Cytochemical Neuron Differentiation *

Knowledge of the mechanisms regulating the sequence of changes whereby neuroblasts differentiate into neurons is fundamental to an understanding of how reflex arcs and higher association pathways become organized. The manner and extent to which the development of neurons is regulated by intra-cellular, presumably genetic, mechanisms and through the interaction between cells, either those of the peripheral field or other neurocellular elements, remain largely unknown. Observations on the normal sequential changes in form' or chemistry2' of these cells have not provided information pertaining to the mechanisms regulating the changes described. However, many experiments based on the enumeration of cells2' 8 11,2,35 show the number of motor neurons ultimately present in the spinal cord of the chick is roughly proportional to the volume of the peripheral field. Further, by correlating the form, spatial arrangement, and sequence of occurrence of unipolar and bipolar neuroblasts, one of the investigators2 arrived at the suggestion that the peripheral field regulates the final number of neurons present in the motor horn by a process of selective induction of indifferent cells to differentiate into neurons. Unipolar neuroblasts in the motor area became bipolar about the time that their axons reached the uninnervated periphery and, as these neuroblasts became bipolar, those indifferent cells medially adjacent to the now bipolar cells began to develop axons. Accordingly, a reduction of the volume of the peripheral field (by limb bud removal) terminates the process of induction earlier by limiting axon outgrowth, and fewer neuroblasts are called upon to differentiate. Additional support for the induction hypothesis has been presented by Mottet.n Our earlier experiments,"' though they indicate the progressive changes in the form of neuroblasts that are induced by the peripheral field, provide no evidence regarding the mechanisms responsible for these changes; that is to say, they give no evidence of the dynamics of cell metabolism associated