Mammalian motoneuron development: effect of peripheral deprivation on motoneuron numbers in a marsupial.

In nonmammalian vertebrates, the survival of developing motoneurons is dependent on their contacting appropriate target cells. It is generally accepted that developing mammalian motoneurons have a similar dependency on their target, but as yet there is little experimental evidence to support this contention. This is mainly because of the difficulty of experimenting on eutherian embryos. We have, therefore, been studying neuronal development in the tammar (an Australian marsupial) as its nervous system is immature at birth. Radical or partial removal of hindlimb buds from newborn tammars resulted in an increased motoneuron cell death. The motoneurons which survived in the operated tammars did so by innervating muscle remnants. In the instances where a group of muscles was totally removed, the corresponding motonuclei appeared to be totally lost. This study supports the hypothesis that mammalian motoneurons must contact their appropriate muscle in order to survive through the period of natural neuronal cell death.