Regeneration of corticospinal axons in the rat.

In the rat, afew long descending motor tracts capable of carrying an impulse and causing apropagated impulse in the ipsilateral sciatic nerve will regenerate after complete spinal cord transection. In this experiment such regeneration was found in both treated and control animals. Orthograde axonal transport of tritiated proline injected into the motor cortex labels only the corticospinal tracts in the rat spinal cord. Scintillation counts of measured lengths of spinal cord can be used as a measure of the number of labeled corticospinal axons. Comparison of radioactivity per unit length of measured cord segments taken from above and below the site of a previous spinal cord transection can give a reliable estimate of the number of labeled axons that regenerated and crossed the site of injury. Using this test we have demonstrated that some corticospinal axons had regenerated six months after spinal cord transection in control animals, animals made tolerant to degenerating spinal cord antigens, and animals treated with cyclophosphamide. A group treated with a single 7 5 mg per kilogram dose of cyclophosphamide 24 hours after spinal cord transection showed the best evidence of corticospinal tract regeneration.