Distribution patterns and individual variations of callosal connections in the albino rat

After complete callosotomy the distribution of degeneration products was re-investigated in adult albino rats. Three to seven days post operation, coronal, horizontal and "flattened" sections were impregnated according to the new methods of Gallyas et al. (1980) which stain degenerating axons and terminals, respectively. The regional distribution patterns of callosal terminals were directly visualized with dark field illumination at low magnification. With this technique the distribution pattern of axons and terminals could be compared between different cortical regions and individuals. Callosal terminals tend to accumulate in patches or bands along the borders of cortical regions and areas. The concentration of callosal terminals was especially high at the common corners of more than two cortical areas. The callosal system shows a rather constant distribution pattern which is composed of column shaped subunits. Considerable individual variations were recognized concerning the number, position, shape, density and contiguity of the columnar units either occupied by callosal connections or empty. Although the laminar distribution of callosal terminals shows some similarities in different areas of the cortex, there is no common laminar pattern characteristic either for the whole neocortex or for any cortical region. The comparison between consecutive sections stained either for degenerating fibers or degenerating axon terminals revealed that the callosal axons do not determine directly the arrangement and packing density of callosal synapses. Whatever determines the position and amount of callosal synapses this influence seems to be exerted via translation into the columnar organization.