Morphology of M-cell axon arbors in striate cortex of monkeys reared with monocular aphakia

Although the effects of visual deprivation on the development of ocular dominance columns have been well described in primates, nothing is known in primates about the impact of the deprivation on the axonal profiles that make up the ocular dominance columns. We now show that the effects of monocular deprivation on the morphology of geniculostriate axons involve not only shifts in terminal arbor sizes, much as would be expected from the ocular dominance data, but also changes in the proliferation of terminal arbor branches. In macaque monkeys reared from birth with unilateral lens removal (aphakia), terminal arbors of geniculostriate axons were bulk-filled with horseradish peroxidase (HRP) in brain-slice preparations and reconstructed from serial sections through striate cortex (area 17). Our focus was on the arbors that terminate in the upper tier of layer IV, the target of cells in the magnocellular (M) layers of the LGN. Of the 26 M-cell arbors reconstructed from three aphakic monkeys, eight were unique in having few very simple terminal arbor branches. These also tended to be smaller in total extent than the average M-cell axons reconstructed from 1 normal monkey. In contrast, eight arbors had very rich terminal branching patterns, and seven of these were larger than any of those from the normal monkey. We propose that the small, sparse axon arbors are related to the deprived eye, and the large, dense arbors are related to the non-deprived eye. These morphological changes reflect abnormalities in the growth patterns of geniculostriate inputs that undoubtedly have important persisting consequences for visual performance.