Postnatal redistribution of pericruciate motor cortical projections within the kitten spinal cord

The anatomical distribution of pericruciate cortical axons in the spinal cord was examined using anterograde transport of WGA-HRP from multiple unilateral injections into defined regions of the pericruciate cortex (PC) in 20 time-mated kittens and 3 adult cats. The gray matter and descending white matter tract distributions of WGA-HRP-labelled densities were analyzed using computerized morphometry and 3-dimensional reconstruction. In kittens older than 16 days postnatal (dPN), PC axon densities were found in dorsolateral column tracts corresponding to those of the adult, indicating that the white matter projections from the PC were largely established by this age. However, in kittens less than 38-44 dPN (about 105-109 days gestation), the spinal gray matter PC axon densities were distributed widely (so as to involve dorsal, intermediate and ventral laminae, e.g., I to IX) and bilaterally at all levels of the spinal cord. This contrasted sharply with the adult spinal cord in which the majority of densities was localized to laminae IV-VII on the contralateral side. The proportion of PC densities counted in the ipsilateral grey matter of neonates was found to average 23% of the total gray matter projection, while in adults this value was 9%. In all animals, by about 44 dPN, the terminal fields became effectively restricted to the adult distribution, that is, focused predominantly in the medial portions of laminae IV-VII in the gray matter contralateral to the cortical injection site.