Differences in the critical period for regenerative growth of supraspinal axons across the lesion site after transection of the thoracic cord in developing opossums

When the spinal cord is transected in adult opossums, regeneration of cut axons does not occur. Regeneration of descending axons has been documented, however, when the lesion is made early in development. In the present study, we asked whether the critical period for regenerative growth is the same for all axons. Opossums are useful for such studies because they are born in a very immature state and, after appropriate anesthesia, their spinal cord can be manipulated in a feta-like stage of development without intrauterine surgery. Supraspinal neurons that innervate the lumbar cord on PD5, 15 and 20 were prelabeled by lumbar injectons of Fast Blue (FB). Five days later, FB was removed and the spinal cord was transected at thoracic levels. Fourteen days later, Rhodamine B Dextran (RBD) was injected between the site of the* FB injection and the lesion. The pups were maintained for an additional 7-10 days before sacrifice and perfusion. We assumed that FB labeled neurons in the brain survived axotomy and that those which also contained RBD supported regenerated axons. In the PD5 group (lesioned at PD10), regenerative growth could be documented for axons originating in all of the supraspinal nuclei that innervate the lumbar cord by PD10. When the injections and lesions were made at the later ages, however, neurons which supported regenerative growth were fewer in number and regionally restricted. In some cases, they were limited primarily to the red nucleus and raphe. Our results support our previous contention that regeneration of cut axons contributes to growth across the lesion after transection of the thoracic cord in developing opossums, but they also show that the critical period for regeneration is not the same for all axons.