DEVELOPMENT OF CORTICOSPINAL TRACT FIBERS AND THEIR PLASTICITY .1. QUANTITATIVE-ANALYSIS OF THE DEVELOPING CORTICOSPINAL TRACT IN MICE

This study was undertaken to elucidate ultrastructurally and quantitat ively the development of the corticospinal tract (CST) axons of mouse at the intumescence level of the cervical cord. An anterograde HRP stu dy showed that the CST was located at the ventral one-third of the dor sal funiculus, and a few HRP-positive fibers were noted at the medialm ost part of the ipsilateral anterior funiculus. Ultrastructurally, the CST was composed of unmyelinated axons, growth cones and a few degene rating axons until postnatal day 10 (P10), then the axons in CST gradu ally increased in size. The number of axons constituting the right CST was calculated at different days of age. The total numbers of axons a t P0, P4, P14, P21 and P56 were 2.3 x 10(4), 6.2 x 10(4), 10.4 x 10(4) , 7.1 x 10(4) and 3.5 x 10(4), respectively. These results indicate th at the number of CST axons at the cervical intumescence of mouse becom es maximum at P14, and then decreases rapidly to reach the adult level of 3.5 x 10(4) (at P56), about 68% of them thus being lost.