurons are generated simultaneously in the abducens nucleus, and suggest that the differentiation of these two neuron types may be controlled by the local environment. The motor pools of the oculomotor nucleus are generated sequentially. This may reflect the mechanism whereby nuclei are constructed.
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