DEVELOPMENT OF THE VISUAL CALLOSAL CELL DISTRIBUTION IN THE RAT - MATURE FEATURES ARE PRESENT AT BIRTH

In the present study, the early postnatal distribution and subsequent fate of visual callosal neurons were studied in neonatal rat pups. Pre vious studies had indicated that the adult pattern of visual callosal neurons was sculpted from an initially uniform distribution in the neo natal cortex. To reexamine this issue, we used a sensitive tracer, lat ex microspheres conjugated either to rhodamine or fluorescein, that wa s injected into the occipital cortex of one hemisphere in pups on the day of birth (PND 1), PND 6, or PND 12. Examination of the resulting r etrograde labeling of cortical neurons in the opposite hemisphere indi cates that features of the mature visual callosal pattern are present as early as PND 1. At all stages of postnatal development, the relativ e density of callosal projection cells varies consistently across the mediolateral extent of primary visual cortex-it is always highest in t he region of the 17/18a border and lowest in the body of area 17. Thes e data strongly suggest that, from the outset, visual cortical neurons in the region of the 17/18a border preferentially make connections wi th the opposite hemisphere. The results of experiments in which callos al neurons were labeled on the day of birth indicate that only those n eurons that have migrated to their final cortical destinations have ex tended callosal axons into the vicinity of the visual cortex in the op posite hemisphere. The initial pattern of callosal neurons resembles a dense, compact version of the mature one, and the present study sugge sts that much of the remaining change in the appearance of this pathwa y may be accounted for by the decrease in the overall density of neuro ns that is due to expansion of the cortical gray matter during postnat al life. Taken together, these results suggest that the development of the visual callosal pathway in the rat may be more similar to that in the monkey than has been reported previously.