Bihemispheric Axonal Bifurcation of the Afferents to the Visual Cortical Areas during Postnatal Development in the...

Numerous cortical neurons in the juvenile and adult rat project to visual areas of both hemispheres whereas the vast majority of subcortical structures projecting to the visual cortex send strictly ipsilateral projections (Dreher et al., 1990). In the present study, the authors have sought to determine whether this pattern of axonal bifurcation in the connectivity of the visual areas undergoes a change during postnatal development. Two retrograde fluorescent dyes were used, fast blue (FB) and diamidino yellow (DY). Large multiple injections of one of the dyes were placed in all visual areas of one hemisphere and a small injection of the other dye was placed in area 17 of the opposite hemisphere. Labelled neurons were observed in subcortical and cortical structures on the side of the small injection. The experiments were performed on ten neonatal albino rat pups aged between 3 and 12 postnatal days (p.n.d.) at the time of injection and the results were compared with those obtained in the juvenile and adult animals, as reported in the preceding paper. In the thalamus of newborn animals, neurons belonging to nuclei located away from the midline send strictly ipsilateral cortical projections. However, in the midline nuclei of the intralaminar thalamic complex, a small region of overlap was observed between neurons projecting ipsilaterally and neurons projecting contralaterally in animals aged less than 9 postnatal days. In addition, in these neonatal animals a small number of bilaterally projecting neurons was detected in this region of overlap. In all other subcortical structures examined (ventral tegmental area, diagonal band of Broca, claustrum), the laterality of the projection was the same in the newborn and the adult animals. In particular, in the claustrum of neonatal animals, as in adult animals, there was a large contingent of contralaterally projecting neurons and only a very small number of bilaterally projecting neurons. The results in the cortex contrast with those observed in subcortical structures. Whereas ipsilaterally projecting neurons were distributed in a broadly similar way in newborn and adult animals, the laminar and areal distribution of contralaterally projecting neurons in newborn animals clearly differed from those observed in the adult animals. Furthermore, double labelled neurons were more numerous in animals aged less than 12 days than in adults. The proportions of such bilaterally projecting neurons were computed with respect to the numbers of neurons sending ipsilateral projections to area 17. These proportions are constant at all ages in the claustrum and cortical area 8. In areas 18a, 29 and 35 on the other hand, the proportions of bilaterally projecting neurons increase after 5 days and reach a peak in the period extending from 9 to 11 days of age when more than half of the neurons projecting ipsilaterally also send an axonal branch to the contralateral cortex. In cortical areas 29 and 35, this peak is followed by a sudden drop to the adult level at 12 postnatal days, whereas the return to the adult level is gradual in area 18a. These results demonstrate that, in subcortical structures and in cortical area 8, the laterality of the afferent connections to the visual cortex does not change during postnatal development. By contrast, cortical areas 18a, 29 and 35 go through a stage when numerous cells send bifurcating connections to both hemispheres. The timing of the decrease in proportions of bilaterally projecting neurons in these areas suggests that numerous neurons retract their callosal axonal branch when the adult pattern of callosal connectivity is established at 9 - 11 days of age.