Timing of motoneuron death in the brachial and lumbar regions of the chick embryo.

Motoneuron death was studied in the brachial and lumbar regions of the chick embryo spinal cord. Degenerating cells were found at the earliest stage examined in both regions (stage 29 in the brachial region and stage 28 in the lumbar region). In the lumbar region this was accompanied by a large decrease in observed motoneuron number from stage 28 until stage 36. In the brachial region only a slight fall was seen before stage 34, suggesting that the observed count may not be a reliable measure of motoneuron death at these early stages. In contrast to the lumbar region, motoneuron death continued until stage 44 in the brachial region. This difference at the later stages was shown, by amputating the early limb bud, to be unrelated to the time at which motoneurons become limb-dependent.