POSTNATAL-DEVELOPMENT OF CORTICOSPINAL PROJECTIONS FROM MOTOR CORTEX TO THE CERVICAL ENLARGEMENT IN THE MACAQUE MONKEY

The postnatal development of corticospinal projections was investigate d in 11 macaques by means of the anterograde transport of wheat germ a gglutin-horseradish peroxidase injected into the primary motor cortex hand area. Although the fibers of the corticospinal tract reached all levels of the spinal cord white matter at birth, their penetration int o the gray matter was far from complete. At birth, as in the adult, co rticospinal projections were distributed to the same regions of the in termediate zone, although they showed marked increases in density duri ng the first 5 months, The unique feature of the primate corticospinal tract, namely direct cortico-motoneuronal projections to the spinal m otor nuclei innervating hand muscles, was not present to a significant extent at birth, The density of these cortico-motoneuronal projection s increased rapidly during the first 5 months, followed by a protracte d period extending into the second year of life. The densest corticosp inal terminations occupied only 40% of the hand motor nuclei in the fi rst thoracic segment at 1 month, 73% at 5 months, and 75.5% at 3 years . A caudo-rostral gradient of termination density within the hand moto r nuclei was present throughout development and persisted into the adu lt. As a consequence, the more caudal the segment within the cervical enlargement, the earlier the adult pattern of projection density was r eached. No transitory corticospinal projections were found, The contin uous postnatal expansion of cortico-motoneuronal projections to hand m otor nuclei in primates is in marked contrast to the retraction of exu berant projections that characterizes the development of other sensory and motor pathways in subprimates.