Morphogenesis of callosal arbors in the parietal cortex of hamsters

The morphogenesis of callosal axons originating in the parietal cortex was studied by anterograde labeling with Phaseolus lectin or biocytin injected in postnatal (P) hamsters aged 7-25 days. Some labeled fibers were serially reconstructed. At P7, some callosal fibers extended as far as the contrala teral rhinal fissure, with simple arbors located in the homotopic region of the opposite cortical gray matter, and two or three unbranched sprouts alo ng their trajectory. From P7 to P13, the homotopic arbors became more compl ex, with branches focused predominantly, but not exclusively, in the supra- and infragranular layers of the homotopic region. Simultaneously, the late ral extension of the trunk axon in the white matter became shorter, finally disappearing by P25. Arbors in the gray matter were either bilaminar (laye rs 2/3 and 5) or supragranular. A heterotopic projection to the lateral cor tex was consistently seen at all ages; the heterotopic arbors follow a simi lar sequence of events to that seen in homotopic regions. These observation s document that callosal axons undergo regressive tangential remodeling dur ing the first postnatal month, as the lateral extension of the trunk fiber gets eliminated. Radially, however, significant arborization occurs in laye r-specific locations. The protracted period of morphogenesis suggests a cor respondingly long plastic period for this system of cortical fibers.