Quantitative aspects of corticocortical connections: a tracer study in the mouse.

This study provides neuroanatomical data relevant to models and simulations of the propagation of activity over the cortex. We administered small injections of the anterograde tracer biotinylated dextran amine to various regions of the mouse cortex (1 per animal). Two-dimensional reconstructions of the cortical surface were made, showing the distribution, size, and density of the terminal fields. Within the injected hemisphere, the largest part of the terminal field always surrounded the injection site and extended over neighboring areas. On average, axons from injection sites of <or=0.1 mm2 (containing several thousand neurons) diverged onto a region about 180 times larger than the injection site. The density of stained fibers in distant terminal fields could reach about 25 m/mm3. More than half of the total terminal field from an individual injection site consisted of weak projections with densities of 3 or 4 m/mm3. The number of main axons entering an individual distant terminal field ranged between 14 and about 890. By indirect arguments we estimate that the density of stained fibers close to the injection site is 3-6 times that in the most densely labeled distant terminal fields. In addition to symmetric projections to the opposite hemisphere, nonhomotopic callosal projections were found.