Development of visual callosal connections in neonatally enucleated rats

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Abstract
The present report extends previous descriptions of the mature distributions of callosal cells and axonal terminations in rats monocularly or binocularly enucleated at birth. It also describes the time course of callosal development in these animals, and establishes the age at which eye removal ceases to alter the normal course of callosal development. Although our results indicate that the callosal pattern is anomalous in adult, neonatally enucleated rats, the major features of the normal callosal pattern are nonetheless clearly recognizable in both monocularly and binocularly enucleated rats. Thus, as in normally reared rats, there are dense accumulations of callosal cells and terminations at the 17/18a border region, at the lateral border of area 18a, and within area 18b in enucleated rats. In addition, several narrow bands of callosal connections bridge the width of area 18a at several rostrocaudal levels, and a ring‐like callosal configuration is located anterolateral to area 17. In monocularly enucleated rats, the most prominent anomaly develops in the hemisphere ipsilateral to the remaining eye, where a dense band of callosal connections runs rostrocaudally through the center of area 17. Periodic fluctuations in the density of labeling along the length of this extra band give it a beaded appearance. The callosal pattern in the hemisphere contralateral to the remaining eye in these rats appears normal. Binocular enucleation causes the appearance of discrete regions of reduced labeling within the 17/18a callosal band and several densely labeled tongue‐like regions that extend medially from this band well into area 17. The laminar distribution of callosal cells and terminations is not significantly altered by loss of one or both eyes at birth. Our data indicate that enucleation does not affect the time course of callosal development. Thus, in enucleated pups, all features of the mature callosal pattern can be recognized by 6–7 days of age, and by 12 days of age the patterns appear virtually mature. Finally, our data reveal that monocular or binocular enucleations performed at 6 days of age or later allow the callosal pattern to develop normally, whereas enucleations performed between birth and 5 days of age produce anomalies similar to those observed in rats enucleated at birth. Thus, at about 6 days of age—just as the earliest features of the mature callosal pattern become discernible, and long before rats first open their eyes—the developing callosal pathway is no longer susceptible to disruptions of visual input.