Zebrafish live imaging reveals a surprisingly small percentage of spinal cord motor neurons die during early development

It is widely accepted that large numbers of neurons die during the early development of vertebrates; however, the tracking of this dying process in live animals remains challenging. Here, we generated sensor zebrafish achieving live imaging of motor neuron apoptosis at single- cell resolution. Using these sensor zebrafish, we observed for the first time that in an apoptotic motor neuron, caspase-3 activation occurred quickly within 5-6 min and at the same time between the cell body and axon. Interestingly, we found that only a surprisingly small percentage of spinal cord motor neurons died during zebrafish early development, which is quite different from the generally believed massive motor neuron death occurred in the embryonic stage of chicks, mice, rats, and humans. We also observed that most of the apoptotic bodies of dead motor neurons were not colocalized with macrophages. These sensor zebrafish can serve as powerful tools to study motor neuron apoptosisin vivo.