Postnatal development of the ipsilateral corticospinal component in rat spinal cord: a light and electron microscopic anterograde HRP study.

Most developmental and regeneration studies using the corticospinal tract (CST) as a model are focussed on its major contralateral component in the ventralmost part of the dorsal funiculus. However, for a correct interpretation of the data in these and future studies, a complete anatomical description of the rat CST, including its ipsilateral uncrossed component, is a prerequisite. In this study we used anterograde horseradish-peroxidase (HRP) tract-tracing at the light and electron microscopic level to describe the development of the uncrossed ipsilateral CST component. In the far caudal portions of the medulla oblongata, most of the CST fibers decussate dorsally and medially to assume positions in the ventralmost part of the dorsal funiculus. However, a small number of labeled fibers continue uncrossed in a position just lateral to the ventral median fissure. Labeled ipsilateral CST fibers caudally extend into the ventral funiculus of the third thoracic segment at postnatal day 1 (P1), into the seventh thoracic segment at P3, and into the first lumbar segment at P5 and P7. After the first postnatal week labeled ipsilateral CST fibers gradually disappear from upper lumbar, lower-, and mid-thoracic spinal cord levels, respectively. From P14 through adulthood the ipsilateral CST projection can not be visualized below upper thoracic spinal cord levels. Quantitative analysis of the amount of label along the length of the outgrowing ipsilateral CST revealed a characteristic staggered pattern of outgrowth: a small number of fibers take the lead and additional fibers are successively added. The outgrowth of labeled ipsilateral CST fibers is mainly restricted to spinal cord white matter. Occasionally a few outgrowing fibers into the adjacent cervical spinal gray can be observed, however, only between P3 and P7. Electron microscopic observations at the cervical enlargement (fifth cervical segment) demonstrate that the first labeled ipsilateral CST fibers, at P1, are characterized by the presence of growth cone-like enlargements at their distal endings. The process of myelination of labeled ipsilateral CST axons starts at about day P14. Although myelination seems to be largely completed at P28, labeled unmyelinated axons are still present in the adult ipsilateral CST component.