Development of the lateral motor column in the limbless mutant chick embryo.

This is a report on the development of the lateral motor column (LMC) in the limbless mutant chick embryo. The limbless mutant was used to study the effects of the absence of a periphery on the developing nervous system. The limbless mutant provides a unique opportunity to compare the effects on the LMC of deletion of a limb caused by the genotype with those seen following surgical removal of the limb primordium. Cell counts of the total number of motoneurons in the LMC at both the brachial and lumbar levels were done in a large series of limbless embryos and on their normal siblings. In normal embryos, a substantial loss of LMC motoneurons was observed during the course of normal development. At the brachial level, 54% of the initial LMC cell population was lost between day 6 and day 18. At the lumbar level, 40% of the initial population was lost between the 6th and 12th days of development with no further loss through day 18. An even more massive cell loss was observed in the limbless mutant LMC at both brachial and lumbar spinal cord levels between day 5 and day 12. This resulted in the elimination of at least 85% of the motoneurons that were initially present in the limbless LMC. Our data demonstrate that the effects of peripheral deprivation on LMC development in the limbless mutant are similar to those seen following surgical removal of the periphery. The initial production of motoneurons and assembly of the LMC did not appear to be significantly affected by the mutation, while the subsequent degeneration of LMC motoneurons is greatly accelerated and increased in comparison to the normal.