Dependence of cranial motor neuron formation on ventromedial brain stem.

The formation of motor neurons in the spinal cord is dependent on inductive signals from the floor plate and notochord. Motor neurons in the brain stem, on the other hand, develop in the absence of both structures. This suggests that either the germinal epithelium is specified intrinsically to form specific cranial motor nuclei or that the inductive signals for the formation of cranial motor neurons arise from some other structure. These possibilities were investigated experimentally by using the formation of trochlear motor neurons in the midbrain of duck embryos as a model system. The trochlear motor neurons, which form the nucleus of the fourth cranial nerve, developed normally after early damage to the prospective germinal epithelium, suggesting that it is unlikely to be specified intrinsically to form these cranial motor neurons. Instead, their development was found to be dependent on the cells within, or associated with, the ventromedial region of the brain stem, as the extirpation of this region results in the absence of motor neuron formation. These results show that structures other than the floor plate and notochord provide inductive signals for the cellular differentiation and patterning of the developing central nervous system. The raise the possibility that the inductive signals for motor neuron differentiation in the spinal cord and the brain stem may not be necessarily identical.