Terminal arbors of individual "feedback" axons projecting from area V2 to V1 in the macaque monkey: a study using immunohistochemistry of anterogradely transported Phaseolus vulgaris-leucoagglutinin.

In the present study, the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected into area V2 in order to demonstrate the precise morphology of individual axons from area V2 to V1. On the basis of 28 complete axon reconstructions, several characteristic features have been identified. 1) Individual axons arborize in multiple layers: 1, 2, 5, and (inconstantly) 3. A single axon may have numerous terminal clusters in layers 1 and 2, but at most one in layer 3. 2) Axons typically ascend to layer 1, turn asymmetrically in one direction, and travel for long distances in this layer (1.10-4.30 mm; dimensions uncorrected for shrinkage). A few axons (three of 28 reconstructed) were found to have a single terminal cluster (0.3-0.5 mm wide) in layers 1 and 2. 3) Collaterals in layer 5 seem to extend over shorter distances (0.60 mm or less). 4) Delicate sprays of boutons (both beads and spines) are clustered along the main trunk. Spacing is variable but usually ranges from 0.35 mm to 0.65 mm. 5) In addition to clustered boutons, there can be linear collaterals, continuously studded with boutons, parallel to the main axon in layer 1. These results indicate that axons from V2 have complex radial and tangential distributions in V1. Terminations in different layers may be directed to different sets of neurons or to different portions of the dendritic tree (for example, distal portions of pyramidal neuron apical dendrites in layers 1 and 2, but more proximal portions in layer 3). Clustered terminations over wide tangential areas may imply a divergent innervation by a single axon of multiple compartmental structures, such as ocular dominance columns or cytochrome oxidase patches.