The development of the corticotectal pathway in the albino rat.

To study the development of the corticotectal pathway, the enzyme horseradish peroxidase (HRP) was injected electrophoretically into the superior colliculus (SC) of rats ranging in age from newborn to adult. In animals younger than postnatal day 3 (P3), collicular injections did not label any cells in the cortex while in animals injected at P3-P4, only a few cortical cells were retrogradely labeled. In contrast, injections made at P5 or later resulted in the labeling of a substantial proportion of lamina V cells in a number of cortical areas ipsilateral to the injected colliculus. Although at P5-P7 the bulk of labeled cells was located in the visual cortices (both striate and extrastriate), a substantial proportion of the labeled cells was located in the somatosensory, motor and association cortices. On the other hand, in animals injected at P12 (or later), the labeled cells were largely restricted to the visual cortices with relatively few corticotectal cells located in somatosensory area I. At all ages studied, labeled cortical cells were confined to lamina V and had clear-cut apical dendrites (pyramidal cells). The dendritic morphologies and somal sizes of the corticotectal cells indicate that in animals younger than P12 these cells are immature. These observations suggest that the axons of cortical cells do not reach the SC before P3 and that these early corticotectal projections (P3-P12) are established by immature cells. Furthermore, although the corticotectal projection exhibits, from its onset, a high degree of specificity in terms of the laminar distribution of its cells of origin, its areal distribution is 'exuberant'. The 'exuberant' projections originating from non-visual cortical areas disappear by P12-P14, that is at the time when young rats open their eyes for the first time.