Development of the brachial lateral motor column in the wingless mutant chick embryo: motoneuron survival under varying degrees of peripheral load.

Abstract

Survival of motoneurons in the lateral motor column (LMC) of the chick embryo is known to depend on the periphery. How this dependence relates to the normally occurring death of motoneurons is unknown. Analysis of the time course of LMC cell loss in the absence of varying amounts of limb musculature could help bring about an understanding of this relationship. We undertook this analysis by studying LMC development in wingless chick embryos. Grossly these embryos lack wings, but we have reported that some of them possess more than 40% of the normal volume of wing bud-derived muscles (M.E. Lanser and J.F. Fallon, Anat. Rec. 217:61-78, 1987). In the present work we compared the time course of LMC development in wingless embryos that possessed varying amounts of wing bud-derived musculature with that in normal embryos. In normal embryos little cell loss occurs from the brachial LMC prior to day 8 (15% of the total cell loss). Most of the normal cell loss occurs between 8 and 10 days (62% of the total cell loss). In the wingless LMC, anywhere from 55% to 70% of the total cell loss occurs before day 8. The death of motoneurons prior to day 8 is proportional to the amount of wing bud musculature eliminated by the mutation. Cell loss after day 8 is proportional to the amount of wing bud musculature spared by the mutation. Therefore, when the limb is missing, most motoneurons die before the major period of cell loss even begins in the normal LMC. Counts of dead cells in the LMC also support this conclusion. In addition, curves plotting the rates at which cells are lost from the brachial LMC provide a suggestion that normal cell loss is biphasic and that limb removal enhances primarily the first phase of cell loss. These data suggest that the majority of motoneurons may die for different reasons in the normal and the limb-deprived LMCs. Overall, the number of motoneurons surviving in the brachial LMC is proportional to the volume of wing bud-derived muscle present. However, as the muscle volume approaches zero, motoneuron number does not. This suggests that most, but not all, motoneurons depend on limb bud-derived muscles for survival. Finally, the decreased motoneuron number in the wingless LMC, when compared to normal after the cell death period, cannot be totally accounted for by the additional loss of cells that occurred during the cell death period in the wingless LMC.(ABSTRACT TRUNCATED AT 400 WORDS)