Topographic sequence of outgrowth of corticospinal axons in the rat: a study using retrograde axonal labeling with Fast blue.

The retrogradely transported dye, Fast blue, was injected into cervical or lumbar segments of the spinal cord of rats during the first days of life in order to label the cell bodies of origin of the corticospinal tract which is growing down the cord during that period. The first corticospinal axons arrive at cervical levels immediately after birth and all arise from a circumscribed group of layer V pyramidal cells in a small region of dorsal parietal cortex. This same cell group provides the corticospinal projection to lumbar segments of the spinal cord, the axons reaching those segments at the end of the first postnatal week. The area of lumbar projecting cells undergoes relatively little expansion and no diminution during subsequent weeks and into adulthood. The area occupied by cortical cells projecting to the spinal cord expands during the first postnatal week, but the axons of all these additional cells do not appear to invade the lower sequents of the spinal cord. By the end of the first week, corticospinal cells can be labeled in a continuous sheet throughout most of the extent of the frontal, parietal and cingulate cortex. During the second and third postnatal weeks, the area sending axons to the upper levels of the spinal cord diminishes and large areas bereft of retrogradely labeled corticospinal cells appear: laterally, in lateral frontal and lateral parietal cortex; dorsally, at the border of frontal and parietal cortex; medially, in medial frontal and cingulate cortex. The more restricted adult pattern is established at the end of the third week. Hence, the first cortical axons to advance down the spinal cord are those that will innervate the lumbar segments in the adult. Later addition of corticospinal axons involves only those projecting to upper cord segments. Within this group there are those which will establish persistent connections from appropriate cortical areas and others that will shortly be eliminated from inappropriate areas.