Myelin patterns in V1 and V2 of normal and monocularly enucleated monkeys.

A pattern of alternating light and dark columns was observed in wet, unstained sections of macaque striate cortex after monocular enucleation. The columns were clearest in layer IV, but could be detected through the full thickness of the cortex. Subsequent processing for cytochrome oxidase (CO) showed that the light columns in wet sections viewed under darkfield illumination matched the ocular dominance columns serving the enucleated eye. These columns labeled preferentially with an antibody to myelin basic protein, suggesting that greater myelin content accounted for their brighter appearance. However, when sections were counterstained with luxol fast blue, Gallyas and Woelcke myelin techniques, the enucleated eye's columns appeared pale. It is unclear why classical myelin stains and myelin basic protein immunohistochemistry yielded opposite results. Discrepant patterns of myelin distribution were also found in normal animals using different myelin stains. Luxol fast blue showed homogeneous staining in layer IVc of macaque striate cortex, but the Gallyas stain revealed a pattern of thin pale bands alternating with wide dark bands, matching the pattern seen with the Liesegang stain. The CO patches in layers II and III fit in register with the wide dark myelin bands. In layers II and III of striate cortex, the Gallyas and luxol fast blue methods both labeled the CO patches. However, in squirrel monkey V2 the Gallyas stain labeled the pale CO stripes, whereas luxol fast blue labeled the dark CO stripes. These results indicate that pattern of myelin staining in visual cortex can vary according to the choice of technique, and may not reflect the true distribution of myelin. Studies of myeloarchitecture should employ a variety of myelin techniques, including examination of unstained sections, to obtain the most accurate picture of cortical myelin content.