Pattern in the cortical distribution of prefrontally directed neurons with divergent axons in the rhesus monkey.

Neurons with divergent branched axons have been noted in several structures, but their organization across cortical systems, cortical types, or cortical layers is not known. The above questions were addressed with the aid of multiple fluorescent retrograde tracers injected in one hemisphere of the prefrontal cortex of rhesus monkeys. The prefrontal cortex is well suited for this study because it receives input from diverse cortical systems. A small number of neurons (integral of 1%) that projected to two different ipsilateral prefrontal sites, and were thus double-labeled, were noted in prefrontal, premotor, visual, somatosensory, auditory, and parietotemporal association cortices, suggesting that this form of projection is general within the cortex. However, double-labeled neurons were noted in significantly higher numbers in transitional than in eulaminate areas. Moreover, neurons with divergent axons were more prevalent in the deep cortical layers than in the upper layers in both transitional and eulaminate areas. Although neurons with divergent axons constitute a small population of those that issue corticocortical projections in several adult mammalian species, they are preponderant in the primary sensory cortices of the fetal or neonatal stages. Transitional areas, which include a higher proportion of neurons with divergent axons, may retain some features observed in development to a greater extent than eulaminate sensory areas. This could help explain the great plasticity of transitional areas and their involvement in learning and memory, and also their preferential vulnerability in several neurologic disorders such as Alzheimer's disease and epilepsy.