Long-distance axon growth ability of corticospinal neurons is lost in a segmentally-distinct manner.

Established experimental lesions in the developing central nervous system (CNS) disrupt microenvironments critical for long-distance axon growth and guidance. Therefore, the precise developmental time at which the CNS ceases its capacity to support long-distance axon growth remains unknown. Accordingly, we established a new microsurgical approach to axotomize developing corticospinal tract (CST) axons in the neonatal spinal cord while leaving their local microenvironments relatively intact. This enables unambiguous investigation of long-distance CST growth ability in the CNS. Surprisingly, we find that CST axons lose their capacity for long-distance growth even during the developmental period of CST extension. While this ability remains intact in spinal locations where normal CST extension is occurring, it is completely abolished at sites distant from these locations. Further, the developmental time window for which this ability is maintained is much shorter than for other forms of axon growth such as sprouting. Long-distance CST growth ability does not correlate with astrocytic or microglial activation, nor with myelination levels. These results indicate that long-distance CST growth is controlled by mechanisms that operate early in development in a time- and region-specific manner.