Cell death of motoneurons in the chick embryo spinal cord. VIII. Motoneurons prevented from dying in the embryo persist after hatching.

A chronic neuromuscular blockade during those embryonic stages when naturally occurring spinal motoneuron death occurs, results in the prevention of this cell loss. The excess motoneurons are maintained as long as the neuromuscular blockade is continued; once embryonic neuromuscular activity resumes, however, the excess motoneurons undergo a delayed period of cell death. By contrast, the resumption of neuromuscular activity in these same preparations after hatching did not result in a delayed cell death. The excess motoneurons, prevented from dying in the embryo, persisted for as long as 4 days postnatally despite the presence of considerable limb motility. The maintenance of motoneurons may be regulated differently before and after hatching.