The organization of immature callosal connections.

In newborn kittens, the anterograde transport of horseradish peroxidase, alone or bound to wheat-germ agglutinin, indicates that callosal axons have entered selectively the restricted portions of the neocortical gray matter (e.g., the area 17/18 border) which receive callosal afferents in adults. The callosal axons do also reach regions where they lack in the adult, but there they seem not to penetrate far into the gray matter. Neonatal injections of retrograde fluorescent tracers restricted to the gray matter in areas 17, 18, and posteromedial lateral suprasylvian area (PMLS) label neurons in the contralateral hemisphere only when the tracers were directed into regions known to receive callosal axons. In particular, injections near the 17/18 border label neurons in the contralateral hemisphere at the homologous site and at restricted, retinotopically corresponding locations in other visual areas: a pattern similar to the adult one. In contrast, an injection reaching the white matter of areas 17 or 18 labels a wider, continuous territory extending mediolaterally over most visual areas from 17 to posterolateral lateral suprasylvian area (PLLS) and including regions which later become acallosal; in addition, labeled neurons are found in the limbic cortex medial to area 17 and in the auditory cortex lateral to PLLS, none of which is known to project to either 17 or 18 in the adult. In flattened reconstructions of the cortex, the shape of the territory labeled by each of these injections is characteristically, although somewhat irregularly, crescent shaped; its rostrocaudal position varies with that of the injection. An injection extending into the white matter of more lateral visual areas (19, 21a, PMLS) labels callosal neurons over a similar territory, which extensively overlaps that labeled by the 17/18 border injections and likewise includes regions which are acallosal in the adult. In spite of the overlapping distribution of labeling obtained from separate injection sites, as in adults, each cytoarchitectonically (or retinotopically) defined area seems to receive from a different set of neurons, although a few neurons send bifurcating axons to more than one area. In conclusion, injections restricted to the cortical gray matter reveal a topographic organization of juvenile callosal connections similar to that of the adult. In contrast, injections extending into the white matter and adequate to reach the transitory callosal axons which appear to be confined there reveal what appears to be an earlier organization. These two organizations probably reflect different morphogenetic factors.