Development of projection-specific interneurons and projection neurons in the embryonic mouse and rat spinal cord

Interneurons and projection neurons in the lumbar spinal cord of mouse and rat embryos were labeled retrogradely with fluorescent dextran amines from a distance of one segment from the segment of origin [lumbar segment (L) 2]. Six classes with specific axonal projections (ipsilateral ascending, descending, and bifurcating, and commissural ascending, descending, and bifurcating) were identified by differential labeling in both species and followed from embryonic day (E)12 to birth in the mouse. Neurons with shorter projections (intrasegmental interneurons) were not studied. We show that the four nonbifurcating neuron classes occupy characteristic, partially overlapping domains in the transverse plane, indicating a systematic pattern of migration and settlement related to axon trajectories. The number of neurons in each of the nonbifurcating classes increased steadily during development. Bifurcating neurons represented a minor fraction of the total throughout development and had relatively scattered positions within the ipsilateral and commissural neuron domains. Combination of retrograde tracing and immunohistochemistry for the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) showed that none of the spinal neurons in the six projection-specific classes was GABA positive, suggesting that all GABA-positive spinal neurons, including previously described GABA-positive commissural neurons, are unlikely to have projections exceeding one or two segments in either direction.