Survival of motoneurons in the brachial lateral motor column of limbless mutant chick embryos depends on the periphery

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Abstract
Motoneuron survival in the embryonic spinal cord is influenced by the presence or absence of the developing limb bud. We have recently begun a reexamination of the relationship between limb absence and motoneuron survival in a nonsurgical limb deletion model, the limbless mutant chick embryo. As in surgically limb-deleted normal embryos, only 10% of the motoneurons that are initially produced in the limbless mutant lateral motor column (LMC) survive the embryonic period (Lanser and Fallon, 1984). We now report that, when supplied with a normal periphery (i.e., a normal limb bud), more than 40% of the motoneurons initially produced in the limbless LMC survive the embryonic period. Motoneuron cell counts in one-winged limbless embryos reveal that over 3.5 times as many motoneurons survive the cell death period in the LMC on the side with the limb than on the opposite, limbless side. This demonstrates the dependence of embryonic LMC motoneurons on the developing limb for survival and indicates that the limbless mutant is an appropriate model for studying the death and survival of LMC motoneurons during development. Using the limbless mutant to study LMC motoneuron survival eliminates the complication of possible direct surgical effects on motoneuron death. In addition, we found that a substantial effect of the wing on rescuing LMC motoneurons was exerted prior to the 6th day of embryonic development. Normally, little cell loss occurs in the brachial LMC during this time. Accordingly, motoneuron death in the limb-deprived brachial LMC, whether in surgically limb-deleted normal embryos or in genetically limbless embryos, is accelerated with respect to cell death in the normal brachial LMC.