Motoneuron death in normal and spinal muscular atrophy-affected human fetuses.

The onset and sequence of motoneuron death in the lumbar part of spinal cord of seven control and five affected fetuses carrying deletion of exon 7 in the SMN gene were studied by light and electron microscopy. Naturally occurring motoneuron death in control fetuses was detected between 8 and 13 weeks of gestation. In affected fetuses motoneuron death was prolonged and also observed at 16 and 20 weeks of gestation. In addition, motoneurons in the affected fetuses displayed nuclear abnormalities as early as 16 weeks of gestation and the changes progressed as the affected fetuses developed and can be considered first signs of motoneuron degeneration.