L1 and GAD65 are expressed on dorsal commissural axons in embryonic rat spinal cord

Using immunocytochemical methods, the cell adhesion molecule L1 was detecte d on axons crossing in the dorsal commissure of developing rat spinal cord. Immunoreactive axons were found in locations similar to fiber bundles illu strated by Ramon y Cajal and designated the anterior, middle and posterior bundles of the dorsal commissure. L1-immunoreactive dorsal commissural axon s were first observed on embryonic day 17 (E17), appeared more numerous by E19, and remained detectable in early postnatal ages. The massive middle ax on bundles extended bilaterally from the dorsolateral funiculi towards the midline and crossed in the central part of the commissure. In horizontal se ctions, bundles of L1-labeled middle axons were observed to traverse the do rsal commissure in a periodic pattern along the entire rostrocaudal extent of the spinal cord. Bundles of glutamic acid decarboxylase (GAD65)-positive axons were detected crossing in the middle and posterior regions of the do rsal commissure between E17 and E20. Results from double-labeling experimen ts demonstrated that GAD65-positive fibers were embedded in larger bundles of L1-labeled axons and that some dorsal commissural axons were double-labe led. To determine if there ware axons crossing in the dorsal commissure tha t did not express L1, double-labeling experiments were conducted using neur ofilament and L1 antibodies. Results indicated that bundles of axons identi fied with anti-neurofilament antibodies were also L1-positive, whereas indi vidually coursing axons within the commissure were L1-negative. The predomi nance of L1 on fiber bundles traversing the dorsal commissure adds to the g rowing evidence that this molecule may play a role in axon outgrowth and fa sciculation. (C) 2000 Elsevier Science B.V. All rights reserved.