Selective elimination of transient corticospinal projections in the rat cervical spinal cord gray matter.

In the present paper a description is given of the development of the rat corticospinal tract (CST) in the lower cervical spinal cord. This area contains, among other cells, the motoneurons innervating the distal forelimb muscles. HRP gels were implanted in the sensorimotor cortex of Wistar rats varying in age from postnatal day 0 (P0) to P60. After a survival period of 48 h, the rats were transcardially perfused, the spinal cords transversely sectioned at 30 microns and the sections reacted for HRP. Labelled CST axons in the dorsal funiculus were first detected at P2, and after a delay of 2 days the first fibres were found in the adjacent gray matter (P4). More labelled fibres were gradually added until maximal number and extension was reached at P10. By then the entire gray matter and large parts of the white matter were covered by labelled CST axons. From P10 onwards, the number of labelled CST fibres as well as their extension decreased. In the adult rat, some areas such as the lateral part of the ventral and dorsal horn and large parts of the ventral and lateral white matter ultimately became devoid of labelled CST axons. It is concluded that a massive overshoot occurs during the development of the terminal field of the rat CST. The results are discussed in conjunction with our previous findings on the development of the motoneurons innervating the rat distal forelimb muscles. The concurrent selective elimination of both CST axons and motoneuron dendrites is suggested to be correlated with progressively more mature, coordinated movements and with high digital skills especially.