Protracted postnatal development of corticospinal projections from the primary motor cortex to hand motoneurones in the macaque monkey.

We have studied the development of corticospinal projections from the hand area of the primary motor cortex to the spinal cord using anterograde transport of WGA-HRP. In the neonate, as in the adult, corticospinal projections to the intermediate zone at the C8/T1 spinal level were clearly present. However, in contrast to the adult, there was only very faint and barely visible labelling in the dorso-lateral motor nuclei which supply the hand muscles. No aberrant projections to other motor nuclei were seen. By 2.5 months, a ring of dense labelling was present around the dorso-lateral motor nuclei, but labelling was still sparse in the central region. This labelling was more pronounced at 11 months, but was still not as heavy as in the adult. There was no labelling among the ventral motoneurones at any age. The conduction velocity (c.v.) of the fastest corticospinal fibres was determined in each of the monkeys. There was an age-related increase in c.v. within the spinal cord. At birth, the fastest axons had a c.v. of only 8 m.s-1. At 11 months c.v. was still substantially slower (55 m.s-1) than the adult value of 73 m.s-1. In contrast, by 11 months, the axonal c.v. within the brain was close to the adult value, suggesting a rostro-caudal maturation of the corticospinal system. Our results demonstrate that corticospinal projections in the macaque monkey mature gradually over a period of at least 11 months, much longer than previously thought.