Zebrafish live imaging reveals only around 2% rather than 50% of motor neurons die through apoptosis during development

It is widely accepted that neurons will die through apoptosis if they cannot receive enough growth factors during development of vertebrates; however, there is still no real-time observation showing this dying process in live animals. 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 around 2% of motor neurons died during zebrafish development, which is much lower than the generally believed 50% cell death occurred in embryonic stage of vertebrates. Our data also showed that most of the apoptotic bodies of these dead motor neurons were not cleared by macrophages. These sensor zebrafish can serve as powerful tools to study motor neuron apoptosis in vivo.