Postnatal growth of corticospinal axons in the spinal cord of developing mice

The corticospinal tract (CST) plays an important role in the control of vol untary movements. Although the development of the CST has been studied exte nsively in other species, limited information is available on its developme nt in mice. In the present study, the growth of corticospinal axons was cha racterized in developing mice using Phaseolus vulgaris leucoagglutinin (PHA -L). Our results indicate that the leading CST axons reach the 8th cervical segment at postnatal day (PD) 2, the 7th thoracic segment at PD4, the 13th thoracic segment at PD7, and the 5th lumbar segment at PD9. The arrival of corticospinal axons at the distal lumbar cord at PD9 was further confirmed by retrograde tracing using fast blue (FB). A waiting period of 2-3 days e xists after the leading CST axons pass a particular segment before sending collaterals into the gray matter of that segment. The CST continues to incr ease in size in lower thoracic and lumbar areas up to PD14 when its adult a ppearance is achieved. In this study, the date of animal's sacrifice was us ed as the specific postnatal date to demonstrate the growth of the CST. Thi s definition gives a more reliable indication of the exact location of the CST at a specific developmental time point since the CST continues to grow after tracer injections and since the dye is transported much faster than a xonal growth. We suggest that these findings can be used as a template for studies on both normal and transgenic mice where some developmental signifi cance is given to the CST. (C) 1999 Elsevier Science B.V. All rights reserv ed.