Postnatal development of the rat corticospinal tract, with special reference to confocal laser scanning microscopy of growing axons.

The corticospinal tract within the spinal cord of the neonatal rats, ranging from postnatal day 1 (P1) to P11, was investigated by using anterograde labeling with DiI optimized by confocal laser scanning microscopy. This method enabled us to observe a thick section containing a large population of the labeled axons and to acquire fine structure images with sufficient resolution. As advantages, novel views of diversity emerged in the developing axons and their collaterals. First, the individual parent axons showed unrestrained trajectories within the main bundle which constantly occupied the ventral part of the dorsal column. Second, there was an aberrant bundle which ran ventrally apart from the main bundle and converged into it. Third, several projection collaterals branched from the parent axons and left the main bundle in various directions, whereas the majority of them branched off at approximately a right angle. In addition, the main bundle consisted of a small number of pioneer axons and numerous follower ones. The pioneer axon tips were first found in the cervical level on P1, in the thoracic level on P2, in the lumbar level on P5 and in the sacral level on P7. The follower axon tips were observed in a wide range away from the pioneer tips to just behind them. Projection collaterals were arborized within the gray matter and their arbors entered the dorsal horn, the intermediate substance or the ventral horn in the transverse plane.