Neurogenesis of spinothalamic and spinocerebellar tract neurons in the lumbar spinal cord of the rat.

The temporal and spatial neurogenic patterns of spinothalamic and spinocerebellar neurons were determined in spinal cord segment L1 of the rat. Neurogenic patterns were demonstrated with [3H]thymidine administered to fetal rats during the period when neurons with supraspinal projections are known to be generated, i.e. on one of embryonic (E) days E13, E14, or E15. The animals were allowed to survive 50 to 100 days postpartum, then neurons with spinothalamic and spinocerebellar projections were retrogradely filled with fluorescent axonal tracers, Fluoro-Gold or True blue, which were pressure injected into the dorsal thalamus and cerebellum in various combinations in the same and in separate animals. Neurons labeled with each retrograde tracer and [3H]thymidine and neurons labeled with retrograde tracers alone were counted in spinal cord segment L1 in each of the animals. Spinothalamic and spinocerebellar neurons were found to be separate and distinct populations. Statistical analysis of the data showed that spinothalamic and spinocerebellar neurons also have distinctly different patterns of neurogenesis which suggest early determination in each cell line. The temporal neurogenic pattern followed a projection-distance gradient, such that spinothalamic neurons, which have longer axons than spinocerebellar neurons, completed neurogenesis prior to spinocerebellar neurons in each region of the spinal gray.