Postnatal Development of the Corticospinal Tract in the Reeler Mouse

Corticospinal tract (CST) neurons are dislocated in the motor cortex of Reelin-deficient mouse, reeler. In the present study, we examined whether postnatal axonal growth arising from these dislocated CST neurons are normal or not with use of anterograde tracer, DiI and retrograde tracer, HRP. A single injection of DiI into the motor cortex of the normal and reeler mice was made during postnatal period and 8-24 hours later, the animals were sacrificed to examine DiI-labeled CST axons at the lower medulla and spinal cord. Both in the normal and reeler mice, CST axons arrived at the pyramidal decussation and entered into the contralateral spinal cord around on postnatal day (P) 0.5, and descend in the ventral area of the contralateral dorsal funiculus at C2 level on P2, at C8 level on P3, at the mid-thoracic level on P4, and at the upper lumbar level on P8. The similar results were also demonstrated by the retrograde labeling of CST neurons with injection of HRP into the C1 level or upper lumbar enlargement. Next, we examined CaMKIIα expression in the CST axons of the adult normal and reeler mice. CaMKIIα-immunopositive fibers were recognized throughout the CST pathway from the internal capsule to the dorsal funiculus of the spinal cord both in the normal and reeler mice. The present study has demonstrated that ectopic location of cell bodies of reeler CST neurons do not affect postnatal development of CST axons in the spinal cord.