Spinal neurogenesis and axon projection: a correlative study in the rat.

The purpose of the present study was to determine the relationship between the duration of a spinal neuron's neurogenic period and the length of its axon or level of projection. Spinal segment L1 was chosen for examination and neurons were divided into four projection groups: 1) supraspinal projection (SSp), 2) long ascending propriospinal (LAPr), 3) short ascending propriospinal (SAPr), and 4) descending propriospinal (DPr). To determine the duration of the neurogenic period for each group, 3H-thymidine was administered to fetal rats during the proliferative period for spinal neuroblasts on one of embryonic (E) days E13 through E16. Between 50 and 100 days after birth neurons in each group were labeled with the retrograde fluorescent tracer Fluoro-Gold. To demonstrate nerve cells with SSp projections, spinal cords were hemisected at spinal segment C3 in one group of animals and Fluoro-Gold was applied to the sectioned surface of the cord. Three additional sets of animals were used to label nerve cells with LAPr, SAPr, and DPr projections by injecting Fluoro-Gold into the gray matter at spinal segments C6, T12, and L5, respectively. Neurons labeled with both Fluoro-Gold and 3H-thymidine and neurons labeled with Fluoro-Gold alone in each animal in each group were counted and the data statistically analyzed. Results showed that within each spinal lamina neurons with different projections were generated, i.e., completed cell division, at significantly different rates. Neurons with the longest axons, those with SSP projections, were generated first. These were followed by those with LAPr projections, and finally those with SAPr and DPr projections. In most laminate there was no significant difference between the neurogenic periods of rostrally projecting short propriospinal (SAPr) neurons versus caudally projecting short propriospinal (DPr) neurons. It was concluded that the duration of the neurogenic period for a given group of neurons within each spinal lamina is inversely related to the distance between the nerve cell and its projection site regardless of the direction of its projection.